LT3433_1 [Linear]
High Voltage Step-Up/Step-Down DC/DC Converter; 高电压升压/降压型DC / DC转换器
| 型号： | LT3433_1 |
| 厂家： | 
Linear |
| 描述： | High Voltage Step-Up/Step-Down DC/DC Converter |
| 文件： | 总16页 (文件大小：234K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3433
High Voltage
Step-Up/Step-Down
DC/DC Converter
U
FEATURES
DESCRIPTIO
The LT®3433 is a 200kHz fixed-frequency current mode
switching regulator that provides both step-up and step-
down regulation using a single inductor. The IC operates
over a 4V to 60V input voltage range making it suitable for
use in various wide input voltage range applications such
as automotive electronics that must withstand both load
dump and cold crank conditions.
■
Automatic Step-Up and Step-Down Conversion
■
Uses a Single Inductor
■
Wide 4V to 60V Input Voltage Range
■
VOUT from 3.3V to 20V
■
Dual Internal 500mA Switches
■
100µA No-Load Quiescent Current
■
Low Current Shutdown
■
±1% Output Voltage Accuracy
Internal control circuitry monitors system conditions and
converts from single switch buck operation to dual switch
bridged operation when required, seamlessly changing
between step-down and step-up voltage conversion.
Optional Burst Mode® operation reduces no-load quies-
cent current to 100µA and maintains high efficiencies with
light loads.
■
200kHz Operating Frequency
■
Boosted Supply Pin to Saturate High Side Switch
■
Frequency Foldback Protection
Current Limit Foldback Protection
■
■
Current Limit Unaffected by Duty Cycle
16-lead Thermally Enhanced TSSOP Package
■
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APPLICATIO S
Current limit foldback and frequency foldback help pre-
vent inductor current runaway during start-up. Program-
mablesoft-starthelpspreventoutputovershootatstart-up.
■
12V Automotive Systems
■
Wall Adapter Powered Systems
■
Battery Power Voltage Buffering
The LT3433 is available in a 16-lead thermally enhanced
TSSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
U.S. patent number: 5731694
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TYPICAL APPLICATIO
Maximum Output
Current vs VIN
4V to 60V to 5V DC/DC Converter
with Burst Mode Operation
Efficiency
1N4148
90
80
70
60
50
40
30
20
500
400
300
200
V
= 5V
OUT
V
V
IN
V
BST
IN
2.2µF
0.01µF
0.1µF
BUCK
V = 13.8V
IN
SHDN
SWH
LT3433
B160A
SS
100µH
330pF
1nF
B120A
V
C
V
SWL
OUT
V
IN
= 4V
+
68k
V
OUT
47µF
1N4148
BRIDGED
V
BIAS
0.1µF
309k
100k
100
0
BURST_EN
V
FB
SGND PGND
0.1
1
10
100
1000
0
20
30
(V)
40
50
60
10
OUTPUT CURRENT (mA)
V
IN
3433 TA01
3433 TA01b
3433 TA01c
3433f
1
LT3433
W W
U W
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
TOP VIEW
Input Supply (VIN) .................................... –0.3V to 60V
Boosted Supply (VBST) .............. –0.3V to VSW_H + 30V
(VBST(MAX) = 80V)
ORDER PART
SGND
1
2
3
4
5
6
7
8
16
SGND
NUMBER
V
15 SW_L
BST
LT3433EFE
LT3433IFE
SW_H
14
13
12
11
10
9
PWRGND
Internal Supply (VBIAS) ............................. –0.3V to 30V
SW_H Switch Voltage.................................. –2V to 60V
SW_L Switch Voltage ............................... –0.3V to 30V
Feedback Voltage (VFB)............................... –0.3V to 5V
Burst Enable Pin (VBURST_EN) ................... –0.3V to 30V
Shutdown Pin (VSHDN) ............................. –0.3V to 60V
Operating Junction Temperature Range (Note 5)
LT3433E (Note 6) ............................ – 40°C to 125°C
LT3433I ........................................... – 40°C to 125°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
V
IN
V
V
OUT
17
BURST_EN
BIAS
V
C
SHDN
SS
V
FB
FE PART MARKING
SGND
SGND
FE PACKAGE
16-LEAD PLASTIC TSSOP
3433EFE
3433IFE
TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 17)
MUST BE SOLDERED TO SGND
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VIN = 13.8V, VFB = 1.25V, VOUT = 5V, VBURST_EN = 0V, VBST – VIN = 5V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
60
UNITS
V
V
Operating Voltage Range
Undervoltage Lockout
Undervoltage Lockout Hysteresis
Operating Voltage Range
Operating Voltage Range
●
●
4
V
V
IN
Enable Threshold
3.4
3.95
IN(UVLO)
160
mV
V
V
V
●
3.3
3.3
20
OUT
BST
V
V
< V
– V
+ 20V
●
●
75
20
V
V
BST
BST
SW_H
SW_H
I
Normal Operation
Burst Mode Operation
Shutdown
(Notes 2, 3)
●
●
●
580
100
10
940
190
25
µA
µA
µA
VIN
V
V
< 0.6V
< 0.4V
VC
SHDN
V
Internal Supply Output Voltage
Operating Voltage Range
●
●
●
2.6
2.9
20
V
V
BIAS
I
Normal Operation
Burst Mode Operation
Shutdown
660
0.1
0.1
4.5
990
µA
µA
µA
VBIAS
V
V
< 0.6V
VC
< 0.4V
SHDN
Short-Circuit Current Limit
mA
R
R
Boost Supply Switch On-Resistance
Output Supply Switch On-Resistance
Shutdown Pin Thresholds
I
I
= 500mA
= 500mA
●
●
0.8
0.6
1.2
1
Ω
Ω
SWH(ON)
SWL(ON)
SHDN
SW
SW
V
Disable
Enable
●
●
0.4
V
V
1
I
I
I
/I
Boost Supply Switch Drive Current
Output Supply Switch Drive Current
Switch Current Limit
High Side Switch On, I = 500mA
●
●
●
30
30
50
50
0.9
mA/A
mA/A
A
VBST SW
SW
/I
Low Side Switch On, I = 500mA
SW
VOUT SW
0.5
3
0.7
0.35
5
LIM
SS
Foldback Current Limit
V
= 0V
A
FB
I
Soft-Start Output Current
●
9
µA
3433f
2
LT3433
ELECTRICAL CHARACTERISTICS
The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VIN = 13.8V, VFB = 1.25V, VOUT = 5V, VBURST_EN = 0V, VBST – VIN = 5V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
Feedback Reference Voltage
1.224
1.215
1.231
1.238
1.245
V
V
FB
●
●
●
●
∆V
Feedback Reference Line Regulation
5.5V ≤ V ≤ 60V
0.002
35
0.01
100
330
%/V
nA
FB
IN
I
V
Pin Input Bias Current
FB
FB
g
Error Amplifier Transconductance
Error Amplifier Voltage Gain
200
270
66
umhos
dB
m
A
V
I
f
/V
Control Voltage to Switch Transconductance
Operating Frequency
0.6
A/V
SW VC
O
V
V
> 1V
= 0V
185
170
200
215
230
kHz
kHz
FB
FB
●
Foldback Frequency
50
0.8
35
kHz
V
V
Burst Enable Threshold
Input Bias Current
BURST_EN
BURST_EN
ON(MIN)
I
t
t
V
≥ 2V
µA
ns
ns
BURST_EN
Minimum Switch On Time
Minimum Switch Off Time
R = 35Ω (Note 4)
●
●
250
500
450
800
L
R = 35Ω (Note 4)
OFF(MIN)
L
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Supply current specification does not include switch drive
currents. Actual supply currents will be higher.
Note 5: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3: “Normal Operation” supply current specification does not include
Note 6: The LT3433E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3433I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
I
currents. Powering the V
pin externally reduces I supply
BIAS
BIAS CC
current.
Note 4: Minimum times are tested using the high side switch with a 35Ω
load to ground.
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TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Output Current
vs VIN
VBIAS Output Voltage
vs Temperature
VIN Supply Current
vs VIN Supply Voltage
2.8
2.6
2.4
2.2
620
590
560
530
500
500
400
300
200
V
A
= 5V
OUT
T
A
= 25°C
T
= 25°C
BUCK
BRIDGED
100
0
SEE TYPICAL APPLICATION
ON THE FIRST PAGE OF
THIS DATA SHEET
0
15
30
(V)
45
60
–50
0
50
100 125
0
20
30
(V)
40
50
60
10
V
TEMPERATURE (°C)
V
IN
IN
3433 G02
3433 G01
3433 G11
3433f
3
LT3433
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TYPICAL PERFOR A CE CHARACTERISTICS
Error Amp Reference
vs Temperature
Soft-Start Current vs Temperature
Switch Current Limit vs VFB
7.0
6.5
6.0
5.5
5.0
4.5
4.0
1.232
1.231
1.230
1.229
1.228
700
600
500
400
300
T
= 25°C
A
–50
0
50
100 125
0.2
0.6
(V)
0.8
–50
0
50
100 125
0
0.4
V
1.0
TEMPERATURE (°C)
TEMPERATURE (°C)
FB
3433 G03
3433 G04
3433 G05
Oscillator Frequency
vs Temperature
Oscillator Frequency vs VFB
Current Limit vs Temperature
210
205
200
195
190
1.0
0.9
0.8
0.7
0.6
0.5
200
150
100
50
T
= 25°C
A
W/C HIGH
TYPICAL
W/C LOW
0
–50
0
50
100 125
–50 –25
0
25
50
75
125
100
0
0.2
0.4
V
0.6
(V)
0.8
1.0
TEMPERATURE (°C)
TEMPERATURE (°C)
FB
3433 G06
3433 G08
3433 G07
Maximum Boost Supply Switch
Drive Current vs Boost Supply
Voltage
Maximum Output Supply Switch
Drive Current vs Output Supply
Voltage
70
65
60
55
50
45
70
65
60
55
50
45
T
= 25°C
T
= 25°C
A
A
8
8
4
5
6
7
9
10 11 12
4
5
6
7
9
10 11 12
V
– V
(V)
V
(V)
OUT
BST
SW_H
3433 G09
3433 G10
3433f
4
LT3433
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TYPICAL PERFOR A CE CHARACTERISTICS
Switch Resistance
vs Temperature (ISW = 500mA)
VBST Supply Switch Drive Current
vs Temperature (ISW = 500mA)
VOUT Supply Switch Drive Current
vs Temperature (ISW = 500mA)
1.1
1.0
40
37
34
31
28
25
40
37
34
31
28
25
0.9
0.8
0.7
0.6
0.5
R
SWH
R
SWL
0.4
50
100 125
0
25
50
75 100 125
0
25
50
75 100 125
–50 –25
0
25
75
–50 –25
–50 –25
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3433 G12
3433 G13
3433 G14
U
U
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PI FU CTIO S
SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference.
the switch transistor.This pin also supplies power to most
of the IC’s internal circuitry if the VBIAS pin is not driven
externally. This supply will be subject to high switching
transientcurrentssothispinrequiresahighqualitybypass
capacitor that meets whatever application-specific input
ripple current requirements exist.
VBST (Pin 2): Boosted Switch Supply. This “boosted” sup-
ply rail is referenced to the SW_H pin. Supply voltage is
maintained by a bootstrap capacitor tied from the VBST pin
to the SW_H pin. A 1µF capacitor is generally adequate for
most applications.
BURST_EN (Pin 5): Burst Mode Enable/Disable. When
this pin is below 0.3V, Burst Mode operation is enabled.
Pin input bias current < 1µA when Burst Mode operation
is enabled. If Burst Mode operation is not desired, pulling
this pin above 2V will disable the burst function. When
Burst Mode operation is disabled, typical pin input current
= 35µA. BURST_EN should not be pulled above 20V. This
pin is typically shorted to SGND for Burst Mode function,
orconnectedtoeitherVBIAS orVOUT todisableBurstMode
operation.
Thechargeonthebootstrapcapacitorisrefreshedthrough
a diode, typically connected from the converter output
(VOUT), during the switch-off period. Minimum off-time
operationassuresthattheboostcapacitorisrefreshedeach
switch cycle. The LT3433 supports operational VBST sup-
ply voltages up to 75V (absolute maximum) as referenced
to ground.
SW_H (Pin 3): Boosted Switch Output. This is the current
returnfortheboostedswitchandcorrespondstotheemitter
of the switch transistor. The boosted switch shorts the
SW_H pin to the VIN supply when enabled. The drive cir-
cuitry for this switch is boosted above the VIN supply
through the VBST pin, allowing saturation of the switch for
maximum efficiency. The “ON” resistance of the boosted
switch is 0.8Ω.
VC (Pin 6): Error Amplifier Output. The voltage on the VC
pincorrespondstothemaximumswitchcurrentperoscil-
latorcycle. Theerroramplifieristypicallyconfiguredasan
integrator circuit by connecting an RC network from this
pin to ground. This circuit typically creates the dominant
polefortheconverterregulationfeedbackloop.Specificin-
tegratorcharacteristicscanbeconfiguredtooptimizetran-
sient response. See Applications Information.
VIN (Pin 4): Input Power Supply. This pin supplies power
to the boosted switch and corresponds to the collector of
3433f
5
LT3433
U
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PI FU CTIO S
VFB (Pin7):ErrorAmplifierInvertingInput.Thenoninvert-
ing input of the error amplifier is connected to an internal
1.231V reference. The VFB pin is connected to a resistor
divider from the converter output. Values for the resistor
connected from VOUT to VFB (RFB1) and the resistor con-
nectedfromVFB toground(RFB2)canbecalculatedtopro-
gram converter output voltage (VOUT) via the following
relation:
The time from VSS = 0V to maximum available current can
be calculated given a capacitor CSS as:
tSS = (2.7 • 105)CSS or 0.27s/µF
SHDN (Pin 11): Shutdown. If the SHDN pin is externally
pulledbelow0.5V,lowcurrentshutdownmodeisinitiated.
Duringshutdownmode,allinternalfunctionsaredisabled,
and ICC is reduced to 10µA. This pin is intended to receive
a digital input, however, there is a small amount of input
hysteresisbuiltintotheSHDNcircuittohelpassureglitch-
free mode switching. If shutdown is not desired, connect
the SHDN pin to VIN.
VOUT = 1.231 • (RFB1 + RFB2)/RFB2
The VFB pin input bias current is 35nA, so use of extremely
high value feedback resistors could cause a converter
output that is slightly higher than expected. Bias current
error at the output can be estimated as:
VBIAS (Pin 12): Internal Local Supply. Much of the LT3433
circuitry is powered from this supply, which is internally
regulated to 2.5V through an on-board linear regulator.
CurrentdriveforthisregulatorissourcedfromtheVIN pin.
The VBIAS supply is short-circuit protected to 5mA.
∆VOUT(BIAS) = 35nA • RFB1
The voltage on VFB also controls the LT3433 oscillator
frequencythrougha“frequency-foldback”function.When
theVFB pinvoltageisbelow0.8V,theoscillatorrunsslower
than the 200kHz typical operating frequency. The oscilla-
torfrequencyslowswithreducedvoltageonthepin, down
to 50kHz when VFB = 0V.
The VBIAS supply only sources current, so forcing this pin
abovetheregulatedvoltageallowstheuseofexternalpower
formuchoftheLT3433circuitry.Whenusingexternaldrive,
this pin should be driven above 3V to assure the internal
supply is completely disabled. This pin is typically diode-
connectedtotheconverteroutputtomaximizeconversion
efficiency. This pin must be bypassed with at least a 0.1µF
ceramic capacitor to SGND.
The VFB pin voltage also controls switch current limit
througha“current-limitfoldback”function.AtVFB=0V,the
maximum switch current is reduced to half of the normal
value. The current limit value increases linearly until VFB
reaches 0.6V when the normal maximum switch current
level is restored. The frequency and current-limit foldback
functions add robustness to short-circuit protection and
help prevent inductor current runaway during start-up.
VOUT (Pin 13): Converter Output Pin. This pin voltage is
compared with the voltage on VIN internally to control
operation in single or 2-switch mode. When the ratios of
thetwovoltagesaresuchthata>75%dutycycleisrequired
forregulation,thelowsideswitchisenabled.Drivebiasfor
the low side switch is also derived directly from this pin.
SS(Pin10):SoftStart. Connectacapacitor(CSS)fromthis
pin to ground. The output voltage of the LT3433 error
amplifier corresponds to the peak current sense amplifier
output detected before resetting the switch output(s). The
soft-start circuit forces the error amplifier output to a zero
peak current for start-up. A 5µA current is forced from the
SS pin onto an external capacitor. As the SS pin voltage
ramps up, so does the LT3433 internally sensed peak cur-
rent limit. This forces the converter output current to ramp
from zero until normal output regulation is achieved. This
function reduces output overshoot on converter start-up.
PWRGND (Pin 14): High Current Ground Reference. This
isthecurrentreturnforthelowsideswitchandcorresponds
to the emitter of the low side switch transistor.
SW_L(Pin15):GroundReferencedSwitchOutput.Thispin
is the collector of the low side switch transistor. The low
sideswitchshortstheSW_LpintoPWRGNDwhenenabled.
The series impedance of the ground-referenced switch is
0.6Ω.
Exposed Pad (Pin 17): Exposed Pad must be soldered to
PCB ground for optimal thermal performance.
3433f
6
LT3433
W
BLOCK DIAGRA
V
BIAS
12
BURST
CONTROL
CIRCUITS
BIAS
BURST_EN
5
4
1.25V
V
IN
SENSE
AMPLIFIER
V
BST
2
3
COMPARATOR
BOOSTED
DRIVER
SW_H
SWITCH
CONTROL
LOGIC
SLOPE
COMP
OSCILLATOR 200kHz
SW_L
GND
15
14
7
FREQUENCY
CONTROL
DRIVER
MODE
CONTROL
V
FB
ERROR
AMPLIFIER
30%
LOAD
1.231V
V
C
6
SHDN
+
–
11
Burst Mode
CONTROL
SHUTDOWN
15%
LOAD
0.7V
SS
10
13
5µA
V
OUT
+
SGND
1, 8, 9,16,17
3433 BD
V
OUT
3433f
7
LT3433
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APPLICATIO S I FOR ATIO
Overview
condition as requiring a duty cycle greater than 75%. If
suchaconditionexists, asecondswitchisenabledduring
theswitchontime,whichactstopulltheoutputsideofthe
inductor to ground. This “bridged” operation allows volt-
age conversion to continue when VOUT approaches or
exceeds VIN.
The LT3433 is a high input voltage range, step-up/step-
down DC/DC converter IC using a 200kHz constant fre-
quency, currentmodearchitecture. Dualinternalswitches
allow the full input voltage to be imposed across the
switched inductor, such that both step-up and step-down
modes of operation can be realized using the same single
inductor topology.
Shutdown
The LT3433 incorporates a low current shutdown mode
where all IC functions are disabled and the VIN current is
reduced to 10µA. Pulling the SHDN pin down to 0.4V or
less activates shutdown mode.
The LT3433 has provisions for high efficiency, low load
operation for battery-powered applications. Burst Mode
operation reduces average quiescent current to 100µA in
noloadconditions.Alowcurrentshutdownmodecanalso
be activated, reducing total quiescent current to 10µA.
Burst Mode Operation
Much of the LT3433’s internal circuitry is biased from an
internal low voltage linear regulator. The output of this
regulatorisbroughtouttotheVBIAS pin, allowingbypass-
ing of the internal regulator. The associated internal
circuitry can be powered directly from the output of the
converter, increasing overall converter efficiency. Using
externally derived power also eliminates the IC’s power
dissipation associated with the internal VIN to VBIAS
regulator.
TheLT3433employslowcurrentBurstModefunctionality
to maximize efficiency during no load and low load condi-
tions. Burst Mode function is disabled by shorting the
BURST_EN pin to either VBIAS or VOUT. Burst Mode
function is enabled by shorting BURST_EN to SGND.
In certain wide current range applications, the IC could
enterburstoperationduringnormalloadconditions. Ifthe
additional output ripple and noise generated by Burst
Mode operation is not desired for normal operation,
BURST_EN can be biased using an external supply that is
disabled during a no-load condition. This enables Burst
Mode operation only when it is required. The BURST_EN
pin typically draws 35µA when Burst Mode operation is
disabled (VBURST_EN ≥ 2V) and will draw no more than
75µA with VBURST_EN = 2V.
Theory of Operation (See Block Diagram)
The LT3433 senses converter output voltage via the VFB
pin. The difference between the voltage on this pin and an
internal 1.231V reference is amplified to generate an error
voltage on the VC pin which is, in turn, used as a threshold
for the current sense comparator.
When the required switch current, sensed via the VC pin
voltage, is below 30% of maximum, the Burst Mode
functionisemployed.WhenthevoltageonVC dropsbelow
the 30% load level, that level of sense current is latched
intotheIC. Iftheoutputloadrequireslessthanthislatched
currentlevel,theconverterwilloverdrivetheoutputslightly
during each switch cycle. This overdrive condition forces
the voltage on the VC pin to continue to drop. When the
voltage on VC drops below the 15% load level, switching
isdisabled,andtheLT3433shutsdownmostofitsinternal
circuitry, reducing quiescent current to 100µA. When the
voltage on the VC pin climbs back to 20% load level, the IC
returns to normal operation and switching resumes.
During normal operation, the LT3433 internal oscillator
runs at 200kHz. At the beginning of each oscillator cycle,
the switch drive is enabled. The switch drive stays enabled
until the sensed switch current exceeds the VC-derived
threshold for the current sense comparator and, in turn,
disables the switch driver. If the current comparator
threshold is not obtained for the entire oscillator cycle, the
switch driver is disabled at the end of the cycle for 250ns.
This minimum off-time mode of operation assures regen-
eration of the VBST bootstrapped supply.
If the converter input and output voltages are close
together, proper operation in normal buck configuration
would require high duty cycles. The LT3433 senses this
3433f
8
LT3433
W U U
APPLICATIO S I FOR ATIO
U
Antislope Compensation
switches the VOUT side of the inductor. The LT3433
bridged topology merges the elements of buck and boost
topologies, providing switches on both sides of the induc-
tor. Operating both switches simultaneously achieves
both step-up and step-down functionality.
Most current mode switching controllers use slope com-
pensationtopreventcurrentmodeinstability. TheLT3433
is no exception. A slope compensation circuit imposes an
artificial ramp on the sensed current to increase the rising
slope as duty cycle increases. Unfortunately, this addi-
tional ramp corrupts the sensed current value, reducing
the achievable current limit value by the same amount as
the added ramp represents. As such, current limit is
typically reduced as duty cycles increase.
Step-Down (V > V
)
IN
OUT
SW
L
V
IN
V
OUT
C
IN
D
C
OUT
TheLT3433containscircuitrytoeliminatethecurrentlimit
reduction associated with slope-compensation, or anti-
slope compensation. As the slope compensation ramp is
addedtothesensedcurrent, asimilarrampisaddedtothe
current limit threshold reference. The end result is that
current limit is not compromised so the LT3433 can
provide full power regardless of required duty cycle.
Step-Up (V < V
IN
)
OUT
D
L
V
IN
V
OUT
C
IN
SW
C
OUT
Step-Up/Step-Down (V > V
or V < V
)
IN
OUT
IN
OUT
D
SW
L
V
IN
V
OUT
Mode Switching
TheLT3433switchesbetweenbuckandbuck/boostmodes
of operation automatically. While in buck mode, if the
converter input voltage becomes close enough to the
output voltage to require a duty cycle greater than 75%,
the LT3433 enables a second switch which pulls the
output side of the inductor to ground during the switch-on
time. This “bridged” switching configuration allows volt-
ageconversiontocontinuewhenVIN approachesorisless
C
D
SW
C
OUT
IN
3433 F01
Maximumdutycyclecapability(DCMAX)gatesthedropout
capabilities of a buck converter. As VIN – VOUT is reduced,
the required duty cycle increases until DCMAX is reached,
beyond which the converter loses regulation. With a
secondswitchbridgingtheswitchedinductorbetweenVIN
and ground, the entire input voltage is imposed across the
inductor during the switch-on time, which subsequently
reduces the duty cycle required to maintain regulation.
Using this topology, regulation is maintained as VIN ap-
than VOUT
.
When the converter input voltage falls to where the duty
cycle required for continuous buck operation is greater
than 75%, the LT3433 enables its ground-referred switch,
changing the converter operation to a dual-switch bridged
configuration. Because the voltage available across the
switched inductor is greater while bridged, operational
duty cycle will decrease. Voltage drops associated with
external diodes and loss terms are estimated internally so
that required operating duty cycle can be calculated re-
gardless of specific operating voltages.
proaches or drops below VOUT
.
Inductor Selection
TheprimarycriterionforinductorvalueselectioninLT3433
applications is the ripple current created in that inductor.
Design considerations for ripple current are converter
output capabilities in bridged mode, output voltage ripple
and the ability of the internal slope compensation wave-
form to prevent current mode instability.
In the simplest terms, a buck DC/DC converter switches
the VIN side of the inductor, while a boost converter
3433f
9
LT3433
W U U
U
APPLICATIO S I FOR ATIO
The requirement for avoiding current mode instability is
that the rising slope of sensed inductor ripple current (S1)
isgreaterthanthefallingslope(S2). Atdutycyclesgreater
than50%thisisnottrue.Toavoidtheinstabilitycondition,
a false signal is added to the sensed current with a slope
(SX) that is sufficient to prevent current mode instability,
or S1 + SX ≥ S2. This leads to the following relations:
Converter Capabilities
The output current capability of an LT3433 converter is
affected by a myriad of variables. The current in the
switches is limited by the LT3433. Switch current is
measured coming from the VIN supply, and does not
directly translate to a limitation in load current. This is
especially true during bridged mode operation when the
converter output current is discontinuous.
SX ≥ S2(2DC – 1)/DC
If the forward voltages of a converter’s catch and pass
diodes are defined as VF1 and VF2, then:
During bridged mode operation, the converter output
current is discontinuous, or only flowing to the output
while the switches are off (not to be confused with discon-
tinuous switcher operation). As a result, the maximum
output current capability of the converter is reduced from
that during buck mode operation by a factor of roughly
1 – DC, not including additional losses. Most converter
losses are also a function of DC, so operational duty cycle
must be accurately determined to predict converter load
capabilities.
S2 = (VOUT + VF1 + VF2)/L
Solving for L yields a relation for the minimum inductance
that will satisfy slope compensation requirements:
LMIN = (VOUT + VF1 + VF2)(2DC – 1)/(DC • SX)
The LT3433 maximizes available dynamic range using a
slope compensation generator that generates a continu-
ously increasing slope as duty cycle increases. The slope
compensationwaveformiscalibratedat80%dutycycleto
generate an equivalent slope of at least 0.05A/µs. The
equation for minimum inductance then reduces to:
V
IN
SW_H
LT3433
SW_L
D1
V
LMIN = (VOUT + VF1 + VF2)(15e-6)
L
D2
For example, with VOUT = 5V and using VF1 + VF2 = 1.1V
(cold):
OUT
LMIN = (5 + 1.1)(15e-6) = 91.5µH
3433 AI02
Slope Compensation Requirements
Typical Minimum Inductor Values vs VOUT
Application variables:
VIN = Converter input supply voltage
VOUT = Converter programmed output voltage
VBST = Boosted supply voltage (VBST – VSWH
DC = Operational duty cycle
fO = Switching frequency
IMAX = Peak switch current limit
∆IL = Inductor ripple current
ISW = Average switch current or peak switch current
less half the ripple current (IMAX – ∆IL/2)
350
300
250
200
150
100
50
)
14 16
4
6
8
10 12
18 20
RSWH = Boosted switch “on” resistance
RSWL = Grounded switch “on” resistance
V
(V)
OUT
3433 AI01
L = Inductor value
3433f
10
LT3433
W U U
APPLICATIO S I FOR ATIO
U
RL = Inductor series resistance
Once DC is determined, maximum output current can be
determined using current conservation on the converter
output:
∆BST = Boosted switch drive currents IVBST/ISW (in A/A)
∆
= Grounded switch drive currents IVOUT/ISW
OUT
(in A/A)
Bridged Operation: IOUT(MAX) = ISW • [1 – DC •
(1 + ∆BST + ∆OUT)] – IBIAS
VF1 = Switch node catch diode forward voltage
VF2 = Pass diode forward voltage
Buck Operation:
IOUT(MAX) = ISW • (1 – DC • ∆BST)
– IBIAS
IVIN = VIN quiescent input current
IIN = VIN switched current
IBIAS = VBIAS quiescent input current
PIN = POUT + PLOSS, where PLOSS = PSWON + PSWOFF + PIC,
correspondingtothepowerlossintheconverter.PIC isthe
quiescent power dissipated by the LT3433. PSWON is the
loss associated with the power path during the switch on
interval, and PSWOFF is the PowerPathTM loss associated
with the switch off interval.
RCESR = Output capacitor ESR
Operational duty cycle is a function of voltage imposed
across the switched inductance and switch on/off times.
Using the relation for change in current in an inductor:
PLOSS equals the sum of the power loss terms:
PVIN = VIN • IVIN
δI = V • δt/L
and putting the application variables into the above rela-
tion yields:
PBIAS = VOUT • IBIAS
PSWON(BRIDGED) = DC • [ISW 2 • (RSWH + RSW2 L+ RL)
δION(BRIDGED) = (DC/fO • L)[VIN – ISW • (RSWH + RSWL
+ RL)]
+ ISW • VOUT • (∆BST + ∆OUT) + RCESR • IOUT
]
PSWON(BUCK) = DC • [ISW 2 • (RSWH + RL) + ISW
VOUT • ∆BST + RCESR • (ISW • (1 – ∆BST) – IBIAS
IOUT)2]
•
–
δION(BUCK) = (DC/fO • L)[VIN – VOUT – VF2 – ISW
• (RSWH + RL + RESR)]
δIOFF = [(1 – DC)/fO • L][VOUT + VF1 + VF2 – ISW
• (RL + RESR)]
PSWOFF = (1 – DC) • [ISW • (VF1 + VF2) + ISW2 • RL +
RCESR • (ISW – IBIAS – IOUT)2]
Current conservation in an inductor dictates δION = δIOFF
sopluggingintheaboverelationsandsolvingforDCyields:
,
Efficiency (E) is described as POUT/PIN, so:
–1
Efficiency = {1 + (PVIN + PBIAS + PSWON + PSWOFF)/POUT
}
DC(BRIDGED) = [VOUT + VF1 + VF2 – ISW • (RL + RESR)]/
[VIN – ISW • (RSWH + RSWL + 2RL + RESR) + VOUT
VF1 + VF2
+
Empirical determination of converter capabilities is ac-
complished by monitoring inductor currents with a cur-
rent probe under various input voltages and load currents.
Decreasing input voltage or increasing load current re-
sults in an inductor current increase. When peak inductor
currents reach the switch current limit value, maximum
output current is achieved. Limiting the inductor currents
to the LT3433 specified W/C current limit of 0.5V (cold)
will allow margin for operating limit variations. These
limitations should be evaluated at the operating tempera-
ture extremes required by the application to assure robust
performance.
]
DC(BUCK) = [VOUT + VF1 + VF2 – ISW • (RL + RESR)]/
[VIN – ISW • (RSWH + 2RL + 2RESR) + VF1]
In order to solve the above equations, inductor ripple
current (∆I) must be determined so ISW can be calculated.
∆I follows the relation:
∆I = (VOUT + VF1 + VF2 – ISW • RL)(1 – DC)/(L • fO)
As ∆I is a function of DC and vice-versa, the solution is
iterative. Seed ∆I and solve for DC. Using the resulting
value for DC, solve for ∆I. Use the resulting ∆I as the new
seed value and repeat. The calculated value for DC can be
usedoncetheresulting∆Iisclose(<1%)totheseedvalue.
PowerPath is a trademark of Linear Technology Corporation
3433f
11
LT3433
W U U
U
APPLICATIO S I FOR ATIO
Design Example
CALCULATED VALUES
ITERATION #
SEED ∆I
0
I
DC
∆I
SW
4V-60V to 5V DC/DC converter (the application on the
1
2
3
0.55
0.503
0.501
0.683
0.674
0.674
0.095
0.098
0.098
frontpageofthisdatasheet), loadcapabilityforTA =85°C.
0.095
0.098
Application Specific
Constants:
LT3433 W/C Constants:
VIN = 4V
IMAX = 0.55A
RSWH = 1.2Ω
RSWL = 1Ω
After iteration, DC = 0.674 and ∆I = 0.098.
V
OUT = 5V
Use iteration result for DC and above design constants to
solve the IOUT(MAX) relation:
L = 100µH
RL = 0.28Ω
fO = 190kHz
IOUT(MAX) = 0.501 • [1 – 0.674 • (1 + 0.05 + 0.05)] –
800µA
V
F1 = 0.45V
∆
BST = 0.05
OUT = 0.05
VF2 = 0.4V
RCESR = 0.01Ω
∆
IOUT(MAX) = 129mA
IVIN = 600µA
IBIAS = 800µA
Increased Output Voltages
The LT3433 can be used in converter applications with
output voltages from 3.3V through 20V, but as converter
output voltages increase, output current and duty cycle
limitations prevent operation with VIN at the extreme low
end of the LT3433 operational range. When a converter
operates as a buck/boost, the output current becomes
discontinuous,whichreducesoutputcurrentcapabilityby
roughly a factor of 1 – DC, where DC = duty cycle. As such,
the output current requirement dictates a minimum input
voltage where output regulation can be maintained.
TheLT3433operatesinbridgedmodewithVIN =4V,sothe
relations used are:
DC = [VOUT + VF1 + VF2 – ISW • (RL + RESR)]/[VIN –
ISW • (RSWH + RSWL + 2RL + RESR) + VOUT + VF1 +
VF2]
∆I = (VOUT + VF1 + VF2 - ISW • RL) • (1 – DC)/(L • fO)
IOUT(MAX) = ISW • [1 – DC • (1 + ∆BST + ∆OUT)] – IBIAS
Iteration procedure for DC:
(1) Set initial seed value for ∆I (this example will set
∆I = 0).
Typical Minimum Input Voltage as a Function of
Output Voltage and Required Load Current
(2) Using seed value for ∆I, determine ISW (ISW = 0.55 –
24
0 = 0.55).
(3) Use calculated ISW and above design constants to
solve the DC relation (DC = 0.683).
20
200mA
16
(4) Use calculated DC to solve the ∆I relation (yields ∆I =
175mA
0.0949).
12
125mA
(5) If calculated ∆I is equal to the seed value, stop.
Otherwise, use calculated ∆I as new seed value and
repeat (2) through (4).
150mA
8
4
4
8
12
16
20
V
(V)
OUT
3433 AI03
3433f
12
LT3433
W U U
APPLICATIO S I FOR ATIO
U
4V-50V to 5V Converter Input Transient Response
1ms 13.8V to 4V Input Transition
Input Voltage Transient Suppression
Not only does a LT3433 converter operate across a large
range of DC input voltages, it also maintains tight output
regulation during significant input voltage transients. The
LT3433 automatic transitioning between buck and buck/
boostmodesofoperationprovidesseamlessoutputregu-
lationovertheseinputvoltagetransients.Inanautomotive
environment, input voltage transients are commonplace,
such as those experienced during a cold crank condition.
During the initiation of cold crank, the battery rail can be
pulleddownto4Vinaslittleas1ms. Ina4V-60Vto5VDC/
DC converter application (shown on the first page of this
data sheet) a cold crank transient condition, simulated
with a 1ms 13.8V to 4V input transition, yields regulation
maintained to 1% with a 125mA load.
VIN
5V/DIV
VOUT
0.1V/DIV
1ms/DIV
3433 AI04
U
TYPICAL APPLICATIO S
4V-60V to 5V Converter with Switched Burst Enable and Shutdown
L1
D
S1
100µH
B160A
COEV DU1352-101M
V
OUT
5V
4V < V < 8.5V: 125mA
D2
IN
8.5V < V < 60V: 350mA
D
+
S2
C7
47µF
10V
1N4148
IN
B120A
Efficiency
C5
1µF
10V
V
SW_L
BST
90
80
R4
20k
V
= 13.8V
IN
V
BATT
(SWITCHED)
SW_H PWRGND
LT3433
V
BATT
4V TO 60V
V
V
70
V
IN
OUT
D1
V
= 4V
C4
2.2µF
100V
IN
= 13.8V
BURST
1N4148
IN
BURST_EN
V
BIAS
60
50
40
30
20
C6
0.1µF 10V
C3 330pF
DZ1
20V
V
C
SHDN
SS
C2
1nF
V
= 4V
R3
100k
R1
68k
IN
V
FB
(BURST)
C1
SGND
0.01µF
R2
100k
1%
R5
309k
1%
0.1
1
10
100
1000
MODE SWITCH:
OUTPUT CURRENT (mA)
V
V
H-L: 7.9V
L-H: 8.3V
IN
IN
3433 TA03b
3433 TA03a
SHDN
3433f
13
LT3433
U
TYPICAL APPLICATIO S
8V-60V to 12V Converter
L1
200µH
TDK SLF12565T-221M1R0
D
S1
B160A
V
OUT
12V
8V < V < 18V: 125mA
D2
IN
D
S2
C5
47µF
25V
+
1N4148
18V < V < 60V: 380mA
IN
B120A
C7
0.47µF
20V
V
SW_L
BST
SW_H PWRGND
LT3433
V
IN
V
V
IN
OUT
8V TO 60V
D1
1N4148
C3 330pF
C6
2.2µF
100V
(BURST)
BURST_EN
V
BIAS
C6
0.1µF
20V
R1 68k
V
C
SHDN
SS
C2 1nF
V
FB
C4
0.01µF
R2
20k
1%
R3
174k
1%
SGND
MODE SWITCH:
V
V
H-L: 16.6V
L-H: 17V
IN
IN
(NO BURST)
3433 TA04a
Efficiency
Minimum Output Current vs VIN
500
400
300
200
100
100
V
= 20V
IN
90
80
70
60
50
40
30
20
BRIDGED
V
= 8V
IN
V
= 20V
IN
(BURST)
V
= 8V
IN
BUCK
(BURST)
0
0
20
30
(V)
40
50
60
10
0.1
1
10
100
1000
V
OUTPUT CURRENT (mA)
IN
3433 TA04b
3433 TA04c
3433f
14
LT3433
U
PACKAGE DESCRIPTIO
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BB
4.90 – 5.10*
(.193 – .201)
3.58
(.141)
3.58
(.141)
16 1514 13 12 1110
9
6.60 ±0.10
4.50 ±0.10
2.94
(.116)
6.40
(.252)
BSC
SEE NOTE 4
2.94
(.116)
0.45 ±0.05
1.05 ±0.10
0.65 BSC
5
7
8
1
2
3
4
6
RECOMMENDED SOLDER PAD LAYOUT
1.10
(.0433)
MAX
4.30 – 4.50*
(.169 – .177)
0.25
REF
0° – 8°
0.65
(.0256)
BSC
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
0.05 – 0.15
(.002 – .006)
0.195 – 0.30
FE16 (BB) TSSOP 0204
(.0077 – .0118)
TYP
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
3433f
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-
tation that the interconnection of its circuits as described herein will notinfringe onexisting patent rights.
15
LT3433
U
TYPICAL APPLICATIO
Burst Only Low Noise 5V Maintenance Supply
L1
D
D
S1
B160A
S2
B120A
33µH
COILCRAFT LPO1704-333
D1
1N4148
V
SW_L
BST
C1
0.1µF
SW_H PWRGND
LT3433
V
IN
V
V
IN
OUT
4V TO 60V
D2
1N4148
2.2µF
BURST_EN
V
BIAS
C6 100pF
C2
0.1µF
V
C
SHDN
SS
V
FB
V
OUT
R2
510k
5%
R1
5V
IN
OUT
BYP
SGND
C4
0.01µF
C5
2.2µF
2.2M
LT1761-5
10mA
5%
SHDN
GND
C3
10µF
3433 TA02
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
V : 7.3V to 45V/64V, V
LT1076/LT1076HV
1.6A (I ), 100kHz High Efficiency Step-Down DC/DC Converters
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Q
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< 10µA, DD5/DD7, TO220-5/TO220-7
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I
< 25µA, TSSOP16/TSSOP16E
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I
< 6µA, MS8/MS8E
LT1776
40V, 550mA (I ), 200kHz High Efficiency Step-Down
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OUT(MIN)
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I
< 30µA, N8, SO-8
LT1976
60V, 1.2A (I ), 200kHz High Efficiency Micropower (I < 100µA)
V : 3.3V to 60V, V
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Q
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Q
IN
SD
Step-Down DC/DC Converter
I
< 1µA, TSSOP16E
LT3010
80V, 50mA Low Noise Linear Regulator
V : 1.5V to 80V, V
= 1.28V, I = 30µA,
Q
IN
SD
I
< 1µA, MS8E
LTC3412/LTC3414
LTC3414
2.5A (I ), 4MHz Synchronous Step-Down DC/DC Converters
V : 2.5V to 5.5V, V
SD
= 0.8V, I = 60µA,
Q
OUT
IN
OUT(MIN)
< 1µA, TSSOP16E
I
4A (I ), 4MHz Synchronous Step-Down DC/DC Converter
V : 2.3V to 5.5V, V
SD
= 0.8V, I = 64µA,
Q
OUT
IN
OUT(MIN)
< 1µA, TSSOP20E
I
LTC3727/LTC3727-1 36V, 500kHz High Efficiency Step-Down DC/DC Controllers
V : 4V to 36V, V
SD
= 0.8V, I = 670µA,
IN
OUT(MIN) Q
< 20µA, QFN32, SSOP28
I
LT3430/LT3431
60V, 2.75A (I ), 200kHz/500kHz High Efficiency Step-Down
V : 5.5V to 60V, V
SD
= 1.20V, I = 2.5mA,
OUT(MIN) Q
OUT
IN
DC/DC Converters
I
< 30µA, TSSOP16E
LTC3440/LTC3441
600mA/1.2A (I ), 2MHz/1MHz Synchronous Buck-Boost DC/DC Converter V : 2.5V to 5.5V, V
= 2.5V, I = 25µA,
OUT(MIN) Q
OUT
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with 95% Efficiency
I
< 1µA, MS10
SD
3433f
LT/TP 0504 1K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
LINEAR TECHNOLOGY CORPORATION 2003
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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