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LTC1261L

Switched Capacitor Regulated Voltage Inverter

Linear Technology 

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.

TO CHARGE

PUMP

RESISTORS ARE

INTERNAL FOR FIXED

OUTPUT VOLTAGE PARTS

COMP1

+

REF

– 1.23V

R1

CC

100pF

ADJ/COMP

R2

VOUT

1261L F02

Figure 2. Regulator Loop Compensation

OUTPUT FILTERING

If extremely low output ripple (<5mV) is required, additional

output filtering is required. Because the LTC1261L uses

a high 650kHz switching frequency, fairly low value RC

or LC networks can be used at the output to effectively

filter the output ripple. A 10Ω series output resistor and

a 3.3µF capacitor will cut output ripple to below 3mV

(Figure 3). Further reductions can be obtained with larger

filter capacitors or by using an LC output filter.

5V

1µF

0.1µF

VCC

2 C1+

6

OUT

LTC1261L-4

3 C1–

COMP 5

GND

4

10Ω

VOUT = – 4V

+ 3.3µF + 3.3µF

100pF

1261L F03

Figure 3. Output Filter Cuts Ripple Below 3mV

CAPACITOR SELECTION

Capacitor Sizing

The performance of the LTC1261L is affected by the

capacitors to which it connects. The LTC1261L requires

bypass capacitors to ground for both the VCC and OUT

pins. The input capacitor provides most of LTC1261L’s

supply current while it is charging the flying capacitors.

This capacitor should be mounted as close to the package

as possible and its value should be at least ten times larger

than the flying capacitor. Ceramic capacitors generally

provide adequate performance. Avoid using a tantalum

capacitor as the input bypass unless there is at least a

0.1µF ceramic capacitor in parallel with it. The charge pump

capacitor is somewhat less critical since its peak current

is limited by the switches inside the LTC1261L. Most ap-

plications should use a 0.1µF as the flying capacitor value.

Conveniently, ceramic capacitors are the most common

type of 0.1µF capacitor and they work well here. Usually

the easiest solution is to use the same capacitor type for

both the input bypass capacitor and the flying capacitor.

In applications where the maximum load current is well-

defined and output ripple is critical or input peak currents

need to be minimized, the flying capacitor value can be

1261lfa

8

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