LTC1041 datasheet(18/24 Pages) LINER

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LTC3109

3109fa

APPLICATIONS INFORMATION

The time for VOUT to charge and reach regulation can be

calculated by the formula below, which assumes VOUT is

programmed to 3.3V and COUT is 330μF:

tVOUT =

3.3V • 330μF

ICHG –IVOUT –ILDO

+ tLDO

With 150μA of charge current available and 5μA of load on

both VOUT and VLDO, the time for VOUT to reach regula-

tion after the initial application of power would be 7.81

seconds.

DESIGN EXAMPLE 2

In most pulsed-load applications, the duration, magnitude

and frequency of the load current pulses are known and

ﬁxed. In these cases, the average charge current required

from the LTC3109 to support the average load must be

calculated, which can be easily done by the following:

ICHG ≥ IQ +

IPULSE •tPULSE

where IQ is the sleep current supplied by VOUT and VLDO

to the external circuitry in-between load pulses, including

output capacitor leakage, IPULSE is the total load current

during the pulse, tPULSE is the duration of the load pulse

and T is the pulse period (essentially the time between

load pulses).

In this example, IQ is 5μA, IPULSE is 100mA, tPULSE is 5ms

and T is one hour. The average charge current required

from the LTC3109 would be:

ICHG ≥ 5μA +

100mA • 0.005sec

3600sec

= 5.14μA

Therefore, if the LTC3109 has an input voltage that allows

it to supply a charge current greater than just 5.14μA, the

application can support 100mA pulses lasting 5ms every

hour. It can be seen that the sleep current of 5μA is the

dominant factor in this example, because the transmit

duty cycle is so small (0.00014%). Note that for a VOUT

of 3.3V, the average power required by this application is

only 17μW (not including converter losses).

Keep in mind that the charge current available from the

LTC3109 has no effect on the sizing of the VOUT capacitor,

and the VOUT capacitor has no effect on the maximum

allowed pulse rate.

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