LTC3109_15 Datasheet, PDF (18/24 Pages) Linear Technology – Auto-Polarity, Ultralow Voltage Step-Up Converter and Power Manager

LTC3109
Applications Information
The time for V_OUT to charge and reach regulation can be
calculated by the formula below, which assumes V_OUT is
programmed to 3.3V and C_OUT is 330µF:
t_VOUT
=
3.3V • 330µF
I_CHG – I_VOUT – I_LDO
+
t_LDO
With 150µA of charge current available and 5µA of load on
both V_OUT  and VLDO, the time for V_OUT 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:
I_CHG
≥ I_Q
+
I_PULSE
• t_PULSE
T
where I_Q is the sleep current supplied by V_OUT and V_LDO
to the external circuitry in-between load pulses, including
output capacitor leakage, I_PULSE is the total load current
during the pulse, t_PULSE is the duration of the load pulse
and T is the pulse period (essentially the time between
load pulses).
In this example, I_Q is 5µA, I_PULSE is 100mA, t_PULSE is 5ms
and T is one hour. The average charge current required
from the LTC3109 would be:
I_CHG
≥
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 V_OUT
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 V_OUT capacitor,
and the V_OUT capacitor has no effect on the maximum
allowed pulse rate.
18
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