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LTC3108-1

31081fa

APPLICATIONS INFORMATION

Design Example 1

This design example will explain how to calculate the

necessary storage capacitor value for VOUT in pulsed

load applications, such as a wireless sensor/transmit-

ter. In these types of applications, the load is very small

for a majority of the time (while the circuitry is in a low

power sleep state), with bursts of load current occur-

ring periodically during a transmit burst. The storage

capacitor on VOUT supports the load during the transmit

burst, and the long sleep time between bursts allows

the LTC3108-1 to recharge the capacitor. A method for

calculating the maximum rate at which the load pulses

can occur for a given output current from the LTC3108-1

will also be shown.

In this example, VOUT is set to 3V, and the maximum al-

lowed voltage droop during a transmit burst is 10%, or

0.3V. The duration of a transmit burst is 1ms, with a total

average current requirement of 40mA during the burst.

Given these factors, the minimum required capacitance

on VOUT is:

COUT(μF) ≥

40mA • 1ms

0.3V

= 133μF

Note that this equation neglects the effect of capacitor

ESR on output voltage droop. For most ceramic or low

ESR tantalum capacitors, the ESR will have a negligible

effect at these load currents.

A standard value of 150μF or larger could be used for COUT

in this case. Note that the load current is the total current

draw on VOUT, VOUT2 and VLDO, since the current for all of

these outputs must come from VOUTduringaburst.Current

contribution from the holdup capacitor on VSTORE is not

considered, since it may not be able to recharge between

bursts. Also, it is assumed that the charge current from

the LTC3108-1 is negligible compared to the magnitude

of the load current during the burst.

To calculate the maximum rate at which load bursts can

occur, determine how much charge current is available

from the LTC3108-1 VOUT pin given the input voltage

source being used. This number is best found empirically,

since there are many factors affecting the efﬁciency of

the converter. Also determine what the total load cur-

rent is on VOUT during the sleep state (between bursts).

Note that this must include any losses, such as storage

capacitor leakage.

Assume, for instance, that the charge current from the

LTC3108-1 is 50μA and the total current drawn on VOUT

in the sleep state is 17μA, including capacitor leakage. In

addition, use the value of 150μF for the VOUT capacitor.

The maximum transmit rate (neglecting the duration of

the transmit burst, which is typically very short) is then

given by:

150μF • 0.3V

(50μA

−17μA)

= 1.36sec or fMAX = 0.73Hz

Therefore, in this application example, the circuit can sup-

port a 1ms transmit burst every 1.3 seconds.

It can be determined that for systems that only need to

transmit every few seconds (or minutes or hours), the

average charge current required is extremely small, as

long as the sleep current is low. Even if the available

charge current in the example above was only 10μA and

the sleep current was only 5μA, it could still transmit a

burst every 9 seconds.

The following formula enables the user to calculate the

time it will take to charge the LDO output capacitor and

the VOUT capacitor the ﬁrst time, from 0V. Here again, the

charge current available from the LTC3108-1 must be

known. For this calculation, it is assumed that the LDO

output capacitor is 2.2μF.

tLDO =

2.2V • 2.2μF

ICHG −ILDO

If there were 50μA of charge current available and a 5μA

load on the LDO (when the processor is sleeping), the time

for the LDO to reach regulation would be 107ms.

If VOUT were programmed to 3V and the VOUT capacitor

was 150μF, the time for VOUT to reach regulation would be:

tVOUT =

3V • 150μF

ICHG −IVOUT −ILDO

+ tLDO

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