9

LTC1261L

3.3

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

2

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

this additional 200mV surge will trip the feedback com-

parator and terminate the charging cycle. The pulse dissi-

pates quickly and the comparator returns to the correct

state, but the RS latch will not allow the charge 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 4). The

resulting output voltage behaves as if a resistor of value

avoid this nasty sequence of events, connect a 0.1

µF

ceramic capacitor in parallel with the larger output capaci-

tor. 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 capaci-

tor supplies output current to the load between charge

cycles.

approaches half of a clock period (the time the capacitors

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

capability of the LTC1261L starts to diminish. For a 0.1

µF

flying capacitor, this gives a maximum total series resis-

tance of:

=

/ 0.1

µF = 7.7Ω

1

2

)

1

2

)

650kHz

Most of this resistance is already provided by the internal

switches in the LTC1261L. More than 1

Ω 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 voltage

versions of the LTC1261L— 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 1.23V at the ADJ pin

reference with respect to ground (Figure 1 shows the

reference connection). 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.

The LTC1261L can be internally configured for other

fixed output voltages. Contact the Linear Technology

Marketing department for details.

LOW ESR

OUTPUT CAP

CLOCK

AVERAGE

COMP1

OUTPUT

HIGH ESR

OUTPUT CAP

AVERAGE

COMP1

OUTPUT

1261L F04

Figure 4. Output Ripple with Low and High ESR Capacitors

Note that ESR in the flying capacitor will not cause the

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

tion 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

LTC1261L

GND

R1

4

5

6

1261L F05

R2

R1 + R2

R2

ADJ

OUT

()

Figure 5. External Resistor Connections