LTC1622CS8_技术文档

LTC1622

Low Input Voltage

Current Mode Step-Down

DC/DC Controller

U

FEATURES

DESCRIPTIO

The LTC^®1622 is a constant frequency current mode step-

downDC/DCcontrollerprovidingexcellentACandDCload

and line regulation. The device incorporates an accurate

undervoltage feature that shuts the LTC1622 down when

the input voltage falls below 2V.

■

High Efficiency

■

Constant Frequency 550kHz Operation

■

V_INRange: 2V to 10V

■

Multiampere Output Currents

OPTI-LOOP^TMCompensation Minimizes C_OUT

■

Selectable, Burst ModeOperation

TheLTC1622boastsa±1.9%outputvoltageaccuracyand

consumes only 350µA of quiescent current. For applica-

tions where efficiency is a prime consideration and the

load current varies from light to heavy, the LTC1622 can

be configured for Burst Mode^TMoperation. Burst Mode

operation enhances low current efficiency and extends

battery run time. Burst Mode operation is inhibited during

synchronizationorwhentheSYNC/MODEpinispulledlow

to reduce noise and possible RF interference.

■

Low Dropout Operation: 100% Duty Cycle

■

Synchronizable up to 750kHz

■

Current Mode Operation for Excellent Line and Load

Transient Response

Low Quiescent Current: 350µA

■

Shutdown Mode Draws Only 15µA Supply Current

±1.9% Reference Accuracy

Available in 8-Lead MSOP

■

U

High constant operating frequency of 550kHz allows the

use of a small inductor. The device can also be synchro-

nized up to 750kHz for special applications. The high

frequency operation and the available 8-lead MSOP pack-

age create a high performance solution in an extremely

small amount of PCB area.

APPLICATIO S

■

1- or 2-Cell Li-Ion Powered Applications

■

Cellular Telephones

■

Wireless Modems

Portable Computers

■

Distributed 3.3V, 2.5V or 1.8V Power Systems

To further maximize the life of the battery source, the

P-channel MOSFET is turned on continuously in dropout

(100% duty cycle). In shutdown, the device draws a mere

15µA.

■

Scanners

■

Battery-Powered Equipment

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode and OPTI-LOOP are a trademarks of Linear Technology Corporation.

U

TYPICAL APPLICATIO

Efficiency vs Load Current with

Burst Mode Operation Enabled

100

V

IN

2.5V TO 8.5V

V

= 4.2V

IN

8

C1

R2

10µF

10V

90

80

70

60

50

40

V

IN

SENSE

0.03Ω

V

= 3.3V

IN

2

1

7

–

I

TH

R1

PDRV

Si3443DV

10k

L1

V

= 6V

IN

LTC1622

SYNC/MODE

4.7µH

C3

220pF

V

2.5V

1.5A

OUT

5

6

V

= 8.4V

IN

D1

IR10BQ015

R3

159k

4

+

C2

RUN/SS

GND

47µF

470pF

V

FB

R4

75k

6V

3

V

R

= 2.5V

OUT

SENSE

= 0.03Ω

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT L1: MURATA LQN6C-4R7

1622 F01a

C2: SANYO POSCAP 6TPA47M

R2: DALE WSL-1206 0-03Ω

1

10

100

1000

5000

D1: INTERNATIONAL RECTIFIER IR10BQ015

LOAD CURRENT (mA)

1622 F01b

Figure 1. High Efficiency Step-Down Converter

1

LTC1622

ABSOLUTE MAXIMUM RATINGS

Input Supply Voltage (V_IN).........................–0.3V to 10V

RUN/SS Voltage .......................................–0.3V to 2.4V

SYNC/MODE Voltage ................................. –0.3V to V_IN

SENSE^–Voltage .......................................... 2.4V to V_IN

PDRV Peak Output Current (<10µs) ......................... 1A

Storage Ambient Temperature Range ... –65°C to 150°C

W W

U W

(Note 1)

Operating Temperature Range

Commercial ............................................ 0°C to 70°C

Industrial ........................................... –45°C to 85°C

Junction Temperature (Note 2)............................. 125°C

Lead Temperature (Soldering, 10 sec).................. 300°C

U

W U

PACKAGE/ORDER INFORMATION

ORDER PART

TOP VIEW

NUMBER

TOP VIEW

–

SENSE

1

2

3

4

8

7

6

5

V

IN

–

LTC1622CS8

LTC1622IS8

LTC1622CMS8

SENSE

1

2

3

8 V

IN

I

PDRV

TH

I

V

7 PDRV

TH

FB

6 GND

5 SYNC/MODE

V

GND

FB

RUN/SS 4

RUN/SS

SYNC/MODE

MS8 PART MARKING

LTDB

S8 PART MARKING

MS8 PACKAGE

8-LEAD PLASTIC MSOP

T_JMAX= 125°C, θ_JA= 250°C/ W

S8 PACKAGE

8-LEAD PLASTIC SO

T_JMAX= 125°C, θ_JA= 150°C/ W

1622

1622I

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS ^The● denotes specifications which apply over the full operating

temperature range, otherwise specifications are at T_A= 25°C. V_IN= 4.2V

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

I

Feedback Current

(Note 3) V = 0.8V

10

70

nA

VFB

FB

V

Regulated Feedback Voltage

(Note 3) Commercial Grade

(Note 3) Industrial Grade

●

0.785

0.780

0.8

0.815

0.820

V

FB

V

Output Overvoltage Lockout

Referenced to Nominal V

4

7.5

10.5

0.08

%

OVL

OUT

∆V

Reference Voltage Line Regulation

Output Voltage Load Regulation

V

= 4.2V to 8.5V (Note 3)

IN

0.04

%/V

OSENSE

V

Measured in Servo Loop; V = 0.2V to 0.625V

Measured in Servo Loop; V = 0.9V to 0.625V

0.3

–0.3

0.5

–0.5

%

LOADREG

ITH

I

Input DC Supply Current

Burst Mode Inhibited

Sleep Mode

(Note 4)

S

V

= 2.3V

450

350

15

µA

IN

= 0V, V

= 2.4V

400

30

10

ITH

SYNC/MODE

= 0V

Shutdown

RUN/SS

= 0V, V = V

– 0.1V

4

IN

UVLO

V

RUN/SS Threshold

Commercial Grade

Industrial Grade

●

0.4

0.3

0.7

0.9

1.0

V

RUN/SS

I

f

Soft-Start Current Source

Oscillator Frequency

V

= 0V

RUN/SS

1

2.5

5

µA

RUN/SS

OSC

V

= 0.8V

475

75

550

110

625

140

kHz

FB

= 0V

V

SYNC/MODE Threshold

Undervoltage Lockout

V

Ramping Down

SYNC/MODE

1

1.5

V

SYNC/MODE

UVLO

V

Ramping Down

Ramping Up

●

1.55

1.92

1.97

2.3

2.36

V

IN

2

LTC1622

The ● denotes specifications which apply over the full operating

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T_A= 25°C. V_IN= 4.2V

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

PDRV t

Gate Drive Rise Time

Gate Drive Fall Time

C

= 3000pF

80

100

140

ns

r

f

LOAD

∆V

SENSE(MAX)

Maximum Current Sense Voltage

●

80

110

140

mV

Note 1: Absolute Maximum Ratings are those values beyond which the life

of a device may be impaired.

Note 3: The LTC1622 is tested in a feedback loop that servos V to the

FB

feedback point for the error amplifier (V = 0.8V).

ITH

Note 2: T is calculated from the ambient temperature T and power

Note 4: Dynamic supply current is higher due to the gate charge being

delivered at the switching frequency.

J

A

dissipation P according to the following formula:

D

LTC1622CS8; T = T + (P • 150°C/W),

J

A

D

LTC1622CMS8; T = T + (P • 250°C/W)

J

A

D

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Shutdown Current

vs Supply Voltage

Maximum Current Sense Voltage

vs Duty Cycle

RUN/SS Current vs Supply Voltage

45

40

35

30

25

20

15

10

5

3.50

3.00

2.50

2.00

1.50

1.00

110

100

90

V

IN

= 4.2V

UNSYNC

80

70

60

50

40

0

30

6

7

2

3

4

5

6

7

8

9

10

60 70

DUTY CYCLE (%)

2

3

4

5

8

9

10

20 30

100

40 50

80 90

SUPPLY VOLTAGE (V)

1622 G01

1622 G02

1622 G03

Normalized Oscillator Frequency

vs Temperature

Reference Voltage

vs Temperature

Undervoltage Lockout Voltage

vs Temperature

10.0

7.5

0.810

0.805

0.800

0.795

0.790

0.785

0.780

0.775

2.10

2.05

2.00

1.95

1.90

1.85

1.80

1.75

V

IN

= 4.2V

V

= 4.2V

IN

5.0

2.5

0

–2.5

–5.0

–7.5

–10.0

25 45 65

5

TEMPERATURE (°C)

–55 –35 –15

25 45 65

5

TEMPERATURE (°C)

85 105 125

25 45 65

5

TEMPERATURE (°C)

–55 –35 –15

85 105 125

–55 –35 –15

85 105 125

1622 G04

1622 G05

1622 G06

3

LTC1622

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency vs Load Current for

Figure 1 with Burst Mode

Operation Defeated

Load Step Transient Response

Burst Enabled

Load Step Transient Response

Burst Inhibited

100

90

80

70

60

50

40

V

= 3.3V

IN

V

= 4.2V

IN

V

= 6V

IN

V

= 8.4V

IN

I_LOAD= 50mA TO 1.2A

V_IN= 4.2V

I_LOAD= 50mA TO 1.2A

V_IN= 4.2V

V

R

= 2.5V

= 0.03Ω

OUT

SENSE

1622 G08

1622 G09

1

10

100

1000

LOAD CURRENT (mA)

1622 G07

U

PIN FUNCTIONS

SENSE^–(Pin 1): The Negative Input to the Current Com-

parator.

SYNC/MODE (Pin 5): This pin performs three functions.

Greater than 2V on this pin allows Burst Mode operation

at low load currents, while grounding or applying a clock

signal on this pin defeats Burst Mode operation. An

external clock between 625kHz and 750kHz applied to this

pin forces the LTC1622 to operate at the external clock

frequency. Do not attempt to synchronize below 625kHz.

Pin 5 has an internal 1µA pull-up current source.

I_TH(Pin 2): Error Amplifier Compensation Point. The

current comparator threshold increases with this control

voltage. Nominal voltage range for this pin is 0V to 1.2V.

V_FB(Pin 3): Receives the feedback voltage from an exter-

nal resistive divider across the output capacitor.

RUN/SS (Pin 4): Combination of Soft-Start and Run

Control Inputs. A capacitor to ground at this pin sets the

ramptimetofulloutputcurrent. Thetimeisapproximately

0.45s/µF. Forcing this pin below 0.4V causes all circuitry

to be shut down.

GND (Pin 6): Ground Pin.

PDRV (PIN 7): Gate Drive for the External P-Channel

MOSFET. This pin swings from 0V to V_IN.

V_IN(Pin 8): Main Supply Pin. Must be closely decoupled

to ground Pin 6.

4

LTC1622

U

U W

FUNCTIONAL DIAGRA

Y = “0” ONLY WHEN X IS A CONSTANT “1”

OTHERWISE Y = “1”

V

IN

V

CC

BURST DEFEAT

Y

X

1µA

SLOPE

COMP

SYNC/

5

MODE

OSC

0.36V

0.3V

–

V

IN

1

+

8

SENSE

V

3

FB

–

+

EN

FREQ

–

+

SHIFT

SLEEP

0.8V

REF

+

V

IN

V

+

–

0.12V

EA

ICOMP

–

BURST

Ω

g

m

= 0.5m

2.5µA

V

IN

SWITCHING

0.8V

REFERENCE

2

LOGIC

AND

S

RUN/

I

TH

RUN/SS

4

V

SOFT-START

R

Q

BLANKING

CIRCUIT

IN

V

REF

0.8V

PDRV

7

R

S1

UVLO

TRIP = 1.97V

+

OV

6

V

+ 60mV

–

REF

GND

SHUTDOWN

1622 BD

U

(Refer to Functional Diagram)

OPERATIO

Main Control Loop

current source to charge up the soft-start capacitor C_SS.

When C_SSreaches 0.7V, the main control loop is enabled

with the I_THvoltage clamped at approximately 5% of its

maximum value. As C_SScontinues to charge, I_THis gradu-

ally released allowing normal operation to resume.

TheLTC1622isaconstantfrequencycurrentmodeswitch-

ing regulator. During normal operation, the external

P-channel power MOSFET is turned on each cycle when

the oscillator sets the R_Slatch (R_S1) and turned off when

the current comparator (I_COMP) resets the latch. The peak

inductor current at which I_COMPresets the R_Slatch is

controlledbythevoltageontheI_THpin, whichistheoutput

of the error amplifier EA. An external resistive divider

connected between V_OUTand ground allows EA to receive

an output feedback voltage V_FB. When the load current

increases, it causes a slight decrease in V_FBrelative to the

0.8V reference, which in turn causes the I_THvoltage to

increase until the average inductor current matches the

new load current.

Comparator OV guards against transient overshoots

>7.5% by turning off the P-channel power MOSFET and

keeping it off until the fault is removed.

Burst Mode Operation

The LTC1622 can be enabled to go into Burst Mode

operationatlowloadcurrentssimplybyleavingtheSYNC/

MODE pin open or connecting it to a voltage of at least 2V.

In this mode, the peak current of the inductor is set as if

V_ITH= 0.36V (at low duty cycles) even though the voltage

at the I_THpin is at lower value. If the inductor’s average

current is greater than the load requirement, the voltage at

The main control loop is shut down by pulling the RUN/SS

pin low. Releasing RUN/SS allows an internal 2.5µA

5

LTC1622

U

(Refer to Functional Diagram)

OPERATIO

the I_THpin will drop. When the I_THvoltage goes below

0.12V, the sleep signal goes high, turning off the external

MOSFET. The sleep signal goes low when the I_THvoltage

rises above 0.22V and the LTC1622 resumes normal

operation. The next oscillator cycle will turn the external

MOSFET on and the switching cycle repeats.

Short-Circuit Protection

Whentheoutputisshortedtoground, thefrequencyofthe

oscillator will be reduced to about 110kHz. This lower

frequency allows the inductor current to safely discharge,

thereby preventing current runaway. The oscillator’s fre-

quency will gradually increase to its nominal value when

the feedback voltage increases above 0.65V. Note that

synchronization is inhibited until the feedback voltage

goes above 0.3V.

Frequency Synchronization

The LTC1622 can be externally driven by a TTL/CMOS

compatibleclocksignalupto750kHz. Donot synchronize

the LTC1622 below its maximum default operating fre-

quency of 625kHz as this may cause abnormal operation

and an undesired frequency spectrum. The LTC1622 is

synchronized to the rising edge of the clock. The external

clock pulse width must be at least 100ns and not more

than the period minus 200ns.

Overvoltage Protection

As a further protection, the overvoltage comparator in the

LTC1622 will turn the external MOSFET off when the

feedback voltage has risen 7.5% above the reference

voltage of 0.8V. This comparator has a typical hysteresis

of 35mV.

Synchronization is inhibited when the feedback voltage is

below 0.3V. This is to prevent inductor current buildup

under short-circuit conditions. Burst Mode operation is

deactivated when the LTC1622 is externally driven by a

clock.

Slope Compensation and Peak Inductor Current

The inductor’s peak current is determined by:

V

ITH

I =

PK

10 R

Dropout Operation

SENSE

(

)

When the input supply voltage decreases towards the

output voltage, the rate of change of inductor current

during the ON cycle decreases. This reduction means that

the P-channel MOSFET will remain on for more than one

oscillator cycle since the inductor current has not ramped

up to the threshold set by EA. Further reduction in input

supplyvoltagewilleventuallycausetheP-channelMOSFET

tobeturnedon100%, i.e., DC. Theoutputvoltagewillthen

be determined by the input voltage minus the voltage drop

across the MOSFET, the sense resistor and the inductor.

when the LTC1622 is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak induc-

torcurrent. Theamountofreductionisgivenbythecurves

in Figure 2.

110

100

90

80

70

60

Undervoltage Lockout

I

= 0.4I

PK

RIPPLE

50

40

30

20

10

AT 5% DUTY CYCLE

= 0.2I

TopreventoperationoftheP-channelMOSFETbelowsafe

input voltage levels, an undervoltage lockout is incorpo-

rated into the LTC1622. When the input supply voltage

drops below 2V, the P-channel MOSFET and all circuitry is

turned off except the undervoltage block, which draws

only several microamperes.

I

RIPPLE

PK

AT 5% DUTY CYCLE

V

IN

= 4.2V

UNSYNC

0

10 20 30 40 50 60 70 80 90 100

DUTY CYCLE (%)

1622 F02

Figure 2. Maximum Output Current vs Duty Cycle

6

LTC1622

U

W U U

APPLICATIONS INFORMATION

The basic LTC1622 application circuit is shown in Figure

V

_OUT. The inductor’s peak-to-peak ripple current is given

1. External component selection is driven by the load

by:

requirementandbeginswiththeselectionofLandR_SENSE

Next, the Power MOSFET and the output diode D1 are

selected followed by C_INand C_OUT

.

V − V

V

+ V

IN

OUT OUT D

I

=

RIPPLE

V + V

f L

( )

.

IN

D

wherefistheoperatingfrequency.Acceptinglargervalues

of I_RIPPLEallows the use of low inductances, but results in

higher output voltage ripple and greater core losses. A

reasonable starting point for setting ripple current is

R_SENSESelection for Output Current

R_SENSEis chosen based on the required output current.

Withthecurrentcomparatormonitoringthevoltagedevel-

oped across R_SENSE, the threshold of the comparator

determines the inductor’s peak current. The output cur-

rent the LTC1622 can provide is given by:

I

_RIPPLE=0.4(I_OUT(MAX)).Remember,themaximumI_RIPPLE

occurs at the maximum input voltage.

With Burst Mode operation selected on the LTC1622, the

ripple current is normally set such that the inductor

current is continuous during the burst periods. Therefore,

the peak-to-peak ripple current should not exceed:

0.08

I

RIPPLE

I

=

−

OUT

R

2

SENSE

where I_RIPPLEis the inductor peak-to-peak ripple current

(see Inductor Value Calculation section).

0.036

I

≤

RIPPLE

R

SENSE

A reasonable starting point for setting ripple current is

I_RIPPLE= (0.4)(I_OUT). Rearranging the above equation, it

becomes:

This implies a minimum inductance of:

V − V + V

V

IN

OUT

D

L

=

1

MIN

R

=

for Duty Cycle < 40%

V + V

SENSE

0.036

IN

D

15 I

f

OUT

( )(

)

R

SENSE

However,foroperationthatisabove40%dutycycle,slope

compensation has to be taken into consideration to select

the appropriate value to provide the required amount of

current. Using Figure 2, the value of R_SENSEis:

(Use V_IN(MAX)= V_IN)

A smaller value than L_MINcould be used in the circuit;

however, the inductor current will not be continuous

during burst periods.

SF

R

=

SENSE

Inductor Core Selection

15

I

100

)(

( )(

)

OUT

Once the value for L is known, the type of inductor must be

selected. High efficiency converters generally cannot

affordthecorelossfoundinlowcostpowderedironcores,

forcing the use of more expensive ferrite, molypermalloy

or Kool Mu^®cores. Actual core loss is independent of core

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requires

more turns of wire and therefore copper losses will

increase. Ferritedesignshaveverylowcorelossesandare

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the use

of a smaller inductor for the same amount of inductor

ripplecurrent. However, thisisattheexpenseofefficiency

due to an increase in MOSFET gate charge losses.

The inductance value also has a direct effect on ripple

current. The ripple current, I_RIPPLE, decreases with higher

inductance or frequency and increases with higher V_INor

Kool Mu is a registered trademark of Magnetics, Inc.

7

LTC1622

U

W U U

APPLICATIONS INFORMATION

In applications where the maximum duty cycle is less than

100%andtheLTC1622isincontinuousmode,theR_DS(ON)

is governed by:

preferred at high switching frequencies, so design goals

canconcentrateoncopperlossandpreventingsaturation.

Ferrite core materials saturate “hard,” which means that

the inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequently, output voltage

ripple. Do not allow the core to saturate!

P

R

DS(ON)

2

OUT

1+ δp

DC I

(

)

(

)

where DC is the maximum operating duty cycle of the

LTC1622.

Molypermalloy (from Magnetics, Inc.) is a very good, low

losscorematerialfortoroids,butitismoreexpensivethan

ferrite. A reasonable compromise from the same manu-

facturer is Kool Mu. Toroids are very space efficient,

especially when you can use several layers of wire.

Because they generally lack a bobbin, mounting is more

difficult. However, newsurfacemountabledesignsthatdo

not increase the height significantly are available.

When the LTC1622 is operating in continuous mode, the

MOSFET power dissipation is:

2

) (

V

_OUT+ V_D

P_MOSFET

=

I_OUT1+ δp R_DS(ON)

(

)

V + V_D

IN

2

+K V

I_OUTC_RSS

f

(

^IN) (

)(

)( )

Power MOSFET Selection

An external P-channel power MOSFET must be selected

for use with the LTC1622. The main selection criteria for

the power MOSFET are the threshold voltage V_GS(TH)and

the “on” resistance R_DS(ON),reverse transfer capacitance

C_RSSand total gate charge.

where K is a constant inversely related to gate drive

current. Because of the high switching frequency, the

second term relating to switching loss is important not to

overlook. The constant K = 3 can be used to estimate the

contributions of the two terms in the MOSFET dissipation

equation.

Since the LTC1622 is designed for operation down to low

inputvoltages,asublogiclevelthresholdMOSFET(R_DS(ON)

guaranteed at V_GS= 2.5V) is required for applications that

workclosetothisvoltage.WhentheseMOSFETsareused,

makesurethattheinputsupplytotheLTC1622islessthan

the absolute maximum MOSFET V_GSrating, typically 8V.

The gate drive voltage levels are from ground to V_IN.

Output Diode Selection

The catch diode carries load current during the off-time.

The average diode current is therefore dependent on the

P-channel switch duty cycle. At high input voltages the

diode conducts most of the time. As V_INapproaches V_OUT

the diode conducts only a small fraction of the time. The

most stressful condition for the diode is when the output

is short circuited. Under this condition the diode must

safelyhandleI_PEAKatcloseto100%dutycycle. Therefore,

itisimportanttoadequatelyspecifythediodepeakcurrent

and average power dissipation so as not to exceed the

diode ratings.

The required minimum R_DS(ON)of the MOSFET is gov-

erned by its allowable power dissipation. For applications

that may operate the LTC1622 in dropout, i.e., 100% duty

cycle, at its worst case the required R_DS(ON)is given by:

P

R

=

DS(ON)_DC=100%

2

I

(

1+ δp

) (

)

OUT(MAX)

Under normal load conditions, the average current con-

ducted by the diode is:

where P_Pis the allowable power dissipation and δp is the

temperature dependency of R_DS(ON). (1 + δp) is generally

given for a MOSFET in the form of a normalized R_DS(ON)vs

temperature curve, but δp = 0.005/°C can be used as an

approximation for low voltage MOSFETs.

V − V

IN

OUT

I =

I

OUT

D

V + V

IN

D

8

LTC1622

U

W U U

APPLICATIONS INFORMATION

The allowable forward voltage drop in the diode is calcu-

lated from the maximum short-circuit current as:

1

∆V_OUT≈I_RIPPLEESR +

8fC_OUT

P

D

where f is the operating frequency, C_OUTis the output

capacitance and I_RIPPLEis the ripple current in the induc-

tor. The output ripple is highest at maximum input voltage

since ∆I_Lincreases with input voltage.

V ≈

F

I

SC(MAX)

where P_Dis the allowable power dissipation and will be

determined by efficiency and/or thermal requirements.

The choice of using a smaller output capacitance in-

creases the output ripple voltage due to the frequency

dependent term, but can be compensated for by using

capacitors of very low ESR to maintain low ripple voltage.

TheI_THpinOPTI-LOOPcompensationcomponentscanbe

optimized to provide stable, high performance transient

response regardless of the output capacitors selected.

A fast switching diode must also be used to optimize

efficiency. Schottky diodes are a good choice for low

forwarddropandfastswitchingtimes. Remembertokeep

lead length short and observe proper grounding (see

Board Layout Checklist) to avoid ringing and increased

dissipation.

C_INand C_OUTSelection

Manufacturers such as Nichicon, United Chemicon and

Sanyoshouldbeconsideredforhighperformancethrough-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR (size)

product of any aluminum electrolytic at a somewhat

higher price. Once the ESR requirement for C_OUThas been

met, the RMS current rating generally far exceeds the

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle (V_OUT+ V_D)/

(V_IN+ V_D). To prevent large voltage transients, a low ESR

input capacitor sized for the maximum RMS current must

beused. ThemaximumRMScapacitorcurrentisgivenby:

1/2

]

I_RIPPLE(P-P)requirement.

V

V − V

OUT

(

)

OUT IN

[

C Required I

≈I

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surfacemountconfigurations. Inthecaseoftantalum, itis

critical that the capacitors are surge tested for use in

switching power supplies. An excellent choice is the AVX

TPS, AVX TPSV and KEMET T510 series of surface mount

tantalum, available in case heights ranging from 2mm to

4mm.OthercapacitortypesincludeSanyoOS-CON,Sanyo

POSCAP, Nichicon PL series and the Panasonic SP series.

IN

RMS MAX

V

IN

This formula has a maximum at V_IN= 2V_OUT, where I_RMS

= I_OUT/2. This simple worst-case condition is commonly

usedfordesignbecauseevensignificantdeviationsdonot

offer much relief. Note that capacitor manufacturer’s

ripplecurrentratingsareoftenbasedon2000hoursoflife.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet the

size or height requirements in the design. Due to the high

operating frequency of the LTC1622, ceramic capacitors

can also be used for C_IN. Always consult the manufacturer

if there is any question.

Low Supply Operation

Although the LTC1622 can function down to 2V, the

maximum allowable output current is reduced when V_IN

decreasesbelow3V.Figure3showstheamountofchange

as the supply is reduced down to 2V. Also shown in

Figure3istheeffectofV_INonV_REFasV_INgoesbelow2.3V.

Remember the maximum voltage on the I_THpin defines

The selection of C_OUTis driven by the required effective

series resistance (ESR). Typically, once the ESR require-

ment is satisfied, the capacitance is adequate for filtering.

The output ripple (∆V_OUT) is approximated by:

9

LTC1622

U

W U U

APPLICATIONS INFORMATION

101

is limiting the efficiency and which change would produce

the most improvement. Efficiency can be expressed as:

V

REF

100

99

98

97

96

95

Efficiency = 100% – (η1 + η2 + η3 + ...)

V

ITH

where η1, η2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1622 circuits: 1) LTC1622 DC bias current,

2) MOSFET gate charge current, 3) I²R losses, 4) voltage

drop of the output diode and 5) transition losses.

2.0

2.2

2.4

2.6

2.8

3.0

INPUT VOLTAGE (V)

1622 F03

1. The V_INcurrent is the DC supply current, given in the

electricalcharacteristics, thatexcludesMOSFETdriver

and control currents. V_INcurrent results in a small loss

which increases with V_IN.

Figure 3. Line Regulation of V_REFand V_ITH

the maximum current sense voltage that sets the maxi-

mum output current.

2. MOSFET gate charge current results from switching

the gate capacitance of the power MOSFET. Each time

a MOSFET gate is switched from low to high to low

again,apacketofchargedQmovesfromV_INtoground.

The resulting dQ/dt is a current out of V_INwhich is

typically much larger than the DC supply current. In

continuous mode, I_GATECHG= f(Qp).

Setting Output Voltage

The LTC1622 develops a 0.8V reference voltage between

thefeedback(Pin3)terminalandground(seeFigure4).By

selecting resistor R1, a constant current is caused to flow

throughR1andR2tosettheoutputvoltage. Theregulated

output voltage is determined by:

3. I²R losses are predicted from the DC resistances of the

MOSFET, inductor and current shunt. In continuous

mode the average output current flows through L but

is “chopped” between the P-channel MOSFET in series

withR_SENSEandtheoutputdiode.TheMOSFETR_DS(ON)

plus R_SENSEmultiplied by duty cycle can be summed

with the resistance of the inductor to obtain I²R losses.

R2

R1

V

= 0.8 1+

OUT

For most applications, a 30k resistor is suggested for R1.

To prevent stray pickup, an optional 100pF capacitor is

suggested across R1 located close to LTC1622.

4. The output diode is a major source of power loss at

high currents and gets worse at high input voltages.

The diode loss is calculated by multiplying the forward

voltage drop times the diode duty cycle multiplied by

theloadcurrent. Forexample, assumingadutycycleof

50% with a Schottky diode forward voltage drop of

0.4V, the loss increases from 0.5% to 8% as the load

current increases from 0.5A to 2A.

V

OUT

R2

R1

LTC1622

3

V

FB

100pF

1622 F04

Figure 4. Setting Output Voltage

Efficiency Considerations

5. Transition losses apply to the external MOSFET and

increase with higher operating frequencies and input

voltages. Transition losses can be estimated from:

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%. It is

oftenusefultoanalyzeindividuallossestodeterminewhat

10

LTC1622

U

W U U

APPLICATIONS INFORMATION

Transition Loss = 3(V_IN)²I_{O(MAX) RSS}

(f)

C

V

OUT

LTC1622

R2

R1

Other losses including C_INand C_OUTESR dissipative

losses, and inductor core losses, generally account for

less than 2% total additional loss.

+

I

V

FB

TH

D

FB

Run/Soft-Start Function

1622 F05

The RUN/SS pin is a dual purpose pin that provides the

soft-startfunctionandameanstoshutdowntheLTC1622.

Soft-start reduces input surge current from V_INby gradu-

ally increasing the internal current limit. Power supply

sequencing can also be accomplished using this pin.

Figure 5. Foldback Current Limiting

Design Example

Assume the LTC1622 is used in a single lithium-ion

battery-poweredcellularphoneapplication.TheV_INwillbe

operating from a maximum of 4.2V down to a minimum of

2.7V. Load current requirement is a maximum of 1.5A but

most of the time it will be on standby mode, requiring only

2mA. Efficiency at both low and high load current is

important. Output voltage is 2.5V.

An internal 2.5µA current source charges up an external

capacitor C_SS. When the voltage on the RUN/SS reaches

0.7V the LTC1622 begins operating. As the voltage on

RUN/SS continues to ramp from 0.7V to 1.8V, the internal

current limit is also ramped at a proportional linear rate.

The current limit begins near 0A (at V_RUN/SS= 0.7V) and

ends at 0.1/R_SENSE(V_RUN/SS≥ 1.8V). The output current

thus ramps up slowly, reducing the starting surge current

required from the input power supply. If the RUN/SS has

been pulled all the way to ground, there will be a delay

before the current limit starts increasing and is given by:

In the above application, Burst Mode operation is enabled

by connecting Pin 5 to V_IN.

V

+ V

D

+ V

D

OUT

Maximum Duty Cycle =

= 93%

V

IN(MIN)

t

_DELAY= 2.8 • 10⁵• C_SSin seconds

From Figure 2, SF = 57%.

Pulling the RUN/SS pin below 0.4V puts the LTC1622 into

a low quiescent current shutdown (I_Q< 15µA).

Use the curve of Figure 2 since the operating frequency is

the free running frequency of the LTC1622.

Foldback Current Limiting

SF

0.57

R_SENSE

=

= 0.0253Ω

As described in the Output Diode Selection, the worst-

case dissipation occurs with a short-circuited output

when the diode conducts the current limit value almost

continuously. To prevent excessive heating in the diode,

foldback current limiting can be added to reduce the

current in proportion to the severity of the fault.

15

100

15 1.5A

( )(

I

( )( _OUT)(

)

In the application, a 0.025Ω resistor is used. For the

inductor, the required value is:

4.2 − 2.5

0.036

2.5 + 0.3

4.2 + 0.3

L

=

= 1.33µH

MIN

Foldback current limiting is implemented by adding diode

D_FB(1N4148orequivalent)betweentheoutputandtheI_TH

pin as shown in Figure 5. In a hard short (V_OUT= 0V), the

current will be reduced to approximately 50% of the

maximum output current.

550kHz

0.025

In the application, a 3.9µH inductor is used to reduce

inductor ripple current and thus, output voltage ripple.

For the selection of the external MOSFET, the R_DS(ON)

mustbeguaranteedat2.5VsincetheLTC1622hastowork

11

LTC1622

U

W U U

APPLICATIONS INFORMATION

down to 2.7V. Let’s assume that the MOSFET dissipation

is to be limited to P_P= 250mW and its thermal resistance

is 50°C/W. Hence the junction temperature at T_A= 25°C

will be 37.5°C and δp = 0.005 (37.5 – 25) = 0.0625. The

required R_DS(ON)is then given by:

layout diagram in Figure 6. Check the following in your

layout:

1. IstheSchottkydiodecloselyconnectedbetweenground

at (–) lead of C_INand drain of the external MOSFET?

2. Does the (+) plate of C_INconnect to the sense resistor

as closely as possible? This capacitor provides AC

current to the MOSFET.

P

2

P

R

= 0.11Ω

DS(ON)

DC I

1+ δp

(

) (

)

OUT

3. Is the input decoupling capacitor (0.1µF) connected

The P-channel MOSFET requirement can be met by an

Si6433DQ.

closely between V_IN(Pin 8) and ground (Pin 6)?

4. Connect the end of R_SENSEas close to V_IN(Pin 8) as

possible. The V_INpin is the SENSE⁺of the current

comparator.

5. Is the trace from the SENSE^–(Pin 1) to the Sense

resistor kept short? Does the trace connect close to

The requirement for the Schottky diode is the most strin-

gent when V_OUT= 0V, i.e., short circuit. With a 0.025Ω

R_SENSEresistor, the short-circuit current through the

Schottky is 0.1/0.025 = 4A. An MBRS340T3 Schottky

diode is chosen. With 4A flowing through, the diode is

rated with a forward voltage of 0.4V. Therefore, the worst-

case power dissipated by the diode is 1.6W. The addition

of D_FB(Figure 5) will reduce the diode dissipation to

approximately 0.8W.

R_SENSE

?

6. Keep the switching node, SW, away from sensitive

small signal nodes.

7. Does the V_FBpin connect directly to the feedback

resistors? The resistive divider R1 and R2 must be

connected between the (+) plate of C_OUTand signal

ground. Optional capacitor C1 should be located as

close as possible to the LTC1622.

The input capacitor requires an RMS current rating of at

least 0.75A at temperature, and C_OUTwill require an ESR

of 0.1Ω for optimum efficiency.

PC Board Layout Checklist

R1andR2shouldbelocatedascloseaspossibletothe

LTC1622. R2 should connect to the output as close to

the load as practicable.

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1622. These items are illustrated graphically in the

V

IN

+

R

SENSE

C

IN

1

2

3

4

8

7

6

5

–

SENSE

V

IN

M1

0.1µF

I

TH

PDRV

LTC1622

GND

L1

SW

V

OUT

V

FB

R

ITH

+

C

SYNC/

MODE

OUT

RUN/

SS

C1

C

ITH

C

SS

QUIET SGND

R1

R2

1622 F06

BOLD LINES INDICATE HIGH CURRENT PATHS

Figure 6. LTC1622 Layout Diagram (See PC Board Layout Checklist)

12

LTC1622

U

TYPICAL APPLICATIONS

LTC1622 1.8V/1.5A Regulator with Burst Mode Operation Disabled

C1

47µF

16V

V

IN

+

2.5V TO

8.5V

U1

R2

0.025Ω

1

2

8

7

6

5

1

2

3

4

8

7

6

5

–

SENSE

V

IN

L1

3.3µH

V

1.8V

1.5A

OUT

I

TH

PDRV

LTC1622

GND

R3

R1

93.1k

+

3

4

V

FB

C2

10K

220µF

C3

6V

SYNC/

MODE

RUN/

SS

R4

75k

220pF

C4

560pF

1622 TA01

C1: AVX TPSD476M016R0150

C2: AVX TPSD227M006R0100

L1: MURATA LQN6C3R3

R2: DALE WSL-1206 0.025Ω

U1: INTERNATIONAL RECTIFIER FETKY ^TMIRF7422D2

LTC1622 2.5V/2A Regulator with Burst Mode Operation Enabled

V

IN

3.3V TO

8.5V

+

C1

R2

47µF

16V

× 2

1

2

3

4

8

7

6

–

SENSE

0.02Ω

V

IN

I

PDRV

LTC1622

TH

M1

L1

4.7µH

D1

V

OUT

V

GND

2.5V

2A

FB

R1

10k

R3

RUN/

SS

158k

C2

+

5

SYNC/

MODE

150µF

6V

C3

220pF

R4

75k

× 2

C4

560pF

1622 TA02

C1: AVX TPSD476M016R0150

C2: SANYO POSCAP 6TPA47M

D1: MOTOROLA MBR320T3

L1: COILCRAFT D03316-472

M1: SILICONIX Si3443DV

R2: DALE WSL-2010 0.02Ω

FETKY is a trademark of International Rectifier Corporation.

13

LTC1622

TYPICAL APPLICATIONS

U

LTC1622 2.5V/3A Regulator with External Frequency Synchronization

V

IN

3.3V TO

8.5V

C1

+

R2

47µF

16V

× 2

1

2

3

4

8

7

6

5

0.01Ω

–

SENSE

V

IN

I

TH

PDRV

LTC1622

GND

M1

L1

4.7µH

D1

V

OUT

V

FB

R1

10k

2.5V

3A

R3

C2

SYNC/

MODE

RUN/

SS

158k

100µF

6.3V

× 2

C3

220pF

650kHz

1.5V

R4

75k

C4

560pF

P-P

1622 TA03

C1: AVX TPSD476M016R0150

C2: AVX TPSD107M010R0065

D1: MOTOROLA MBR320T3

L1: COILCRAFT D03316-472

M1: SILICONIX Si3443DV

R2: DALE WSL-2512 0.01Ω

Zeta Converter with Foldback Current Limit

V

IN

2.5V TO

8.5V

C1

+

R2

D2

1N4818

47µF

16V

× 2

1

2

3

4

8

7

6

5

–

0.04Ω

SENSE

V

IN

I

PDRV

LTC1622

TH

Si3441DV

L1B

6.2µH

V

OUT

V

GND

FB

3.3V

+

R1

47k

+

C2

R3

D1

47µF

16V

100µF

RUN/

SS

SYNC/

MODE

232k

L1A

6.2µH

10V

C3

470pF

C4

0.1µF

R4

75k

1622 TA04

V

I

IN

OUT(MAX)

(A)

C1: AVX TPSD476M016R0150

C2: AVX TPSD107M010R0080

D1: MOTOROLA MBRS320T3

2

3

(V)

2.5

3.3

5.0

6.0

8.4

0.45

0.70

0.95

1.00

1.05

L1A

L1B

TOP VIEW

L1A, L1B: BH ELECTRONICS BH511-1012

4

1

R2: DALE WSL-1206 0.04Ω

•

14

LTC1622

U

PACKAGE DESCRIPTION ^{Dimensions in inches (millimeters) unless otherwise noted.}

MS8 Package

8-Lead Plastic MSOP

(LTC DWG # 05-08-1660)

0.118 ± 0.004*

(3.00 ± 0.102)

8

7

6

5

0.118 ± 0.004**

(3.00 ± 0.102)

0.193 ± 0.006

(4.90 ± 0.15)

1

2

3

4

0.040 ± 0.006

(1.02 ± 0.15)

0.034 ± 0.004

(0.86 ± 0.102)

0.007

(0.18)

0° – 6° TYP

SEATING

PLANE

0.012

(0.30)

REF

0.021 ± 0.006

(0.53 ± 0.015)

0.006 ± 0.004

(0.15 ± 0.102)

0.0256

(0.65)

BSC

MSOP (MS8) 1098

* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,

PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.189 – 0.197*

(4.801 – 5.004)

7

5

8

6

0.150 – 0.157**

(3.810 – 3.988)

0.228 – 0.244

(5.791 – 6.197)

1

3

4

2

0.010 – 0.020

(0.254 – 0.508)

× 45°

0.053 – 0.069

(1.346 – 1.752)

0.004 – 0.010

(0.101 – 0.254)

0.008 – 0.010

(0.203 – 0.254)

0°– 8° TYP

0.016 – 0.050

(0.406 – 1.270)

0.050

(1.270)

BSC

0.014 – 0.019

(0.355 – 0.483)

TYP

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

SO8 1298

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.

15

LTC1622

TYPICAL APPLICATION

U

Small Footprint 3.3V/1A Regulator

Efficiency vs Load Current

100

90

80

70

60

50

V

IN

3.3V TO

8.5V

V

= 3.5V

IN

+

C1

R2

10µF

16V

0.025Ω

1

2

3

4

8

7

6

5

–

SENSE

V

IN

CERAMIC

L1

2.2µH

I

PDRV

V

= 4.2V

IN

M1

TH

LTC1622

GND

V

3.3V

1A

OUT

V

= 6V

IN

V

FB

R1

R3

232k

10k

SYNC/

MODE

D1

RUN/

SS

C2

47µF

6V

C3

220pF

C4

560pF

R4

75k

V

R

= 3.3V

OUT

SENSE

= 0.025Ω

1622 TA05

1

10

100

1000

C1: MURATA CERAMIC GRM235Y5V106Z

C2: SANYO POSCAP 6TPA47M

D1: MOTOROLA MBRS120LT3

L1: COILCRAFT D01608C-222

M1: SILICONIX Si3443DY

R2: DALE WSL-2010 0.025Ω

LOAD CURRENT (mA)

1622 TA05b

Efficiency vs Load Current With LTC1622

Configured as Boost Converter

Boost Converter 3.3V/2.5A

100

V

R

= 5V

SENSE

OUT

1

8

V

–

IN

3.3V

= 0.015Ω

SENSE

V

IN

+

C1

R2

C6

90

80

70

60

50

100µF

10V

0.015Ω

2

3

4

7

6

5

0.1µF

V

= 3.3V

I

TH

PDRV

IN

LTC1622

C3

V

5V

2.5A

OUT

470pF

L1

4.6µH

V

FB

GND

R3

M1

SYNC/

MODE

RUN/

SS

105k

C2

R1

33k

+

220µF

10V

D1

R4

20k

C5

150pF

C4

0.1µF

×2

Si6801DQ

1622 TA06a

C1, C2: SANYO POSCAP TPB SERIES M1: SILICONIX Si3442DV

0.001

0.01

0.1

1

D1: MOTOROLA MBRD835L

L1: SUMIDA CEP123-4R6

R2: DALE WS-L2512 0.015Ω

LOAD CURRENT (mA)

1622 TA06b

RELATED PARTS

PART NUMBER

LTC1147 Series

LT1375/LT1376

DESCRIPTION

COMMENTS

High Efficiency Step-Down Switching Regulator Controllers

1.5A, 500kHz Step-Down Switching Regulators

100% DC, 3.5V ≤ V ≤ 16V, HV Version Has 20V

IN

High Frequency, Small Inductor, High Efficiency

LTC1436/LTC1436-PLL High Efficiency, Low Noise, Synchronous Step-Down Converters

24-Pin Narrow SSOP, 3.5V ≤ V ≤ 36V

IN

LTC1438/LTC1439

LTC1474/LTC1475

LTC1624

Dual, Low Noise, Synchronous Step-Down Converters

Low Quiescent Current Step-Down DC/DC Converters

High Efficiency SO-8 N-Channel Switching Regulator Controller

Low Voltage, High Efficiency Step-Down DC/DC Converter

Low Voltage, Monolithic Synchronous Step-Down Regulator

Dual High Efficiency 2-Phase Step-Down Controller

SOT-23 Current Mode Step-Down Controller

Multiple Output Capability, 3.5V ≤ V ≤ 36V

IN

Monolithic, MSOP, I

= 10µA

OUT

8-Pin N-Channel Drive, 3.5V ≤ V ≤ 36V

IN

LTC1626

Monolithic, Constant Off-Time, 2.5V ≤ V ≤ 6V

IN

LTC1627/LTC1707

LTC1628

Low Supply Voltage Range: 2.65V to 8V, 0.5A

Antiphase Drive, 3.5V ≤ V ≤ 36V, Protection

IN

LTC1772

6-Lead SOT-23, 2.5V ≤ V ≤ 9.8V, 550kHz

IN

LTC1735

High Efficiency, Low Noise Synchronous Switching Controller

Burst Mode Operation, Protection, 3.5V ≤ V ≤ 36V

IN

1622f LT/TP 0100 4K • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

16

●

LINEAR TECHNOLOGY CORPORATION 1998

(408)432-1900 FAX:(408)434-0507 www.linear-tech.com


型号：	LTC1622CS8
厂家：	Linear
描述：	Low Input Voltage Current Mode Step-Down DC/DC Controller 低输入电压电流模式降压型DC / DC控制器开关光电二极管控制器
文件：	总16页 (文件大小：222K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC1622CS8 [Linear]

相关型号：

SI9130DB

SI9135LG-T1

SI9135LG-T1-E3

SI9135_11

SI9136_11

SI9130CG-T1-E3

SI9130LG-T1-E3

SI9130_11

SI9137

SI9137DB

SI9137LG

SI9122E