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LTC1515

Step-Up/Step-Down Switched Capacitor DC/DC Converters with Reset

Linear Technology 

LTC1515 Series

APPLICATIONS INFORMATION

(equivalent series resistance) in the output capacitor.

Typical output ripple (VIN < 8V) under maximum load is

100mV peak-to-peak with a low ESR, 10µF output capaci-

tor. For applications requiring VIN to exceed 8V, a 22µF or

larger COUT capacitor is recommended to maintain maxi-

mum ripple in the 100mV range.

The magnitude of the ripple voltage depends on several

factors. High input voltages increase the output ripple

since more charge is delivered to COUT per charging cycle.

A large C1 flying capacitor (> 0.22µF) also increases ripple

in step-up mode for the same reason. Large output

current load and/or a small output capacitor (<10µF)

results in higher ripple due to higher output voltage dV/dt.

High ESR capacitors (ESR > 0.5Ω) on the output pin cause

high frequency voltage spikes on VOUT with every clock

cycle.

There are several ways to reduce the output voltage ripple.

A large 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 typi-

cally found with higher value (larger case size) capacitors.

A low ESR (<0.5Ω) ceramic output capacitor will mini-

mize the high frequency ripple, but will not reduce the low

frequency ripple unless a high capacitance value is cho-

sen. 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 4).

LTC1515/

LTC1515-X

VOUT 8 +

LTC1515/

LTC1515-X

VOUT 8 +

15µF

TANTALUM

2Ω

10µF

+

TANTALUM

VOUT

1µF

CERAMIC

VOUT

10µF

TANTALUM

LT1515 • F04

Figure 4. Output Ripple Reduction Techniques

Inrush Currents

A common problem with switched capacitor regulators is

inrush current—particularly during power-up and com-

ing 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, especially if the input

supply is a low power DC/DC converter or linear regulator.

The LTC1515 family minimizes inrush currents both at

start-up and under 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 cur-

rent 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.

Power-On Reset

The POR pin is an open-drain output that pulls low when

the output voltage is out of regulation. When the VOUT

rises to within 6.5% of regulation, an internal timer is

started which releases POR after 200ms (typ). In shut-

down, the POR output is pulled low. In normal operation,

an external pull-up resistor is generally used between the

POR pin and VOUT.

Protection Features

All of the parts 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.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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