ETC LTC1751EMS8-3.3

LTC1751/LTC1751-3.3/LTC1751-5
Micropower, Regulated
Charge Pump
DC/DC Converters
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FEATURES
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DESCRIPTIO
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
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APPLICATIO S
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Li-Ion Battery Backup Supplies
Local 3V and 5V Conversion
Smart Card Readers
PCMCIA Local 5V Supplies
White LED Backlighting
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 applications.
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.
The LTC1751 family is available in an 8-pin MSOP
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
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TYPICAL APPLICATIO
Output Voltage vs Input Voltage
5.2
VIN
2.7V
TO 5.5V
3
C2
10µF
OFF ON
VIN
VOUT
2
7
1
SHDN
PGOOD
LTC1751-5
8
6
SS
C+
4
GND
C–
5
R1
100k
PGOOD
CFLY
1µF
VOUT
5V ±4%
C1
I
10µF OUT ≤ 100mA, VIN ≥ 3V
IOUT ≤ 50mA, VIN ≥ 2.7V
OUTPUT VOLTAGE (V)
Regulated 5V Output from a 2.7V to 5.5V Input
IOUT = 50mA
CFLY = 1µF
COUT = 10µF
5.1
TA = 85°C
TA = 25°C
5.0
TA = –40°C
4.9
1751 TA01
CFLY = MURATA GRM39X5R105K6.3AJ
C1, C2 = MURATA GRM40X5R106K6.3AJ
4.8
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
5.0
5.5
1751 TA02
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LTC1751/LTC1751-3.3/LTC1751-5
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
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
IOUT (Note 2)....................................................... 125mA
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
ORDER PART
NUMBER
TOP VIEW
FB/PGOOD*
VOUT
VIN
GND
1
2
3
4
8
7
6
5
SS
SHDN
C+
C–
MS8 PACKAGE
8-LEAD PLASTIC MSOP
LTC1751EMS8
LTC1751EMS8-3.3
LTC1751EMS8-5
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
4.4
V
3.3
3.3
3.43
3.43
V
V
18
40
µA
LTC1751-3.3
VIN
Input Supply Voltage
●
2
VOUT
Output Voltage
2V ≤ VIN ≤ 4.4V, IOUT ≤ 40mA
2.5V ≤ VIN ≤ 4.4V, IOUT ≤ 80mA
●
●
3.17
3.17
ICC
Operating Supply Current
2V ≤ VIN ≤ 4.4V, IOUT = 0mA, SHDN = VIN
●
VR
Output Ripple
VIN = 2.5V, IOUT = 40mA
68
mVP-P
η
Efficiency
VIN = 2V, IOUT = 40mA
80
%
LTC1751-5
VIN
Input Supply Voltage
●
2.7
VOUT
Output Voltage
ICC
VR
η
5.5
V
2.7V ≤ VIN ≤ 5.5V, IOUT ≤ 50mA
3V ≤ VIN ≤ 5.5V, IOUT ≤ 100mA
●
●
4.8
4.8
5
5
5.2
5.2
V
V
Operating Supply Current
2.7V ≤ VIN ≤ 5.5V, IOUT = 0mA, SHDN = VIN
●
Output Ripple
VIN = 3V, IOUT = 50mA
20
75
50
mVP-P
Efficiency
VIN = 3V, IOUT = 50mA
82
%
µA
LTC1751
VIN
Input Supply Voltage
ICC
Operating Supply Current
2V ≤ VIN ≤ 5.5V, IOUT = 0mA, SHDN = VIN (Note 4 )
●
VFB
FB Regulation Voltage
2V ≤ VIN ≤ 5.5V, IOUT ≤ 20mA
●
1.157
IFB
FB Input Current
VFB = 1.3V
●
– 50
ROUT
Open-Loop Charge Pump Strength
VIN = 2V, VOUT = 3.3V (Note 5)
VIN = 2.7V, VOUT = 5V (Note 5)
●
●
2
●
2
5.5
V
16
40
µA
1.205
1.253
V
50
nA
8.5
6.0
20
12
Ω
Ω
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
PGOOD Undervoltage Low Threshold
Relative to Regulated VOUT (Note 6)
●
–11
–7
–3
%
UVH
PGOOD Undervoltage High Threshold Relative to Regulated VOUT (Note 6)
●
–8
– 4.5
–2
%
VOL
PGOOD Low Output Voltage
IPGOOD = – 500µA
●
0.4
V
IOH
PGOOD High Output Leakage
VPGOOD = 5.5V
●
1
µA
VIN ≤ 3.6V, VOUT = 0V, VSHDN = 0V
3.6V < VIN, VOUT = 0V, VSHDN = 0V
●
●
2
5
µA
µA
LTC1751/LTC1751-3.3/LTC1751-5
ISHDN
Shutdown Supply Current
VIH
SHDN Input Threshold (High)
●
VIL
SHDN Input Threshold (Low)
●
IIH
SHDN Input Current (High)
SHDN = VIN
●
IIL
SHDN Input Current (Low)
SHDN = 0V
●
tr
VOUT Rise Time
VIN = 3V, IOUT = 0mA, 10% to 90% (Note 6)
fOSC
Switching Frequency
Oscillator Free Running
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Based on long term current density limitations.
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.
0.01
1.5
V
0.3
V
–1
1
µA
–1
1
µA
0.6ms/nF • CSS
sec
800
kHz
Note 4: The no load input current will be approximately ICC plus twice the
standing current in the resistive output divider.
Note 5: ROUT ≡ (2VIN – VOUT)/IOUT.
Note 6: See Figure 2.
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TYPICAL PERFOR A CE CHARACTERISTICS
(LTC1751-3.3)
3.40
TA = 25°C
CFLY = 1µF
3.35
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
3.40
VIN = 2.5V
3.30
VIN = 2V
3.25
3.20
0
25
50
75
100
LOAD CURRENT (mA)
125
No Load Supply Current
vs Input Voltage
Output Voltage vs Input Voltage
150
1751 G01
IOUT = 40mA
CFLY = 1µF
COUT = 10µF
3.35
40
TA = –40°C
TA = 25°C
TA = 85°C
3.30
3.25
3.20
2.0
2.5
3.5
4.0
3.0
INPUT VOLTAGE (V)
4.5
1751 G02
SUPPLY CURRENT (µA)
Output Voltage vs Load Current
IOUT = 0mA
CFLY = 1µF
VSHDN = VIN
30
TA = 85°C
20
TA = 25°C
TA = –40°C
10
0
2.0
2.5
3.5
4.0
3.0
INPUT VOLTAGE (V)
4.5
1751 G03
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LTC1751/LTC1751-3.3/LTC1751-5
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TYPICAL PERFOR A CE CHARACTERISTICS
(LTC1751-3.3)
Short-Circuit Output Current
vs Input Voltage
Power Efficiency vs Load Current
100
250
VIN = 2V
70
OUTPUT CURRENT (mA)
EFFICIENCY (%)
TA = 25°C
90 CFLY = 1µF
= 10µF
C
80 OUT
VIN = 2.75V
60
50
VIN = 3.3V
40
VIN = 4.4V
30
20
TA = 25°C
CFLY = 1µF
200
150
100
10
0
0.001
0.01
0.1
1
10
LOAD CURRENT (mA)
50
2.0
100
2.5
3.5
4.0
3.0
INPUT VOLTAGE (V)
1751 G04
Start-Up
1751 G05
Output Ripple
SHDN
2V/DIV
PGOOD
5V/DIV
4.5
Load Transient Response
IOUT
40mA/DIV
VOUT
AC COUPLED
50mV/DIV
VOUT
1V/DIV
VOUT
AC COUPLED
50mV/DIV
CSS = 10nF
2ms/DIV
VIN = 2.5V
IOUT = 80mA
COUT = 10µF
1751 G06
5µs/DIV
1751 G07
VIN = 2.5V
50µs/DIV
(LTC1751-5)
No Load Supply Current
vs Input Voltage
Output Voltage vs Output Current
40
TA = 25°C
CFLY = 1µF
5.1
5.0
VIN = 3V
VIN = 2.7V
4.9
4.8
SUPPLY CURRENT (µA)
OUTPUT VOLTAGE (V)
5.2
0
100
150
50
OUTPUT CURRENT (mA)
200
1751 G09
4
CFLY = 1µF
IOUT = 0
VSHDN = VIN
30
TA = 85°C
TA = 25°C
20
TA = –40°C
10
0
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
5.0
5.5
1751 G10
1751 G08
LTC1751/LTC1751-3.3/LTC1751-5
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TYPICAL PERFOR A CE CHARACTERISTICS
(LTC1751-5)
Short-Circuit Output Current
vs Input Voltage
Power Efficiency vs Load Current
100
250
VIN = 2.7V
70
OUTPUT CURRENT (mA)
EFFICIENCY (%)
TA = 25°C
90 CFLY = 1µF
COUT = 10µF
80
VIN = 4.1V
60
VIN = 5.5V
50
40
30
20
TA = 25°C
CFLY = 1µF
200
150
100
10
0
0.001
0.01
0.1
1
10
LOAD CURRENT (mA)
50
100
2.0
2.5
3.0 3.5 4.0 4.5
INPUT VOLTAGE (V)
1751 G11
Start-Up
5.0
5.5
1751 G12
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
2ms/DIV
1751 G13
VIN = 3V
IOUT = 100mA
COUT = 10µF
5µs/DIV
1751 G14
VIN = 3V
50µs/DIV
1751 G15
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CSS = 10nF
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PI FU CTIO S
PGOOD (Pin 1) (LTC1751-3.3/LTC1751-5): Output Voltage Status Indicator. On start-up, this open-drain pin remains low until the output voltage, VOUT, is within 4.5%
(typ) of its final value. Once VOUT is valid, PGOOD becomes
high-Z. If, due to a fault condition, VOUT falls 7% (typ) below
its correct regulation level, PGOOD pulls low. PGOOD may
be pulled up through an external resistor to any appropriate reference level.
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 program the output voltage.
VOUT (Pin 2): Regulated Output Voltage. For best performance, VOUT should be bypassed with a 6.8µF (min) low
ESR capacitor as close to the pin as possible .
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.
SS (Pin 8): Soft-Start Programming Pin. A capacitor on SS
programs the start-up time of the charge pump so that
large start-up input current is eliminated.
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LTC1751/LTC1751-3.3/LTC1751-5
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SI PLIFIED BLOCK DIAGRA S
LTC1751-3.3/LTC1751-5
READY
PGOOD
+
1
–+
2µA
–
8
SS
7
SHDN
–+
–
VREF
+
UNDERV
VOUT
2
+
–
CONTROL
COMP1
CHARGE PUMP
VIN
3
6
C+
GND
4
5
C–
1751 BD1
LTC1751
2µA
FB
1
8
SS
7
SHDN
VREF
VOUT
2
+
–
CONTROL
COMP1
CHARGE PUMP
VIN
3
6
C+
GND
4
5
C–
1751 BD2
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LTC1751/LTC1751-3.3/LTC1751-5
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APPLICATIO S I FOR ATIO
Operation (Refer to Simplified Block Diagrams)
The LTC1751 family uses a switched capacitor charge
pump to boost VIN to a regulated output voltage. Regulation 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
nonoverlapping clock activates the charge pump switches.
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 and connected to VOUT. This sequence of charging and
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.
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 approximately 0.8V. The LTC1751 is in shutdown when a logic low
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
impedance CMOS input it should never be allowed to float.
To ensure that its state is defined it must always be driven
with a valid logic level.
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 approximately twice the output current. In an ideal regulated
doubler the power efficiency would be given by:
η=
POUT VOUT • IOUT VOUT
=
=
PIN
VIN • 2IOUT
2VIN
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
with the theoretical 83.3% calculation. The LTC1751 product family continues to maintain good efficiency even at
fairly light loads because of its inherently low power
design.
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1751 will draw
between 200mA and 400mA from VIN causing a rise in the
junction temperature. On-chip thermal shutdown circuitry
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 approximately 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.
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.
These capacitors should be either ceramic or tantalum and
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.
The value of COUT controls the amount of output ripple.
Increasing the size of COUT to 10µF or greater will reduce
the output ripple at the expense of higher minimum turn on
time and higher start-up current. See the section Output
Ripple.
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LTC1751/LTC1751-3.3/LTC1751-5
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APPLICATIO S I FOR ATIO
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitor
since its voltage can reverse upon start-up of the LTC1751.
Low ESR ceramic capacitors should always be used for
the flying capacitor.
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
flying capacitor. Capacitors of different materials lose their
capacitance with higher temperature and voltage at different 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 determine what value of capacitor is needed to ensure 0.6µF at
all temperatures and voltages.
Generally an X7R ceramic capacitor is recommended for
the flying capacitor with a minimum value of 1µF. For very
low load applications, it may be reduced to 0.01µF-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.
The theoretical minimum output resistance of a voltage
doubling charge pump is given by:
ROUT (MIN) ≡
2VIN – VOUT
1
=
IOUT
fC
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 applications, the above expression can be used as a guideline
in determining a starting capacitor value.
8
Below is a list of ceramic capacitor manufacturers and
how to contact them:
AVX
www.avxcorp.com
Kemet
www.kemet.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay
www.vishay.com
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 substantial because the increased strength of the charge pump
causes fast edges that may outpace the regulation circuitry. 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 bursting always occurs, thus mitigating possible DC problems.
Generally the regulation ripple has a sawtooth shape
associated with it.
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.
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.
LTC1751/LTC1751-3.3/LTC1751-5
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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
larger COUT capacitor (22µF or greater) is recommended.
A larger capacitor will reduce both the low and high
frequency ripple due to the lower charging and discharging slew rates as well as the lower ESR typically found
with higher value (larger case size) capacitors. A low ESR
ceramic output capacitor will minimize the high frequency 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).
1Ω
VOUT
LTC1751-X
+
10µF
TANT
+
VOUT
5V
10µF
TANT
1751 F01
Figure 1. Output Ripple Reduction Technique
Note that when using a larger output capacitor the minimum turn-on time of the device will increase.
Soft-Start
The LTC1751 family has built-in soft-start circuitry to
prevent excessive current flow at VIN during start-up. The
soft-start time is programmed by the value of the capacitor
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:
various parameters such as temperature, output loading,
charge pump and flying capacitor values and input
voltage.
PGOOD and Undervoltage Detection
The PGOOD pin on the LTC1751-3.3/LTC1751-5 performs
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 loading, 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
by following the SS pin. The PGOOD pin switches from low
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
indicating that a fault exists. Alternatively, if there is a short
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, is preprogrammed at – 4.5% below nominal. Figure 2 shows an example of the PGOOD pin with a
normal start-up followed by an undervoltage fault.
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.
If PGOOD is not used it may be connected to GND.
SHDN
PGOOD
tr = 0.6ms/nF • CSS
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
for an LTC1751-5 will be 4V/2.8ms • 10µF or 14mA, giving
28mA at the VIN pin.
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
tr
VOUT
90%
UVL
UVH
10%
TIME
17515 F02
Figure 2. PGOOD During Start-Up and Undervoltage
9
LTC1751/LTC1751-3.3/LTC1751-5
U
W
U U
APPLICATIO S I FOR ATIO
While the LTC1751-3.3/LTC1751-5 versions have internal
resistive dividers to program the output voltage, the
programmable LTC1751 may be set to an arbitrary voltage
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.
The voltage divider ratio is given by the expression:
Typical ROUT values as a function of input voltage are
shown in Figure 5.
10
TA = 25°C
CFLY = 1µF
8
OUTPUT RESISTANCE (Ω)
Programming the LTC1751 Output Voltage (FB Pin)
IOUT = 100mA
6
IOUT = 50mA
4
2
R1
V
= OUT – 1
R2 1.205V
0
2.0
2
VOUT
R1
1
FB
( )
4
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
reflected by a factor of 2 in the input current. Typical values
for total voltage divider resistance can range from several
kΩs up to 1MΩ.
Maximum Available Output Current
For the adjustable LTC1751, the maximum available output current and voltage can be calculated from the effective open-loop output resistance, ROUT, and effective
output voltage, 2VIN(MIN).
From Figure 4 the available current is given by:
IOUT =
2VIN – VOUT
ROUT
Figure 5. Typical ROUT vs Input Voltage
Due to high switching frequency and high transient currents produced by the LTC1751 product family, careful
board layout is necessary. A true ground plane and short
connections to all capacitors will improve performance and
ensure proper regulation under all conditions. Figure 6
shows the recommended layout configuration.
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 connection 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.
VIN
ROUT
+
–
4.5
Layout Considerations
R2
GND
3.5
4.0
3.0
INPUT VOLTAGE (V)
1751 F05
VOUT
1.205V 1 + R1
R2
COUT
2.5
2VIN
+
VOUT
SHDN
VOUT
–
1751 F04
Figure 4. Equivalent Open-Loop Circuit
10
GND
Figure 6. Recommended Layout
17515 F03
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
1Ω
3
10µF
6
C–
C+
VIN
VOUT
5
2
2-CELL
NiCd OR
NiMH
10µF
LTC1751-5
7
3
VOUT = 5V
100k
SHDN
+
+
10µF
7
OFF ON
8
1nF
1
SS
PGOOD
8
PGOOD
GND
6
C–
C+
VIN
VOUT
2
3.3V
40mA
10µF
LTC1751-3.3
100k
SHDN
1
SS
PGOOD
PGOOD
GND
1nF
4
4
1751 TA04
1751 TA06
Boosted Constant Current Source
1µF
5
VIN
3
C+
VIN
10µF
OFF ON
6
C–
VOUT
IL = 1.205V
RX
VOUT ≤ 2 VIN
2
10µF
LTC1751
7
8
LOAD
SHDN
1
FB
SS
GND
RX
4
1751 TA07
Low Power Battery Backup with Auto Switchover and No Reverse Current
Si4435DY
1µF
5
75k
3
VIN = 5V
10µF
IN4148
3-CELL
NiCd
BATTERY
+ 10µF
+
+
6
C–
C+
VIN
VOUT
2
VOUT = 5V
IOUT ≤ 100mA
10µF
LTC1751-5
100k
1
PGOOD
8
HIGH = BACKUP MODE
7
SS
SHDN
GND
330pF
4
1.43M
BAT54C
7
4
3
LTC1540
–
8
+
6
475k
10k 5 HYST
2
1
1M
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LTC1751/LTC1751-3.3/LTC1751-5
U
TYPICAL APPLICATIO
Current Mode White or Blue LED Driver with PWM Brightness Control
C4
1µF
6
3V TO 4.5V
Li-Ion
BATTERY
3
C1
10µF
17ms
VSHDN
7
t
8
5
C+
C–
VIN
VOUT
LTC1751
SHDN
SS
FB
GND
UP TO 6 LEDS
2
1
C2
10µF
82Ω
4
82Ω
82Ω
82Ω
82Ω
82Ω
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)
0.043
(1.10)
MAX
0.007
(0.18)
0.034
(0.86)
REF
0.118 ± 0.004*
(3.00 ± 0.102)
8
7 6
5
0° – 6° TYP
0.021 ± 0.006
(0.53 ± 0.015)
SEATING
PLANE
0.009 – 0.015
(0.22 – 0.38)
0.0256
(0.65)
BSC
0.118 ± 0.004**
(3.00 ± 0.102)
0.193 ± 0.006
0.005 ± 0.002 (4.90 ± 0.15)
(0.13 ± 0.05)
MSOP (MS8) 1100
* 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
1
2 3
4
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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Charge Pump Inverter with Shutdown
VIN = 2V to 18V, 15V to –15V Supply
LTC1262
12V, 30mA Flash Memory Prog. Supply
Regulated 12V ±5% Output, IQ = 500µA
LTC1514/LTC1515
Buck/Boost Charge Pumps with IQ = 60µA
50mA Output at 3V, 3.3V or 5V; 2V to 10V Input
LTC1516
Micropower 5V Charge Pump
IQ = 12µA, Up to 50mA Output, VIN = 2V to 5V
LTC1517-5/LTC1517-3.3
Micropower 5V/3.3V Doubler Charge Pumps
IQ = 6µA, Up to 20mA Output
LTC1522
Micropower 5V Doubler Charge Pump
IQ = 6µA, Up to 20mA Output
LTC1555/LTC1556
SIM Card Interface
Step-Up/Step-Down Charge Pump, VIN = 2.7V to 10V
LTC1682
Low Noise Doubler Charge Pump
Output Noise = 60µVRMS, 2.5V to 5.5V Output
LTC1754-5
Micropower 5V Doubler Charge Pump
IQ = 13µA, Up to 50mA Output, SOT-23 Package
LTC1755
Smart Card Interface
Buck/Boost Charge Pump, IQ = 60µA, VIN = 2.7V to 6V
LTC3200
Constant Frequency Doubler Charge Pump
Low Noise, 5V Output or Adjustable
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
1751f LT/TP 0401 4K • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2000