MB39A113 Datasheet, PDF (19/51 Pages) Fujitsu Component Limited. – ASSP for Power Supply Applications (Secondary Battery)

MB39A113
(2) Setting constant current mode soft-start
To prevent rush currents when the IC is turned on, the IC allows soft-start using the capacitor (CS) connected
to the CS terminal (pin 22) .
When the CTL terminal (pin 14) is placed under “H” level and IC is activated (threshold voltage of VREF ≥
UVLO) , and Q2 is turned off and the external soft-start capacitor (CS) connected to the CS terminal is charged
at 10 µA.
The Error Amp1 output potential (FB12 terminal (pin 7) ) is determined through comparison between either of
the lower potentials at two non-inverting input terminals ( + INE1 terminal (pin 9) voltage and CS terminal voltage),
and the inverting input terminal voltage ( − INE1 terminal (pin 8) ) . Within the soft-start period (CS terminal
voltage < + INE1) , FB12 is determined by comparison between − INE1 terminal voltage and CS terminal voltage,
and DC/DC converter output voltage goes up proportionately with the increase of CS terminal voltage caused
by charging on the soft-start capacitor.
The Error Amp1 output potential (FB12 terminal (pin 7) ) is determined through comparison between either of
the potentials at two non-inverting input terminals ( + INE2 terminal (pin 3) ) voltage and CS terminal voltage),
and the inverting input terminal voltage ( − INE2 terminal (pin 4) ) . Within the soft-start period (CS terminal
voltage < + INE2) , FB12 is determined by comparison between − INE2 terminal voltage and CS terminal voltage,
and DC/DC converter output voltage goes up proportionately with the increase of CS terminal voltage caused
by charging on the soft-start capacitor.
The soft-start time is obtained from the following formula.
Soft-start time : ts (time until output voltage 100%)
ts (s) =: + INE1 ( + INE2) /10 µA × CS (µF)
=: 4.9 V
+ INE1
( + INE2)
=: 0 V
CS terminal voltage
Error Amp1 block
Comparison voltage with −INE1 voltage
(Error Amp2 block
Comparison voltage with −INE2 voltage)
Soft-start time : ts
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