LTC1261L
Switched Capacitor
Regulated Voltage Inverter
FEATURES
DESCRIPTION
Regulated Negative Voltage from a
Single Positive Supply
n REG Pin Indicates Output is in Regulation
n Adjustable or Fixed Output Voltages
n Output Regulation: ±4.5%
n Supply Current: 650µA Typ
n Shutdown Mode Drops Supply Current to 5µA
n Up to 20mA Output Current
n Requires Only Three or Four External Capacitors
n Available in MS8 and SO-8 Packages
The LTC®1261L is a switched-capacitor voltage inverter
designed to provide a regulated negative voltage from a
single positive supply. The LTC1261L operates from a
single 2.7V to 5.25V supply and provides an adjustable
output voltage from –1.23V to – 5V. The LTC1261L-4/
LTC1261L-4.5 needs a single 4.5V to 5.25V supply and
provides a fixed output voltage of –4V to –4.5V respectively. Three external capacitors are required: a 0.1µF flying
capacitor and an input and output bypass capacitors. An
optional compensation capacitor at ADJ (COMP) can be
used to reduce the output voltage ripple.
APPLICATIONS
Each version of the LTC1261L will supply up to 20mA
output current with guaranteed output regulation of ±4.5%.
The LTC1261L includes an open-drain REG output that
pulls low when the output is within 5% of the set value.
Quiescent current is typically 650µA when operating and
5µA in shutdown.
n
n
n
n
n
GaAs FET Bias Generators
Negative Supply Generators
Battery-Powered Systems
Single Supply Applications
The LTC1261L is available in 8-pin MSOP and SO packages.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Waveforms for –4V Generator with Power Valid
–4V Generator with Power Valid
0V
5V
1
5V
2
C1
1µF
C2
0.1µF
VCC
SHDN
8
OUT
10k
7
C1 +
REG
LTC1261L-4
3
6
OUT
C1 –
4
GND
COMP
–4V
POWER VALID
5
C3*
100pF
+
C4
3.3µF
VOUT = –4V
AT 10mA
SHDN
POWER VALID
*OPTIONAL
5V
0V
5V
0V
1261L TA01
0.1ms/DIV
1261L TA02
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LTC1261L
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Supply Voltage (Note 2)............................................5.5V
Output Voltage (Note 3).............................0.3V to – 5.3V
Total Voltage, VCC to VOUT (Note 2).........................10.8V
SHDN Pin.......................................–0.3V to (VCC + 0.3V)
REG Pin......................................................... –0.3V to 6V
ADJ Pin..............................(VOUT – 0.3V) to (VCC + 0.3V)
Output Short-Circuit Duration.......................... Indefinite
Commercial Temperature Range (Note 4)..... 0°C to 70°C
Industrial Temperature Range (Note 4).... –40°C to 85°C
Storage Temperature Range.................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
VCC
C1+
C1–
GND
1
2
3
4
8
7
6
5
SHDN
REG
OUT
ADJ (COMP)
MS8 PACKAGE
8-LEAD PLASTIC MSOP
VCC 1
8
SHDN
C1+
2
7
REG
C1– 3
6
OUT
GND 4
5
ADJ (COMP)
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 200°C/W
TJMAX = 150°C, θJA = 135°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC1261LCMS8#PBF
LTC1261LCMS8#TRPBF
LTFM
8-Lead Plastic MSOP
0°C to 70°C
LTC1261LIMS8#PBF
LTC1261LIMS8#TRPBF
LTFM
8-Lead Plastic MSOP
–40°C to 85°C
LTC1261LCMS8-4#PBF
LTC1261LCMS8-4#TRPBF
LTFN
8-Lead Plastic MSOP
0°C to 70°C
LTC1261LIMS8-4#PBF
LTC1261LIMS8-4#TRPBF
LTFN
8-Lead Plastic MSOP
–40°C to 85°C
LTC1261LCMS8-4.5#PBF
LTC1261LCMS8-4.5#TRPBF LTFP
8-Lead Plastic MSOP
0°C to 70°C
LTC1261LIMS8-4.5#PBF
LTC1261LIMS8-4.5#TRPBF
LTFP
8-Lead Plastic MSOP
–40°C to 85°C
LTC1261LCS8#PBF
LTC1261LCS8#TRPBF
1261L
8-Lead Plastic SO
0°C to 70°C
LTC1261LIS8#PBF
LTC1261LIS8#TRPBF
1261L
8-Lead Plastic SO
–40°C to 85°C
LTC1261LCS8-4#PBF
LTC1261LCS8-4#TRPBF
1261L4
8-Lead Plastic SO
0°C to 70°C
LTC1261LIS8-4#PBF
LTC1261LIS8-4#TRPBF
1261L4
8-Lead Plastic SO
–40°C to 85°C
LTC1261LCS8-4.5#PBF
LTC1261LCS8-4.5#TRPBF
261L45
8-Lead Plastic SO
0°C to 70°C
LTC1261LIS8-4.5#PBF
LTC1261LIS8-4.5#TRPBF
261L45
8-Lead Plastic SO
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on nonstandard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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2
LTC1261L
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, C1 = 0.1µF, COUT = 3.3µF unless otherwise noted. (Notes 2, 4)
SYMBOL PARAMETER
VCC
CONDITIONS
MIN
Supply Voltage
LTC1261LCMS8/LTC1261LCS8
LTC1261LCMS8-4/LTC1261LCS8-4
(Note 5)
LTC1261LCMS8-4.5/LTC1261LCS8-4.5 (Note 5)
l
l
l
2.7
4.35
4.75
VREF
Reference Voltage
ICC
Supply Current
VCC = 5.25V, No Load, SHDN Floating
VCC = 5.25V, No Load, VSHDN = VCC
fOSC
Internal Oscillator Frequency
VCC = 5V, VOUT = –4V
VOL
REG Output Low Voltage
IREG = 1mA, VCC = 5V, VOUT = –4V
l
IREG
REG Sink Current
VREG = 0.8V, VCC = 3.3V
VREG = 0.8V, VCC = 5V
l
l
IADJ
Adjust Pin Current
VADJ = 1.23V
l
VIH
SHDN Input High Voltage
VCC = 5V
l
VIL
SHDN Input Low Voltage
VCC = 5V
l
IIN
SHDN Input Current
VSHDN = VCC
l
tON
Turn-On Time
VCC = 5V, IOUT = 10mA, –1.5V ≤ VOUT ≤ –4V (LTC1261L)
VCC = 5V, IOUT = 5mA, VOUT = –4.5V (LTC1261L)
VCC = 5V, IOUT = 10mA, VOUT = –4V (LTC1261L-4)
VCC = 5V, IOUT = 5mA, VOUT = –4.5V (LTC1261L-4.5)
l
l
l
l
VOUT
Output Regulation (LTC1261L)
2.70V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
3.25V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 20mA
l –1.552
l –1.552
2.70V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 5mA
2.95V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
3.50V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 20mA
ISC
TYP
MAX
UNITS
5.25
5.25
5.25
V
V
V
1500
20
µA
µA
1.23
650
5
l
l
V
650
0.1
4
5
kHz
0.8
8
12
±0.01
V
mA
mA
±1
2
µA
V
0.8
V
2.5
25
µA
250
250
250
250
1500
1500
1500
1500
µs
µs
µs
µs
–1.5
–1.5
–1.448
–1.448
V
V
l –2.070
l –2.070
l –2.070
–2.0
–2.0
–2.0
–1.930
–1.930
–1.91
V
V
V
2.95V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 5mA
3.30V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
3.85V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 20mA
l –2.587
l –2.587
l –2.587
–2.5
–2.5
–2.5
–2.413
–2.413
–2.41
V
V
V
3.40V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 5mA
3.70V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
4.25V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 20mA
l –3.105
l –3.105
l –3.105
–3.0
–3.0
–3.0
–2.895
–2.895
–2.885
V
V
V
3.85V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 5mA
4.10V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
4.60V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 20mA
l –3.622
l –3.622
l –3.622
–3.5
–3.5
–3.5
–3.378
–3.378
–3.365
V
V
V
Output Regulation
(LTC1261L/LTC1261L-4)
4.35V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 5mA
4.60V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
5.10V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 20mA
l –4.140
l –4.140
l –4.140
–4.0
–4.0
–4.0
–3.860
–3.860
–3.83
V
V
V
Output Regulation
(LTC1261L/LTC1261L-4.5)
4.75V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 5mA
5.05V ≤ VCC ≤ 5.25V, 0mA ≤ IOUT ≤ 10mA
l –4.657
l –4.657
–4.5
–4.5
–4.343
–4.343
V
V
Output Short-Circuit Current
VOUT = 0V, VCC = 5.25V
l
100
220
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to ground unless otherwise
specified.
Note 3: The output should never be set to exceed VCC – 10.8V.
mA
Note 4: The LTC1261LC is guaranteed to meet specifications from 0°C
to 70°C and is designed, characterized and expected to meet industrial
temperature limits, but is not tested at –40°C and 85°C. The LTC1261LI
is guaranteed to meet specifications from –40°C and 85°C.
Note 5: The LTC1261L-4 and LTC1261L-4.5 will operate with less than the
minimum VCC specified in the electrical characteristics table, but they are
not guaranteed to meet the ±4.5% VOUT specification.
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LTC1261L
TYPICAL PERFORMANCE CHARACTERISTICS
Output Voltage vs Output Current
0
Output Voltage vs Output Current
– 3.0
TA = 25°C
VOUT = – 2V
– 0.25
– 3.2
– 3.4
VCC = 2.7V
–1.00
VCC = 4.5V
– 3.5
– 3.6
–1.25
– 3.7
– 3.8
–1.50
VCC = 5V
– 3.9
–1.75
VCC = 3V
– 2.00
10
15
25
20
OUTPUT CURRENT (mA)
– 4.2
30
–2.00
TA = 85°C
–2.05
0
10
15
20
25
OUTPUT CURRENT (mA)
5
TA = – 40°C
TA = 25°C
– 4.00
TA = 85°C
– 4.05
– 4.10
4.6
4.7 4.8 4.9 5.0 5.1
SUPPLY VOLTAGE (V)
5.2
80
5.2
70
4.8
4.4
IOUT = 20mA
4.0
3.6
3.2
IOUT = 5mA
2.8
IOUT = 10mA
–5
–4
–3
–2
–1
OUTPUT VOLTAGE (V)
1261L G04
3.5
3.0
2.5
2.0
1.5
TA = 85°C
1.0
TA = 25°C
0.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
1261L G07
30
20
10
VOUT = – 3V
0
TA = – 40°C
4.5 4.6
4.7 4.8 4.9 5.0 5.1
SUPPLY VOLTAGE (V)
5.2
5.3
1261L G08
VOUT = – 4V
1261L G06
Reference Voltage
vs Temperature
REFERENCE VOLTAGE (V)
SUPPLY CURRENT (mA)
0.5
TA = – 40°C
40
1.25
4.0
TA = 85°C
0
2.5
0
VOUT = – 4.5V
4.5 IOUT = 0
1.0
VOUT = – 2V
50
0
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 5.1 5.4
SUPPLY VOLTAGE (V)
5.0
TA = 25°C
60
Supply Current vs Supply Voltage
VOUT = – 2V
IOUT = 0
5.0
TA = 25°C
1261L G05
Supply Current vs Supply Voltage
1.5
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
1261L G03
5.6
2.0
5.3
2.0
3.0
Maximum Output Current
vs Supply Voltage
2.4
– 4.15
4.5
–2.10
2.5
Minimum Required VCC
vs VOUT and IOUT
VOUT = – 4V
– 3.95
30
1261L G02
POSITIVE SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
TA = 25°C
TA = – 40°C
MAXIMUM OUTPUT CURRENT (mA)
5
0
– 3.90
SUPPLY CURRENT (mA)
–1.95
– 4.1
Output Voltage vs Supply Voltage
2.5
VOUT = – 2V
– 4.0
1261L G01
3.0
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
– 3.3
– 0.75
– 3.85
Output Voltage vs Supply Voltage
–1.90
TA = 25°C
VOUT = – 4V
– 3.1
– 0.50
– 2.25
(See Test Circuits)
VCC = 5V
ADJ = 0V
1.24
1.23
1.22
1.21
– 55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1261L G09
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LTC1261L
TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency
vs Temperature
0.6
675
650
625
600
575
0.5
VOUT = – 4V
0.4
0.3
VOUT = – 2V
0.2
0.1
550
525
–40 –25 –10
160
TA = 25°C
IOUT = 10mA
SHORT-CIRCUIT CURRENT (mA)
700
0.7
TA = 25°C
VOUT = – 4V
VCC = 5V
START-UP TIME (ms)
OSCILLATOR FREQUENCY (kHz)
725
5 20 35 50 65
TEMPERATURE (°C)
Output Short-Circuit Current
vs Temperature
Start-Up Time vs Supply Voltage
80
95
0
140
120
100
3.0
3.5
4.5
4.0
SUPPLY VOLTAGE (V)
1261L G10
5.0
1261L G11
VCC = 5V
80
60
VCC = 3V
40
20
2.5
VCC = 5.25V
VCC = 2.7V
0
– 40 – 25 –10
5 20 35 50 65
TEMPERATURE (°C)
80
95
1261L G12
PIN FUNCTIONS
VCC (Pin 1): Power Supply. This requires an input voltage
between 2.7V and 5.25V. VCC must be bypassed to ground
with at least a 1µF capacitor placed in close proximity to the
chip. See the Applications Information section for details.
C1+ (Pin 2): C1 Positive Input. Connect a 0.1µF capacitor
between C1+ and C1–.
C1– (Pin 3): C1 Negative Input. Connect a 0.1µF capacitor
from C1+ to C1–.
GND (Pin 4): Ground. Connect to a low impedance ground.
A ground plane will help to minimize regulation errors.
ADJ (COMP for Fixed Versions) (Pin 5): Output Adjust/
Compensation Pin. For adjustable parts this pin is used to
set the output voltage. The output voltage is divided down
with an external resistor divider and fed back to this pin
to set the regulated output voltage. Typically the resistor
string should draw ≥10µA from the output to minimize
errors due to the bias current at the adjust pin. Fixed output
voltage parts have the internal resistor string connected
to this pin inside the package. The pin can be used to
trim the output voltage if desired. It can also be used as
an optional feedback compensation pin to reduce output
ripple on both the adjustable and fixed output voltage
parts. See the Applications Information section for more
information on compensation and output ripple.
OUT (Pin 6): Negative Voltage Output. This pin must be
bypassed to ground with a 1µF or larger capacitor. The
value of the output capacitor and its ESR have a strong
effect on output ripple. See the Applications Information
section for more details.
REG (Pin 7): This is an open-drain output that pulls low
when the output voltage is within 5% of the set value. It
will sink 5mA to ground with a 5V supply. The external
circuitry must provide a pull-up or REG will not swing high.
The voltage at REG may exceed VCC and can be pulled up
to 6V above ground without damage.
SHDN (Pin 8): Shutdown. When this pin is at ground the
LTC1261L operates normally. An internal 5µA pull-down
keeps SHDN low if it is left floating. When SHDN is pulled
high, the LTC1261L enters shutdown mode. In shutdown,
the charge pump is disabled, the output collapses to 0V
and the quiescent current drops to 5µA typically.
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5
LTC1261L
TEST CIRCUITS
Adjustable Output
Fixed Output
5V
1
+
10µF
0.1µF
VCC
SHDN
8
VCC
2
7
REG
C1 +
LTC1261L-X
3
6
OUT
C1 –
4
GND
COMP
5
1
VCC
SHDN
8
2
VOUT = – 4V (LTC1261L-4)
VOUT = – 4.5V (LTC1261L-4.5)
+
3.3µF
1261L TCO1
0.1µF
7
REG
C1 +
LTC1261L
3
6
OUT
C1 –
4
GND
ADJ
5
VOUT
+
3.3µF
1261L TCO2
APPLICATIONS INFORMATION
The LTC1261L uses an inverting charge pump to generate
a regulated negative output voltage that is either equal to
or less than the supply voltage. The LTC1261L needs only
three external capacitors and is available in the MSOP and
SO-8 packages
THEORY OF OPERATION
A block diagram of the LTC1261L is shown in Figure 1.
The heart of the LTC1261L is the charge pump core shown
in the dashed box. It generates a negative output voltage
by first charging the flying capacitor (C1) between VCC
and ground. It then connects the top of the flying capacitor to ground, forcing the bottom of the flying capacitor
to a negative voltage. The charge on the flying capacitor
is transferred to the output bypass capacitor, leaving it
charged to the negative output voltage. This process is
driven by the internal 650kHz clock.
Figure 1 shows the charge pump configuration. With the
clock low, C1 is charged to VCC by S1 and S3. At the next
rising clock edge, S1 and S3 are open and S2 and S4
closed. S2 connects C1+ to ground, C1– is connected to
the output by S4. The charge in C1 is transferred to COUT,
setting it to a negative voltage.
The output voltage is monitored by COMP1 which compares a divided replica of the output at ADJ (COMP for
fixed output voltage parts) to the internal reference. At the
beginning of a cycle the clock is low, forcing the output
of the AND gate low and charging the flying capacitor.
The next rising clock edge sets the RS latch, setting the
charge pump to transfer charge from the flying capacitor
to the output capacitor. As long as the output is below the
set point, COMP1 stays low, the latch stays set and the
charge pump runs at the full 50% duty cycle of the clock
gated through the AND gate. As the output approaches the
set voltage, COMP1 will trip whenever the divided signal
exceeds the internal 1.23V reference relative to OUT.
This resets the RS latch and truncates the clock pulses,
reducing the amount of charge transferred to the output
capacitor and regulating the output voltage. If the output
exceeds the set point, COMP1 stays high, inhibiting the
RS latch and disabling the charge pump.
COMP2 also monitors the divided signal at ADJ but it is
connected to a 1.17V reference, 5% below the main reference voltage. When the divided output exceeds this lower
reference voltage indicating that the output is within 5%
of the set value, COMP2 goes high turning on the REG
output transistor. This is an open-drain N-channel device
capable of sinking 4mA with a 3.3V VCC and 5mA with a
5V VCC. When in the “off” state (divided output is more
than 5% below VREF) the drain can be pulled above VCC
without damage up to a maximum of 6V above ground.
Note that the REG output only indicates if the magnitude of
the output is below the magnitude of the set point by 5%
(i.e., VOUT > –4.75V for a –5V set point). If the magnitude
of the output is forced higher than the magnitude of the
set point (i.e., to –5.25V when the output is set for –5V)
the REG output will stay low.
1261lfa
6
LTC1261L
APPLICATIONS INFORMATION
VCC
CLK
650kHz
S1
OUT
C1+
S
C1
Q
S2
R
+
R2
COUT
S4
C1–
R1
INTERNALLY
CONNECTED FOR
FIXED OUTPUT
VOLTAGE PARTS
S3
ADJ (COMP)
+
COMP1
REG
+
–
COMP2
60mV
–
VREF = 1.23V
1261L F01
1.17V
VOUT
Figure 1. Block Diagram
OUTPUT RIPPLE
Output ripple in the LTC1261L is present from two sources;
voltage droop at the output capacitor between clocks and
frequency response of the regulation loop. Voltage droop
is easy to calculate. With a typical clock frequency of
650kHz, the charge on the output capacitor is refreshed
once every 1.54µs. With a 15mA load and a 3.3µF output
capacitor, the output will droop by:
∆t
1.54µs
ILOAD
= 7mV
= 15mA
3.3µF
COUT
This can be a significant ripple component when the output
is heavily loaded, especially if the output capacitor is small.
If absolute minimum output ripple is required, a 10µF or
greater output capacitor should be used.
Regulation loop frequency response is the other major
contributor to output ripple. The LTC1261L regulates the
output voltage by limiting the amount of charge transferred to the output capacitor on a cycle-by-cycle basis.
The output voltage is sensed at the ADJ pin (COMP for
fixed output voltage versions) through an internal or
external resistor divider from the OUT pin to ground. As
the flying capacitor is first connected to the output, the
output voltage begins to change quite rapidly. As soon as
it exceeds the set point COMP1 trips, switching the state
of the charge pump and stopping the charge transfer.
Because the RC time constant of the capacitors and the
switches is quite short, the ADJ pin must have a wide AC
bandwidth to be able to respond to the output in time.
External parasitic capacitance at the ADJ pin can reduce
the bandwidth to the point where the comparator cannot
respond by the time the clock pulse finishes. When this
happens the comparator will allow a few complete pulses
through, then overcorrect and disable the charge pump
until the output drops below the set point. Under these
conditions the output will remain in regulation but the
output ripple will increase as the comparator “hunts” for
the correct value.
To prevent this from happening, an external capacitor can
be connected from ADJ (or COMP for fixed output voltage
parts) to ground to compensate for external parasitics and
1261lfa
7
LTC1261L
APPLICATIONS INFORMATION
increase the regulation loop bandwidth (Figure 2). This
sounds counter intuitive until we remember that the internal
reference is generated with respect to OUT, not ground. The
feedback loop actually sees ground as its “output,” thus
the compensation capacitor should be connected across
the “top” of the resistor divider, from ADJ (or COMP) to
ground. By the same token, avoid adding capacitance
between ADJ (or COMP) and VOUT. This will slow down
the feedback loop and increase output ripple. A 100pF
capacitor from ADJ or COMP to ground will compensate
the loop properly under most conditions for fixed voltage
versions of the LTC1261L. For the adjustable LTC1261L,
the capacitor value will be dependent upon the values of
the external resistors in the divider network.
5V
1µF
VCC
2
0.1µF
C1
+
OUT
10Ω
6
VOUT = – 4V
LTC1261L-4
3
C1–
COMP
5
GND
+
3.3µF
+
3.3µF
100pF
4
1261L F03
Figure 3. Output Filter Cuts Ripple Below 3mV
CAPACITOR SELECTION
Capacitor Sizing
TO CHARGE
PUMP
RESISTORS ARE
INTERNAL FOR FIXED
OUTPUT VOLTAGE PARTS
COMP1
CC
100pF
R1
REF
+
1.23V
–
R2
ADJ/COMP
VOUT
1261L F02
Figure 2. Regulator Loop Compensation
OUTPUT FILTERING
If extremely low output ripple (
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