6

LTC1514-3.3/LTC1514-5

with every clock cycle.

There are several ways to reduce the output voltage

reduce both the low and high frequency ripple due to the

ESR typically found with higher value (larger case size)

capacitors. A low ESR ceramic output capacitor will

minimize the high frequency ripple, but will not reduce

the low frequency ripple unless a high capacitance value

is chosen. A reasonable compromise is to use a 10

µF to

22

µF tantalum capacitor in parallel with a 1µF to 3.3µF

high frequency ripple. An RC or LC filter may also be used

to reduce high frequency voltage spikes (see Figure 1).

may be charged from 0V to its final regulated value. The

12.5mA for a 10

µF output capacitor (27.5mA for 22µF,

etc). Note that any output current load present during

start-up will add directly to the charging currents men-

tioned above. The soft start circuitry limits start-up

current both at initial power-up and when coming out of

shutdown.

grows, the effective output impedance of the charge

pump is automatically increased by internal voltage

sensing circuitry. This feature minimizes the current

enabled and helps to reduce both input and output ripple.

Protection Features

The LTC1514-X contain thermal shutdown and short-

circuit protection features. The parts will shut down when

the junction temperature reaches approximately 150

°C

and will resume operation once the junction temperature

has dropped back to approximately 140

°C. The parts will

limit output current to 12mA (typ) when a short-circuit

an indefinite short to GND. The LTC1514-X devices use

a low thermal resistance SO-8 package (110

°C/W vs

150

°C/W for standard SO-8). This permits full output

current, even at high input supply voltages.

Low-Battery Comparator

The internal low-battery comparator trips at 1.145

±3%

(LBI ramping negative). Programming the comparator to

trip at a higher voltage can easily be done with an external

LTC1514-X

8

15

µF

TANTALUM

2

Ω

1

µF

CERAMIC

LTC1514-X

1514 F01

8

10

µF

TANTALUM

+

+

10

µF

TANTALUM

+

Figure 1. Output Ripple Reduction Techniques

Inrush Currents

A common problem with switched capacitor regulators

is inrush current — particularly during power-up and

most charge pumps will pull large current spikes from

the input supply. Only the effective charge pump output

impedance limits the current while the charge pump is

enabled. This may disrupt input supply regulation, espe-

cially if the input supply is a low power DC/DC converter

or linear regulator. The LTC1514-3.3/LTC1514-5 mini-

mize inrush currents both at start-up and under normal

Figure 2. Programming the Low-Battery Comparator Trip Voltage

1

2

3

4

8

7

6

5

SHDN

LBO

LBI

GND

LTC1514-X

R2

R1

(LBI RAMPING NEGATIVE)

1514 F02