LTC1751EMS8-5#PBF [Linear]
LTC1751 - Micropower, Regulated Charge Pump DC/DC Converters; Package: MSOP; Pins: 8; Temperature Range: -40°C to 85°C;
| 型号： | LTC1751EMS8-5#PBF |
| 厂家： | 
Linear |
| 描述： | LTC1751 - Micropower, Regulated Charge Pump DC/DC Converters; Package: MSOP; Pins: 8; Temperature Range: -40°C to 85°C 转换器 光电二极管 泵 |
| 文件： | 总12页 (文件大小：212K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1751/LTC1751-3.3/LTC1751-5
Micropower, Regulated
Charge Pump
DC/DC Converters
U
FEATURES
DESCRIPTIO
The LTC®1751 family are micropower charge pump DC/
DC converters that produce a regulated output voltage at
up to 100mA. The input voltage range is 2V to 5.5V.
Extremely low operating current (20µA typical with no
load) and low external parts count (one flying capacitor
and two small bypass capacitors at VIN and VOUT) make
them ideally suited for small, battery-powered applica-
tions.
The LTC1751 family operate as Burst ModeTM switched
capacitor voltage doublers to achieve ultralow quiescent
current. They have thermal shutdown capability and can
survive a continuous short circuit from VOUT to GND. The
PGOOD pin on the LTC1751-3.3 and LTC1751-5 indicates
when the output voltage has reached its final value and if
the output has an undervoltage fault condition. The FB pin
of the adjustable LTC1751 can be used to program the
desired output voltage or current. An optional soft-start
capacitor may be used at the SS pin to prevent excessive
inrush current during start-up.
■
5V Output Current: 100mA (VIN ≥ 3V)
■
3.3V Output Current: 80mA (VIN ≥ 2.5V)
■
Ultralow Power: 20µA Quiescent Current
■
Regulated Output Voltage: 3.3V ±4%, 5V ±4%, ADJ
■
No Inductors
■
Short-Circuit/Thermal Protection
■
VIN Range: 2V to 5.5V
■
800kHz Switching Frequency
■
Very Low Shutdown Current: <2µA
■
Shutdown Disconnects Load from VIN
■
PowerGood/Undervoltage Output
■
Adjustable Soft-Start Time
■
Available in an 8-Pin MSOP Package
U
APPLICATIO S
■
Li-Ion Battery Backup Supplies
■
Local 3V and 5V Conversion
■
Smart Card Readers
■
PCMCIA Local 5V Supplies
The LTC1751 family is available in an 8-pin MSOP
package.
■
White LED Backlighting
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
U
TYPICAL APPLICATIO
Output Voltage vs Input Voltage
5.2
I
C
C
= 50mA
= 1µF
OUT
OUT
FLY
Regulated 5V Output from a 2.7V to 5.5V Input
= 10µF
5.1
5.0
4.9
V
IN
3
7
8
4
2
1
6
5
V
OUT
2.7V
V
V
OUT
T
A
= 85°C
IN
5V ±4%
R1
C2
10µF
C1
10µF
TO 5.5V
T
= 25°C
I
I
≤ 100mA, V ≥ 3V
≤ 50mA, V ≥ 2.7V
A
OUT
OUT
IN
IN
100k
OFF ON
PGOOD
SHDN PGOOD
LTC1751-5
+
SS
C
T
= –40°C
A
C
FLY
1µF
–
GND
C
1751 TA01
C
= MURATA GRM39X5R105K6.3AJ
FLY
4.8
C1, C2 = MURATA GRM40X5R106K6.3AJ
2.5
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
1751 TA02
1
LTC1751/LTC1751-3.3/LTC1751-5
W W
U W
U W
U
ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
ORDER PART
VIN to GND.................................................. –0.3V to 6V
PGOOD, FB, VOUT to GND ........................... –0.3V to 6V
SS, SHDN to GND........................ –0.3V to (VIN + 0.3V)
VOUT Short-Circuit Duration............................. Indefinite
NUMBER
TOP VIEW
FB/PGOOD* 1
8 SS
LTC1751EMS8
LTC1751EMS8-3.3
LTC1751EMS8-5
V
2
3
7 SHDN
OUT
IN
+
6 C
V
–
5 C
GND 4
I
OUT (Note 2)....................................................... 125mA
MS8 PACKAGE
8-LEAD PLASTIC MSOP
Operating Temperature Range (Note 3) .. –40°C to 85°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
MS8 PART MARKING
TJMAX = 150°C, θJA = 160°C/W
LTKL
LTKN
LTKP
*PGOOD ON LTC1751-3.3/LTC1751-5
FB ON LTC1751
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full specified
temperature range, otherwise specifications are at TA = 25°C. CFLY = 1µF, CIN = 10µF, COUT = 10µF unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
LTC1751-3.3
V
V
Input Supply Voltage
Output Voltage
●
2
4.4
V
IN
2V ≤ V ≤ 4.4V, I ≤ 40mA
OUT
●
●
3.17
3.17
3.3
3.3
3.43
3.43
V
V
OUT
IN
2.5V ≤ V ≤ 4.4V, I
≤ 80mA
OUT
IN
I
Operating Supply Current
Output Ripple
2V ≤ V ≤ 4.4V, I
= 0mA, SHDN = V
IN
●
18
68
80
40
µA
CC
IN
OUT
V
V
V
= 2.5V, I
= 40mA
mV
R
IN
IN
OUT
P-P
η
Efficiency
= 2V, I
= 40mA
%
OUT
LTC1751-5
V
V
Input Supply Voltage
Output Voltage
●
2.7
5.5
V
IN
2.7V ≤ V ≤ 5.5V, I ≤ 50mA
OUT
●
●
4.8
4.8
5
5
5.2
5.2
V
V
OUT
IN
3V ≤ V ≤ 5.5V, I
≤ 100mA
IN
OUT
I
Operating Supply Current
Output Ripple
2.7V ≤ V ≤ 5.5V, I
= 0mA, SHDN = V
IN
●
20
75
82
50
µA
CC
IN
OUT
V
V
V
= 3V, I
= 3V, I
= 50mA
= 50mA
mV
R
IN
IN
OUT
OUT
P-P
η
Efficiency
%
LTC1751
V
Input Supply Voltage
●
●
●
●
2
5.5
40
V
µA
V
IN
I
Operating Supply Current
FB Regulation Voltage
FB Input Current
2V ≤ V ≤ 5.5V, I
= 0mA, SHDN = V (Note 4 )
16
CC
IN
OUT
IN
V
2V ≤ V ≤ 5.5V, I
≤ 20mA
1.157
–50
1.205
1.253
50
FB
IN
OUT
I
V
= 1.3V
nA
FB
FB
R
OUT
Open-Loop Charge Pump Strength
V
V
= 2V, V = 3.3V (Note 5)
OUT
●
●
8.5
6.0
20
12
Ω
Ω
IN
IN
= 2.7V, V
= 5V (Note 5)
OUT
2
LTC1751/LTC1751-3.3/LTC1751-5
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full specified
temperature range, otherwise specifications are at TA = 25°C. CFLY = 1µF, CIN = 10µF, COUT = 10µF unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
LTC1751-3.3/LTC1751-5
UVL
UVH
PGOOD Undervoltage Low Threshold Relative to Regulated V
PGOOD Undervoltage High Threshold Relative to Regulated V
(Note 6)
(Note 6)
●
●
●
●
–11
–8
–7
–3
–2
0.4
1
%
%
V
OUT
–4.5
OUT
V
OL
PGOOD Low Output Voltage
PGOOD High Output Leakage
I
= –500µA
PGOOD
I
V
= 5.5V
µA
OH
PGOOD
LTC1751/LTC1751-3.3/LTC1751-5
I
Shutdown Supply Current
V
≤ 3.6V, V
= 0V, V
= 0V
= 0V
●
●
0.01
2
5
µA
µA
SHDN
IN
OUT
SHDN
SHDN
3.6V < V , V
= 0V, V
IN OUT
V
V
SHDN Input Threshold (High)
SHDN Input Threshold (Low)
SHDN Input Current (High)
SHDN Input Current (Low)
●
●
●
●
1.5
V
V
IH
0.3
1
IL
I
I
t
f
SHDN = V
–1
–1
µA
µA
sec
kHz
IH
IN
SHDN = 0V
= 3V, I
1
IL
V
OUT
Rise Time
V
IN
= 0mA, 10% to 90% (Note 6)
0.6ms/nF • C
800
r
OUT
SS
Switching Frequency
Oscillator Free Running
OSC
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 4: The no load input current will be approximately I plus twice the
standing current in the resistive output divider.
CC
Note 2: Based on long term current density limitations.
Note 5: R
≡ (2V – V )/I
.
OUT
IN
OUT OUT
Note 3: The LTC1751EMS8-X is guaranteed to meet performance
specifications from 0°C to 70°C. Specifications over the –40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 6: See Figure 2.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(LTC1751-3.3)
No Load Supply Current
vs Input Voltage
Output Voltage vs Load Current
Output Voltage vs Input Voltage
3.40
3.35
3.30
3.25
3.40
3.35
3.30
3.25
3.20
40
30
20
10
0
T
C
= 25°C
= 1µF
I
C
C
= 40mA
= 1µF
OUT
I
C
V
= 0mA
= 1µF
= V
A
FLY
OUT
FLY
OUT
FLY
SHDN
= 10µF
IN
T
= –40°C
A
T
= 25°C
A
T
T
= 85°C
= 25°C
A
A
V
= 2.5V
T = 85°C
A
IN
V
IN
= 2V
T
= –40°C
A
3.20
0
25
50
75
100
125
150
2.0
2.5
3.0
3.5
4.0
4.5
2.0
2.5
3.0
3.5
4.0
4.5
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1751 G01
1751 G02
1751 G03
3
LTC1751/LTC1751-3.3/LTC1751-5
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(LTC1751-3.3)
Short-Circuit Output Current
vs Input Voltage
Power Efficiency vs Load Current
100
90
250
200
150
100
50
T
C
C
= 25°C
= 1µF
OUT
T
C
= 25°C
= 1µF
A
FLY
A
FLY
V
= 2V
IN
= 10µF
80
70
V
= 2.75V
IN
60
50
V
= 3.3V
= 4.4V
IN
V
IN
40
30
20
10
0
0.001
0.01
0.1
1
10
100
2.0
2.5
3.0
3.5
4.0
4.5
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
1751 G04
1751 G05
Start-Up
Output Ripple
Load Transient Response
SHDN
2V/DIV
PGOOD
5V/DIV
IOUT
40mA/DIV
VOUT
AC COUPLED
50mV/DIV
VOUT
1V/DIV
VOUT
AC COUPLED
50mV/DIV
CSS = 10nF
2ms/DIV
1751 G06
VIN = 2.5V
5µs/DIV
1751 G07
VIN = 2.5V
50µs/DIV
1751 G08
IOUT = 80mA
C
OUT = 10µF
(LTC1751-5)
No Load Supply Current
vs Input Voltage
Output Voltage vs Output Current
5.2
5.1
5.0
4.9
4.8
40
30
20
10
T
C
= 25°C
= 1µF
C
I
SHDN
= 1µF
A
FLY
FLY
OUT
= 0
V
= V
IN
T
T
= 85°C
= 25°C
A
A
V
IN
= 3V
T
A
= –40°C
V
IN
= 2.7V
0
100
150
0
200
50
2.5
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
1751 G09
1751 G10
4
LTC1751/LTC1751-3.3/LTC1751-5
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(LTC1751-5)
Short-Circuit Output Current
vs Input Voltage
Power Efficiency vs Load Current
100
90
250
200
150
100
T
C
C
= 25°C
= 1µF
OUT
A
FLY
T
= 25°C
FLY
A
C
= 1µF
V
= 2.7V
= 10µF
IN
80
70
V
V
= 4.1V
= 5.5V
IN
60
50
IN
40
30
20
10
0
50
0.001
0.01
0.1
1
10
100
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
1751 G11
1751 G12
Start-Up
Output Ripple
Load Transient Response
SHDN
2V/DIV
IOUT
50mA/DIV
PGOOD
5V/DIV
VOUT
AC COUPLED
50mV/DIV
VOUT
AC COUPLED
50mV/DIV
VOUT
2V/DIV
CSS = 10nF
2ms/DIV
1751 G13
VIN = 3V
5µs/DIV
1751 G14
VIN = 3V
50µs/DIV
1751 G15
IOUT = 100mA
COUT = 10µF
U
U
U
PI FU CTIO S
PGOOD (Pin 1) (LTC1751-3.3/LTC1751-5): Output Volt-
age Status Indicator. On start-up, this open-drain pin re-
mains low until the output voltage, VOUT, is within 4.5%
(typ)ofitsfinalvalue.OnceVOUT isvalid,PGOODbecomes
high-Z.If,duetoafaultcondition,VOUT falls7%(typ)below
itscorrectregulationlevel,PGOODpullslow.PGOODmay
be pulled up through an external resistor to any appropri-
ate reference level.
VIN (Pin 3): Input Supply Voltage. VIN should be bypassed
with a 6.8µF (min) low ESR capacitor.
GND (Pin 4): Ground. Should be tied to a ground plane for
best performance.
C– (Pin 5): Flying Capacitor Negative Terminal.
C+ (PIN 6): Flying Capacitor Positive Terminal.
SHDN (Pin 7): Active Low Shutdown Input. A low on
SHDN disables the device. SHDN must not be allowed to
float.
FB (Pin 1) (LTC1751):The voltage on this pin is compared
to the internal reference voltage (1.205V) by the error
comparator to keep the output in regulation. An external
resistor divider is required between VOUT and FB to pro-
gram the output voltage.
SS(Pin8):Soft-StartProgrammingPin.AcapacitoronSS
programs the start-up time of the charge pump so that
large start-up input current is eliminated.
VOUT (Pin 2): Regulated Output Voltage. For best perfor-
mance, VOUT should be bypassed with a 6.8µF (min) low
ESR capacitor as close to the pin as possible .
5
LTC1751/LTC1751-3.3/LTC1751-5
W
W
SI PLIFIED BLOCK DIAGRA S
LTC1751-3.3/LTC1751-5
READY
+
–
2µA
+
–
SS
PGOOD
1
8
–
+
+
–
V
REF
UNDERV
+
V
OUT
CONTROL
2
7
SHDN
COMP1
–
CHARGE PUMP
+
V
3
4
6
5
C
IN
–
GND
C
1751 BD1
LTC1751
2µA
SS
FB
1
8
V
REF
+
V
OUT
CONTROL
2
7
SHDN
COMP1
–
CHARGE PUMP
+
V
3
4
6
5
C
IN
–
GND
C
1751 BD2
6
LTC1751/LTC1751-3.3/LTC1751-5
W U U
U
APPLICATIO S I FOR ATIO
At moderate to high output power, the switching losses
and quiescent current of the LTC1751 are negligible and
the expression is valid. For example, an LTC1751-5 with
VIN = 3V, IOUT = 50mA and VOUT regulating to 5V, has a
measured efficiency of 82% which is in close agreement
withthetheoretical83.3%calculation.TheLTC1751prod-
uct family continues to maintain good efficiency even at
fairly light loads because of its inherently low power
design.
Operation (Refer to Simplified Block Diagrams)
The LTC1751 family uses a switched capacitor charge
pump to boost VIN to a regulated output voltage. Regula-
tion is achieved by sensing the output voltage through a
resistor divider and enabling the charge pump when the
divided output drops below the lower trip point of COMP1.
When the charge pump is enabled, a 2-phase
nonoverlappingclockactivatesthechargepumpswitches.
The flying capacitor is charged to VIN on phase 1 of the
clock. On phase 2 of the clock, it is stacked in series with
VIN andconnectedtoVOUT. Thissequenceofchargingand
discharging the flying capacitor continues at the clock
frequency until the divided output voltage reaches the
upper trip point of COMP1. Once this happens the charge
pump is disabled. When the charge pump is disabled the
device typically draws less than 20µA from VIN thus
providing high efficiency under low load conditions.
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1751 will draw
between 200mA and 400mA from VIN causing a rise in the
junctiontemperature. On-chipthermalshutdowncircuitry
disables the charge pump once the junction temperature
exceeds approximately 160°C and re-enables the charge
pump once the junction temperature drops back to ap-
proximately 150°C. The device will cycle in and out of
thermal shutdown indefinitely without latchup or damage
until the short circuit on VOUT is removed.
In shutdown mode all circuitry is turned off and the
LTC1751 draws only leakage current from the VIN supply.
Furthermore, VOUT is disconnected from VIN. The SHDN
pin is a CMOS input with a threshold voltage of approxi-
mately0.8V.TheLTC1751isinshutdownwhenalogiclow
is applied to the SHDN pin. The quiescent supply current
of the LTC1751 will be slightly higher if the SHDN pin is
driven high with a voltage that is below VIN than if it is
driven all the way to VIN. Since the SHDN pin is a high
impedanceCMOSinputitshouldneverbeallowedtofloat.
To ensure that its state is defined it must always be driven
with a valid logic level.
VIN, VOUT Capacitor Selection
The style and value of capacitors used with the LTC1751
family determine several important parameters such as
output ripple, charge pump strength and minimum
start-up time.
To reduce noise and ripple, it is recommended that low
ESR (<0.1Ω) capacitors be used for both CIN and COUT
.
Thesecapacitorsshouldbeeitherceramicortantalumand
should be 6.8µF or greater. Aluminum capacitors are not
recommended because of their high ESR. If the source
impedance to VIN is very low, up to several megahertz, CIN
may not be needed. Alternatively, a somewhat smaller
value of input capacitor may be adequate, but will not be
as effective in preventing ripple on the VIN pin.
Power Efficiency
The efficiency (η) of the LTC1751 family is similar to that
of a linear regulator with an effective input voltage of twice
the actual input voltage. This occurs because the input
current for a voltage doubling charge pump is approxi-
mately twice the output current. In an ideal regulated
doubler the power efficiency would be given by:
The value of COUT controls the amount of output ripple.
Increasing the size of COUT to 10µF or greater will reduce
theoutputrippleattheexpenseofhigherminimumturnon
time and higher start-up current. See the section Output
Ripple.
POUT
P
IN
VOUT •IOUT VOUT
η =
=
=
V • 2IOUT
2V
IN
IN
7
LTC1751/LTC1751-3.3/LTC1751-5
W U U
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APPLICATIO S I FOR ATIO
Flying Capacitor Selection
Below is a list of ceramic capacitor manufacturers and
how to contact them:
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitor
sinceitsvoltagecanreverseuponstart-upoftheLTC1751.
Low ESR ceramic capacitors should always be used for
the flying capacitor.
AVX
Kemet
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
Murata
Taiyo Yuden
Vishay
The flying capacitor controls the strength of the charge
pump. In order to achieve the rated output current, it is
necessary to have at least 0.6µF of capacitance for the
flyingcapacitor.Capacitorsofdifferentmaterialslosetheir
capacitance with higher temperature and voltage at differ-
ent rates. For example, a ceramic capacitor made of X7R
material will retain most of its capacitance from –40°C to
85°C, whereas, a Z5U or Y5V style capacitor will lose
considerable capacitance over that range. Z5U and Y5V
capacitors may also have a very strong voltage coefficient
causing them to lose 50% or more of their capacitance
when the rated voltage is applied. The capacitor
manufacturer’s data sheet should be consulted to deter-
mine what value of capacitor is needed to ensure 0.6µF at
all temperatures and voltages.
Output Ripple
Low frequency regulation mode ripple exists due to the
hysteresis in the sense comparator and propagation
delays in the charge pump control circuits. The amplitude
and frequency of this ripple are heavily dependent on the
load current, the input voltage and the output capacitor
size. For large VIN the ripple voltage can become substan-
tial because the increased strength of the charge pump
causes fast edges that may outpace the regulation cir-
cuitry. In some cases, rather than bursting, a single
output cycle may be enough to boost the output voltage
into or possibly beyond regulation. In these cases the
average output voltage will climb slightly. For large input
voltages a larger output capacitor will ensure that burst-
ingalwaysoccurs, thusmitigatingpossibleDCproblems.
Generally the regulation ripple has a sawtooth shape
associated with it.
Generally an X7R ceramic capacitor is recommended for
the flying capacitor with a minimum value of 1µF. For very
lowloadapplications, itmaybereducedto0.01µF-0.68µF.
A smaller flying capacitor delivers less charge per clock
cycle to the output capacitor resulting in lower output
ripple. The output ripple is reduced at the expense of
maximum output current and efficiency.
A high frequency ripple component may also be present
on the output capacitor due to the charge transfer action
of the charge pump. In this case, the output can display a
voltage pulse during the output-charging phase. This
pulse results from the product of the charging current and
the ESR of the output capacitor. It is proportional to the
input voltage, the value of the flying capacitor and the ESR
of the output capacitor.
The theoretical minimum output resistance of a voltage
doubling charge pump is given by:
2V – VOUT
1
fC
IN
ROUT(MIN)
≡
=
IOUT
For example, typical combined output ripple for an
LTC1751-5 with VIN = 3V under maximum load is
75mVP-P with a low ESR 10µF output capacitor. A smaller
output capacitor and/or larger output current load will
result in higher ripple due to higher output voltage slew
rates.
Where f if the switching frequency and C is the value of the
flying capacitor. (Using units of MHz and µF is convenient
since they cancel each other.) Note that the charge pump
will typically be weaker than the theoretical limit due to
additional switch resistance. However, for light load appli-
cations, the above expression can be used as a guideline
in determining a starting capacitor value.
8
LTC1751/LTC1751-3.3/LTC1751-5
W U U
U
APPLICATIO S I FOR ATIO
There are several ways to reduce output voltage ripple.
For applications requiring VIN to exceed 3.3V or for
applications requiring <100mV of peak-to-peak ripple, a
largerCOUT capacitor(22µForgreater)isrecommended.
A larger capacitor will reduce both the low and high
frequency ripple due to the lower charging and discharg-
ing slew rates as well as the lower ESR typically found
withhighervalue(largercasesize)capacitors. AlowESR
ceramic output capacitor will minimize the high fre-
quency ripple, but will not reduce the low frequency
ripple unless a high capacitance value is used. An R-C
filter may also be used to reduce high frequency voltages
spikes (see Figure 1).
variousparameterssuchastemperature, outputloading,
charge pump and flying capacitor values and input
voltage.
PGOOD and Undervoltage Detection
ThePGOODpinontheLTC1751-3.3/LTC1751-5performs
two functions. On start-up, it indicates when the output
has reached its final regulation level. After start-up, it
indicates when a fault condition, such as excessive load-
ing, has pulled the output out of regulation.
Once the LTC1751-3.3/LTC1751-5 are enabled via the
SHDN pin, VOUT ramps to its final regulation value slowly
byfollowingtheSSpin.ThePGOODpinswitchesfromlow
impedance to high impedance after VOUT reaches its
regulation value. If VOUT is subsequently pulled below its
correct regulation level, the PGOOD pin pulls low again
indicatingthatafaultexists.Alternatively,ifthereisashort
circuit on VOUT preventing it from ever reaching its correct
regulation level, the PGOOD pin will remain low. The lower
fault threshold, UVL, is preprogrammed to recognize
errors of –7% below nominal VOUT. The upper fault
threshold, UVH, ispreprogrammedat–4.5%belownomi-
nal. Figure 2 shows an example of the PGOOD pin with a
normal start-up followed by an undervoltage fault.
1Ω
V
OUT
V
OUT
5V
+
+
LTC1751-X
10µF
TANT
10µF
TANT
1751 F01
Figure 1. Output Ripple Reduction Technique
Note that when using a larger output capacitor the mini-
mum turn-on time of the device will increase.
Soft-Start
Using an external pull-up resistor, the PGOOD pin can be
pulled high from any available voltage supply, including
the LTC1751-3.3/LTC1751-5 VOUT pin.
The LTC1751 family has built-in soft-start circuitry to
prevent excessive current flow at VIN during start-up. The
soft-starttimeisprogrammedbythevalueofthecapacitor
at the SS pin. Typically a 2µA current is forced out of SS
causing a ramp voltage on the SS pin. The regulation loop
follows this ramp voltage until the output reaches the
correct regulation level. SS is automatically pulled to
ground whenever SHDN is low. The typical rise time is
given by the expression:
If PGOOD is not used it may be connected to GND.
SHDN
PGOOD
tr = 0.6ms/nF • CSS
t
r
90%
10%
UVL
V
OUT
For example, with a 4.7nF capacitor the 10% to 90% rise
time will be approximately 2.8ms. If the output charge
storage capacitor is 10µF, then the average output current
foranLTC1751-5willbe4V/2.8ms•10µFor14mA, giving
28mA at the VIN pin.
UVH
17515 F02
TIME
The soft-start feature is optional. If there is no capacitor
on SS, the output voltage of the LTC1751 will ramp up as
quickly as possible. The start-up time will depend on
Figure 2. PGOOD During Start-Up and Undervoltage
9
LTC1751/LTC1751-3.3/LTC1751-5
W U U
U
APPLICATIO S I FOR ATIO
Programming the LTC1751 Output Voltage (FB Pin)
Typical ROUT values as a function of input voltage are
shown in Figure 5.
While the LTC1751-3.3/LTC1751-5 versions have internal
resistive dividers to program the output voltage, the
programmableLTC1751maybesettoanarbitraryvoltage
via an external resistive divider. Since it employs a voltage
doubling charge pump, it is not possible to achieve output
voltages greater than twice the available input voltage.
Figure 3 shows the required voltage divider connection.
10
T
= 25°C
FLY
A
C
= 1µF
8
6
4
2
0
I
= 100mA
= 50mA
OUT
I
OUT
The voltage divider ratio is given by the expression:
R1
VOUT
=
– 1
R2 1.205V
2.0
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
1751 F05
V
2
1
4
OUT
V
OUT
FB
R1
R2
Figure 5. Typical ROUT vs Input Voltage
1.205V 1 +
(
)
R1
R2
C
OUT
Layout Considerations
GND
Due to high switching frequency and high transient cur-
rents produced by the LTC1751 product family, careful
board layout is necessary. A true ground plane and short
connectionstoallcapacitorswillimproveperformanceand
ensure proper regulation under all conditions. Figure 6
shows the recommended layout configuration.
1751 F03
Figure 3. Programming the Adjustable LTC1751
The sum of the voltage divider resistors can be made large
to keep the quiescent current to a minimum. Any standing
current in the output divider (given by 1.205V/R2) will be
reflectedbyafactorof2intheinputcurrent.Typicalvalues
for total voltage divider resistance can range from several
kΩs up to 1MΩ.
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the
LTC1751. If the junction temperature increases above
approximately 160°C, the thermal shutdown circuitry
will automatically deactivate the output. To reduce the
maximum junction temperature, a good thermal connec-
tion to the PC board is recommended. Connecting the
GND pin (Pin 4) to a ground plane, and maintaining a
solid ground plane under the device on two layers of the
PC board, will reduce the thermal resistance of the
package and PC board system considerably.
Maximum Available Output Current
For the adjustable LTC1751, the maximum available out-
put current and voltage can be calculated from the effec-
tive open-loop output resistance, ROUT, and effective
output voltage, 2VIN(MIN)
.
From Figure 4 the available current is given by:
2V – VOUT
IN
IOUT
=
ROUT
V
IN
R
OUT
+
OUT
–
SHDN
V
OUT
+
2V
V
IN
–
1751 F04
GND
17515 F03
Figure 4. Equivalent Open-Loop Circuit
Figure 6. Recommended Layout
10
LTC1751/LTC1751-3.3/LTC1751-5
U
TYPICAL APPLICATIO S
USB Port to Regulated 5V Power Supply with Soft-Start
2-Cell NiCd or NiMH to 3.3V with Low Standby Current
1µF
1µF
5
6
5
6
–
+
–
+
C
C
V
C
C
V
1Ω
3
7
2
1
3
2
1
3.3V
V
V
= 5V
OUT
V
IN
OUT
IN
OUT
40mA
2-CELL
NiCd OR
NiMH
+
+
10µF
10µF
10µF
10µF
LTC1751-5
LTC1751-3.3
100k
100k
SHDN
7
8
SHDN
SS
OFF
ON
8
SS
PGOOD
GND
PGOOD
PGOOD
GND
PGOOD
1nF
1nF
4
4
1751 TA04
1751 TA06
Boosted Constant Current Source
1µF
5
6
–
+
1.205V
X
C
C
V
I
=
L
3
2
1
R
V
V
IN
IN
OUT
V
≤ 2 V
OUT
IN
10µF
10µF
LOAD
LTC1751
7
8
SHDN
SS
OFF
ON
FB
GND
4
R
X
1751 TA07
Low Power Battery Backup with Auto Switchover and No Reverse Current
Si4435DY
1µF
5
6
–
+
C
C
V
75k
3
8
2
V
= 5V
≤ 100mA
OUT
OUT
V
IN
= 5V
V
IN
OUT
I
+
+
+
IN4148
10µF
10µF
10µF
3-CELL
NiCd
BATTERY
LTC1751-5
PGOOD
SHDN
100k
1
7
HIGH = BACKUP MODE
SS
GND
4
330pF
1.43M
BAT54C
7
LTC1540
4
3
6
5
–
+
8
10k
1M
HYST
475k
2
1
1751 TA05
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.
11
LTC1751/LTC1751-3.3/LTC1751-5
U
TYPICAL APPLICATIO
Current Mode White or Blue LED Driver with PWM Brightness Control
C4
1µF
6
5
+
–
C
C
UP TO 6 LEDS
3V TO 4.5V
Li-Ion
3
2
V
V
IN
OUT
C1
C2
10µF
LTC1751
BATTERY
10µF
17ms
t
7
8
1
4
V
SHDN
SHDN
FB
82Ω
82Ω
82Ω
82Ω
82Ω
82Ω
SS
GND
1751 TA03
C3
680pF
U
PACKAGE DESCRIPTIO
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)
0.043
(1.10)
MAX
0.034
(0.86)
REF
8
7
6
5
0.007
(0.18)
0° – 6° TYP
0.118 ± 0.004**
(3.00 ± 0.102)
0.193 ± 0.006
(4.90 ± 0.15)
SEATING
PLANE
0.009 – 0.015
(0.22 – 0.38)
0.021 ± 0.006
(0.53 ± 0.015)
0.005 ± 0.002
(0.13 ± 0.05)
0.0256
(0.65)
BSC
MSOP (MS8) 1100
1
2
3
4
* 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
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RMS
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Low Noise, 5V Output or Adjustable
1751f LT/TP 0401 4K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
LINEAR TECHNOLOGY CORPORATION 2000
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
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