MB39A113 Datasheet, PDF (18/51 Pages) Fujitsu Component Limited. – ASSP for Power Supply Applications (Secondary Battery)

MB39A113
s SETTING OF SOFT-START TIME
(1) Setting constant voltage 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 VCC ≥ 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 Amp output potential (FB3 terminal (pin 15)) is determined through comparison between either of the
lower potentials at two non-inverting input terminals (internal reference voltage (4.2 V Typ) , CS terminal voltage),
and the inverting input terminal voltage ( − INE3 terminal (pin 16)) . Within the soft-start period (CS terminal
voltage < 4.2 V) , FB3 is determined by comparison between − INE3 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) =: 0.42 × CS (µF)
=: 4.9 V
=: 4.2 V
CS terminal voltage
Error Amp block
internal reference voltage
=: 0 V
Soft-start time : ts
VREF
10 µA
10 µA
FB3 ¹⁵
−INE3 16
CS
22
CS
Error
Amp3
−
+
+
4.2 V
Q2
UVLO
Soft-start circuit
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