Fujitsu MB39A105 manual Setting the SOFT-START Times

MB39A105

■SETTING THE SOFT-START TIMES

To prevent rush currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors (CSCP) to the CSCP terminal (pin 2). When IC starts (VCC ≥ UVLO threshold voltage), the external soft-start capacitors (CSCP) connected to CSCP terminal are charged at 11 ∝A. The error amplifier output (FB (pin 8) ) is determined by comparison between the lower one of the potentials at two non-inverted input terminals (0.5 V in an internal reference voltage, CSCP terminal voltages) and the inverted input terminal voltage (−INE (pin 1) voltage).

The FB terminal voltage is decided for the soft-start period by the comparison between 0.5 V in an internal reference voltage and the voltages of the CSCP terminal. The DC/DC converter output voltage rises in proportion to the CSCP terminal voltage as the soft-start capacitor connected to the CSCP terminal is charged.

The soft-start time is obtained from the following formula:

Soft-start time: ts (time to output 100%)

ts (s) =: 0.045 ⋅ CSCP (∝F)

=: 0.8 V

=: 0.5 V

CSCP terminal voltage

Error Amp block −INE voltage

=: 0 V t

Soft-start time (ts)

• Soft-Start Circuit

VO
	VREF
	11 ∝A
R1
−INE
1
	L priority
R2	Error Amp
	−
2	+
CSCP	+
	(0.5 V)
CSCP
8
FB	UVLO

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