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LTC3109

3109fa

APPLICATIONS INFORMATION

DESIGN EXAMPLE 1

This design example will explain how to calculate the

necessary reservoir capacitor value for VOUT in pulsed-

load applications, such as a wireless sensor/transmitter.

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 pulses of load current occurring periodi-

cally during a transmit burst.

The reservoir capacitor on VOUT supports the load during

the transmit pulse; the long sleep time between pulses

allows the LTC3109 to accumulate energy and recharge

the capacitor (either from the input voltage source or the

storage capacitor). A method for calculating the maximum

rate at which the load pulses can occur for a given output

current from the LTC3109 will also be shown.

In this example, VOUT is set to 3.3V, and the maximum

allowed voltage droop during a transmit pulse is 10%, or

0.33V. The duration of a transmit pulse is 5ms, with a total

average current requirement of 20mA during the pulse.

Given these factors, the minimum required capacitance

on VOUT is:

COUT μF

() ≥

20mA • 5ms

0.33V

= 303μF

Note that this equation neglects the effect of capacitor ESR

on output voltage droop. For ceramic capacitors and low

ESR tantalum capacitors, the ESR will have a negligible

effect at these load currents. However, beware of the voltage

coefﬁcient of ceramic capacitors, especially those in small

case sizes. This greatly reduces the effective capacitance

when a DC bias is applied.

A standard value of 330μF 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 VOUT during a pulse.

Current contribution from the capacitor on VSTORE is not

considered, since it may not be able to recharge between

pulses. Also, it is assumed that the harvested charge

current from the LTC3109 is negligible compared to the

magnitude of the load current during the pulse.

To calculate the maximum rate at which load pulses can

occur, you must know how much charge current is avail-

able from the LTC3109 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. You must also know what the total load cur-

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

Note that this must include any losses, such as storage

capacitor leakage.

Let’s assume that the charge current available from the

LTC3109 is 150μA and the total current draw on VOUT and

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

We’ll also use the value of 330μF for the VOUT capacitor.

The maximum transmit rate (neglecting the duration of

the transmit pulse, which is very short compared to the

period) is then given by:

330μF • 0.33V

150μA – 17μA

= 0.82sec or fMAX = 1.2Hz

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

port a 5ms transmit pulse of 20mA every 0.82 seconds.

It can be seen 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 or standby current is low. Even if the available

charge current in the example above was only 21μA, if the

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

every seven seconds.

The following formula will allow you to calculate the time

it will take to charge the LDO output capacitor and the

VOUT capacitor the ﬁrst time, from zero volts. Here again,

the charge current available from the LTC3109 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 was 150μA 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 only 33ms.

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