BD64550EFV

Technical Note

10/16

www.rohm.com

2010.06 - Rev.A

© 2010 ROHM Co., Ltd. All rights reserved.

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Soft start

As shown in Fig.24, VOUTDCDC output voltage becomes up and run step by step with soft start at the time of power-on.

Fig.24 Soft Starting Time Timing Chart

This soft start method is realized by changing comparator positive side voltage that determines output duty of switching

regulator to linear using DAC.

Soft start time T1 is constant value regardless of VM voltage.

Soft start time

T1=23.6msec(typ.)

Count finish time

T2=32.8msec(typ.)

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Series regulator operation

Inputting switching regulator output into RIN pin (pin 16) enables to drive series regulator circuit.

At the time of power-on, output voltage start up step by step with soft starting at the same timing as switching regulator

circuit.^Soft start time is 23.6ms (Typ.).

Regarding external capacitor of ROUT pin (pin 14), it works normally without setting. But switching noise of switching

regulator becomes easy to get in due to dragging on board pattern and the like. Pay attention to switching noise.

Fig.25 Series Regulator Block Diagram

SS

Regulator

0.9V

RIN

Switching regulator

20kΩ

(typ.)

30kΩ

(typ.)

ROUT

DAC

COUNTER

Internal CLK

1.95kHz

CLK195

(Internal)

Counter output

DAC output

VOUTDCDC output voltage

Oscillation

1.21V

T1=23.6[msec]

T2=32.8[msec]

0.90V

0V

5.0Vor3.3V

0V

SWOUT

~Duty increase~

VM voltage

Constant ON Duty