MB3821 データシートの表示（PDF） - Fujitsu

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MB3821

2-ch DC/DC Converter IC With Synchronous Rectifier

Fujitsu 

MB3821

s METHOD OF SETTING SOFT START TIME

To provide a soft start by preventing current surges at power-on, soft start capacitors (Cs1, Cs2) are connected to

both channels, the CS1 pin (pin 4) for CH1 and the CS2 pin (pin 21) for CH2.

When the IC is started (when the CTL pin (pin 18) goes to “H” level, and Vcc ≥ UVLO threshold voltage), transistors

Q2 and Q3 switch off and the CS1 and CS2 pins begin charging the external soft start capacitors (Cs1, Cs2) at 1 µA.

The error amplifier contributes to a soft start with the proportionate output voltage to the CS1 and CS2 pin voltage

regardless of the load current on the DC/DC converter.

The soft start time can be calculated by the following formula.

Soft start time (time to 100% output)

tS(sec) =: 1.26 × CS (µF)

Soft start circuit

1 µA

1 µA

FB1 6

−IN1 5

4

CS1

Cs1

Error

− Amp.1

+

+

1.26 V

Q2

Output stage

8

OUT1−1

Output stage

11

OUT2−1

FB2 19

−IN2 20

21

CS2

Cs2

Q3

CSCP

22

1 µA

CSCP

Q1

Q4

Error

− Amp.2

+

+

1.26 V

SCP Comp.

bias

S

R

Latch

2.1 V

UVLO

Output stage

17

OUT1−2

Output stage

14

OUT2−2

bias

Ref Power

(2.5V) ON/OFF

18

CTL

14

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