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LTC1261CS-PBF

Switched Capacitor Regulated Voltage Inverter

Linear Technology 

LTC1261

APPLICATIONS INFORMATION

pump to respond until the next clock edge. This prevents

the charge pump from going into very high frequency

oscillation under such conditions but it also creates an

output error as the feedback loop regulates based on

the top of the spike, not the average value of the output

(Figure 5). The resulting output voltage behaves as if a

resistor of value CESR • (IPK/IAVE)Ω was placed in series

with the output. To avoid this nasty sequence of events

connect a 0.1µF ceramic capacitor in parallel with the

larger output capacitor. The ceramic capacitor will “eat”

the high frequency spike, preventing it from fooling the

feedback loop, while the larger but slower tantalum or

aluminum output capacitor supplies output current to the

load between charge cycles.

CLOCK

LOW ESR

OUTPUT CAP

VOUT

HIGH ESR

OUTPUT CAP

VOUT

VSET

VOUT

AVERAGE

COMP1

OUTPUT

VSET

VOUT

AVERAGE

COMP1

OUTPUT

LTC1261 • F05

Figure 5. Output Ripple with Low and High ESR Capacitors

Note that ESR in the flying capacitors will not cause the

same condition; in fact, it may actually improve the situ-

ation by cutting the peak current and lowering the ampli-

tude of the spike. However, more flying capacitor ESR is

not necessarily better. As soon as the RC time constant

approaches half of a clock period (the time the capaci-

tors have to share charge at full duty cycle) the output

current capability of the LTC1261 will begin to diminish.

For 0.1µF flying capacitors, this gives a maximum total

series resistance of:

1

2





tCLK

CFLY





=

1

2





1

550kHz





/

0.1µF

=

9.1Ω

Most of this resistance is already provided by the internal

switches in the LTC1261 (especially in tripler mode). More

than 1Ω or 2Ω of ESR on the flying capacitors will start

to affect the regulation at maximum load.

RESISTOR SELECTION

Resistor selection is easy with the fixed output versions

of the LTC1261— no resistors are needed! Selecting

the right resistors for the adjustable parts is only a little

more difficult. A resistor divider should be used to divide

the signal at the output to give 1.24V at the ADJ pin with

respect to VOUT (Figure 6). The LTC1261 uses a positive

reference with respect to VOUT, not a negative reference

with respect to ground (Figure 2 shows the reference con-

nection). Be sure to keep this in mind when connecting

the resistors! If the initial output is not what you expected,

try swapping the two resistors.

6 (4*)

GND

LTC1261

R1

10 (5*)

ADJ

R2

11 (6*)

OUT

*LTC1261CS8

( ) R1 + R2

VOUT = –1.24V R2

LTC1261 • F06

Figure 6. External Resistor Connections

The 14-lead adjustable parts include a built-in resistor

string which can provide an assortment of output voltages

by using different pin-strapping options at the R0, R1,

and RADJ pins (Table 2). The internal resistors are roughly

124k, 226k, 100k, and 50k (see Figure 2) giving output

options of –3.5V, –4V, –4.5V, and –5V. The resistors

are carefully matched to provide accurate divider ratios,

but the absolute values can vary substantially from part

to part. It is not a good idea to create a divider using an

external resistor and one of the internal resistors unless

the output voltage accuracy is not critical.

10

For more information www.linear.com/LTC1261

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