BD9853AFV

Technical Note

11/16

www.rohm.com

2009.05 - Rev.A

© 2009 ROHM Co., Ltd. All rights reserved.

Setting the Short Circuit Protection (SCP) delay time

When the soft start is complete for each channel, the output voltage stabilizes at the set value 1.35V(typ) for the

SCP/SOFT1, 2 pins.

When any type of short circuit occurs, the voltage at the point of the fault is reduced. In this case, when voltage at the INV

pin falls to 0.56V(typ) or below, the SCP comparator sensor detects the short and further charges the SCP/SOFT pins for

the shorted channel from the 1.35V(typ) level. When the SCP/SOFT pins are charged to 2.3V(typ), the Pch/Nch MOSFET

is switched OFF.

The elapsed time from the occurrence of the output short to the point the external FET switches OFF is calculated with the

following formula:

tscp : SCP delay time, Cscp/soft : SCP/SOFT pin connection capacitance,Isoscp : charge current

* If the current more than the capacity of power supply when the short between VOUT and GND occurs, input voltage starts to fall and under voltage lockout

(UVLO) is activated at Vin < 4.15 V (typ.). The UVLO has FET driver output (OUT1H, OUT2H, OUT1L, OUT2L) off and external FETs become off. When

external FETs are not operating, input voltage returns because of the short between VOUT and GND and the UVLO is canceled. But input voltage starts to

fall again because the UVLO is not activated.

There is possibility that a series of behavior “output short → input voltage fall → UVLO activated → external FETs off → input voltage returns → UVLO

canceled → output short”. So please make provision like inserting FUSE in input line.

Pin conditions with the only single channel use

Pin conditions are shown in the following, when the only single channel out of 2 channels is used.

Use only CH1

Use only CH2

・

SCP/SOFT2, INV2 ⇒ GND short

・

SCP/SOFT1, INV1 ⇒ GND short

・

FB2, OUT2H, OUT2L ⇒ OPEN

・

FB1, OUT1H, OUT1L ⇒ OPEN

●

Application example

Isosofc (typ: 2.3μA)[A]

0.95[V](typ)×Cscp/soft［F］

VCC

RB081

L-20

(ROHM)

10μF

(ceramic）

22kΩ

100kΩ

39kΩ

10kΩ

120pF

RSS065

NO3

(ROHM)

RSS040P03

(ROHM)

4.7μF

1μF

8.2kΩ

10kΩ 680pF

0.015μF

0.015μF

47kΩ

100kΩ

15kΩ

68pF

VCC

VCC

4.7μF

RSS040P03

(ROHM)

RSS065

NO3

(ROHM)

220pF 3.3kΩ

REGB

OUT2H

OUT2L

STB

GND

FB2

INV2

SCP/SOFT2

16

15

14

13

12

11

10

9

VCC

OUT1H

OUT1L

REGA

RT

FB1

INV1

SCP/SOFT1

1

2

3

4

5

6

7

8

0.1μF

VOUT2

(2.5V)

10μF

(ceramic）

RB081

L-20

(ROHM)

1μF

VCC

4.7μH

C6-K3L

(MITSUMI)

4.7μH

C6-K3L

(MITSUMI)

VOUT1

(3.3V)