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LTC1261CS-PBF

Switched Capacitor Regulated Voltage Inverter

Linear Technology 

LTC1261

APPLICATIONS INFORMATION

5V

1µF

0.1µF

VCC

2 C1+

6

OUT

LTC1261CS8-4

3 C1–

COMP 5

GND

4

100pF

10Ω

3.3µF

VOUT = –4V

3.3µF

LTC1261 • F04

Figure 4. Output Filter Cuts Ripple Below 3mV

CAPACITOR SELECTION

Capacitor Sizing

The performance of the LTC1261 can be affected by the

capacitors it is connected to. The LTC1261 requires bypass

capacitors to ground for both the VCC and OUT pins. The

input capacitor provides most of LTC1261’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 five times larger than the

flying capacitor. Ceramic capacitors generally provide

adequate performance but avoid using a tantalum capaci-

tor as the input bypass unless there is at least a 0.1µF

ceramic capacitor in parallel with it. The charge pump

capacitors are somewhat less critical since their peak

currents are limited by the switches inside the LTC1261.

Most applications should use 0.1µF as the flying capaci-

tor 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 and the flying capacitors.

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 values can be

tailored to the application. Reducing the value of the flying

capacitors reduces the amount of charge transferred with

each clock cycle. This limits maximum output current, but

also cuts the size of the voltage step at the output with

each clock cycle. The smaller capacitors draw smaller

pulses of current out of VCC as well, limiting peak cur-

rents and reducing the demands on the input supply.

Table 1 shows recommended values of flying capacitor

vs maximum load capacity.

Table 1. Typical Max Load (mA) vs Flying Capacitor Value at

TA = 25°C, VOUT = – 4V

FLYING

CAPACITOR

MAX LOAD (mA)

MAX LOAD (mA)

VALUE (µF) VCC = 5V DOUBLER MODE VCC = 3.3V TRIPLER MODE

0.1

22

20

0.047

16

15

0.033

8

11

0.022

4

5

0.01

1

3

The output capacitor performs two functions: it provides

output current to the load during half of the charge pump

cycle and its value helps to set the output ripple voltage.

For applications that are insensitive to output ripple, the

output bypass capacitor can be as small as 1µF. To achieve

specified output ripple with 0.1µF flying capacitors, the

output capacitor should be at least 3.3µF. Larger output

capacitors will reduce output ripple further at the expense

of turn-on time.

Capacitor ESR

Output capacitor Equivalent Series Resistance (ESR) is

another factor to consider. Excessive ESR in the output

capacitor can fool the regulation loop into keeping the

output artificially low by prematurely terminating the

charging cycle. As the charge pump switches to recharge

the output a brief surge of current flows from the flying

capacitors to the output capacitor. This current surge can

be as high as 100mA under full load conditions. A typical

3.3µF tantalum capacitor has 1Ω or 2Ω of ESR; 100mA

• 2Ω = 200mV. If the output is within 200mV of the set

point this additional 200mV surge will trip the feedback

comparator and terminate the charging cycle. The pulse

dissipates quickly and the comparator returns to the

correct state, but the RS latch will not allow the charge

For more information www.linear.com/LTC1261

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