9

LTC1429

tions can be obtained with larger filter capacitors or by

lower value (< 0.1

µF) of flying capacitor. Also see the

section on Output Capacitor ESR. For applications requir-

ing ripple below 1mV, see the LTC1550/LTC1551 data

sheet.

CAPACITOR SELECTION

Capacitor Sizing

The performance is dependent on the type of capacitors

used. The LTC1429 requires bypass caps to ground for

of the LTC1429’s supply current while it is charging the

flying caps. It should be mounted as close to the package

as possible, its value should be equal to or larger than the

flying cap in doubling mode and at least twice the value of

the flying caps in tripling mode. Ceramic capacitors

generally provide adequate performance; avoid using a

tantalum capacitor as the input bypass unless there is at

least a 0.1

µF ceramic cap in parallel with it. The charge

pump caps are somewhat less critical, since their peak

currents are limited by the switches inside the LTC1429.

Most applications should use 0.1

µF as the flying cap

value; conveniently, ceramic caps are the most common

type of 0.1

µF cap and they work well here. Usually the

easiest solution is to use the same type of capacitor for

both the input bypass and flying caps.

The output cap 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 cap can be as small as 1

µF. To achieve

specified low output ripple, a 3.3

µF or greater output

(< 0.1

µF)offlyingcapacitorshouldbeused.Largeroutput

caps will reduce output ripple further, at the expense of

turn on time.

In an application 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. The smaller capacitors draw smaller

and reducing the demands on the input supply. Tables 1

and 2 show recommended values of flying capacitors vs

respectively.

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

MAX LOAD (mA)

MAX LOAD (mA)

FLYING CAPACITOR

VALUE (

µF)

DOUBLER MODE

TRIPLER MODE

0.1

22

20

0.047

16

15

0.033

8

11

0.022

4

5

0.01

1

3

Table 2. Typical Max Load (mA) vs Flying Capacitor Value at

MAX LOAD (mA)

MAX LOAD (mA)

FLYING CAPACITOR

VALUE (

µF)

DOUBLER MODE

TRIPLER MODE

0.1

18

25

0.047

17

22

0.033

14

20

0.022

12

17

0.01

3

9

Output Capacitor ESR

Output capacitor the 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 charg-

ing cycle. As the charge pump switches to recharge the

output, a brief surge of current flows from the flying caps

to the output cap. This current surge can be as high as

100mA under full load conditions. A typical 3.3

µFtantalum

capacitor has 1

Ω 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 termi-

nate the charging cycle. The pulse dissipates 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