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LTC1514

Step-Up/Step-Down Switched Capacitor DC/DC Converters with Low-Battery Comparator

Linear Technology 

LTC1514-3.3/LTC1514-5

APPLICATIONS INFORMATION

output pin cause high frequency voltage spikes on VOUT

with every clock cycle.

There are several ways to reduce the output voltage

ripple. A larger COUT capacitor (22µF or greater) will

reduce both the low and high frequency ripple due to the

lower COUT charging and discharging dV/dt and the lower

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 10µF to

22µF tantalum capacitor in parallel with a 1µF to 3.3µF

ceramic capacitor on VOUT to reduce both the low and

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

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

8

VOUT +

LTC1514-X

15µF

TANTALUM

VOUT

1µF

CERAMIC

8

VOUT +

LTC1514-X

2Ω

10µF

TANTALUM

+

10µF

VOUT

TANTALUM

1514 F01

Figure 1. Output Ripple Reduction Techniques

Inrush Currents

A common problem with switched capacitor regulators

is inrush current — particularly during power-up and

coming out of shutdown mode. Whenever large VIN (or

boosted VIN) to VOUT voltage differentials are present,

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

high VIN to VOUT operation.

Internal soft start circuitry controls the rate at which VOUT

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

typical start-up time from VOUT = 0V to 5V is 4ms. This

corresponds to an effective VOUT charging current of only

12.5mA for a 10µF 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.

As the VIN (or boosted VIN) to VOUT voltage differential

grows, the effective output impedance of the charge

pump is automatically increased by internal voltage

sensing circuitry. This feature minimizes the current

spikes pulled from VIN whenever the charge pump is

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

condition (VOUT < 100mV) exists. The parts can survive

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

VBAT

R1

R2

LTC1514-X

1

SHDN

2

LBO

3

LBI

4

GND

8

VOUT

7

VIN

C1+ 6

C1– 5

1514 F02

VTRIP = 1.145V(1 + R1/R2)

(LBI RAMPING NEGATIVE)

Figure 2. Programming the Low-Battery Comparator Trip Voltage

6

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