BD9853AFV Datasheet, PDF (11/17 Pages) Rohm – Single/Dual-output High-frequency Step-down Switching Regulato (Controller type)

BD9853AFV
Technical Note
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＝
0.95[V](typ)×Cscp/soft［F］
Isosofc (typ: 2.3μA)[A]
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
・SCP/SOFT2, INV2 ⇒ GND short
・FB2, OUT2H, OUT2L ⇒ OPEN
Use only CH2
・SCP/SOFT1, INV1 ⇒ GND short
・FB1, OUT1H, OUT1L ⇒ OPEN
●Application example
VCC
VCC
VCC
4.7μF
VOUT1
(3.3V)
4.7μH
C6-K3L
(MITSUMI)
10kΩ
120pF
22kΩ
10μF
(ceramic）
100kΩ
RB081
L-20
(ROHM)
39kΩ
1μF
0.1μF
RSS040P03
(ROHM)
1 ^VCC
2 ^OUT1H
REGB ₁₆
OUT2H ₁₅
RSS040P03
(ROHM)
1μF
8.2kΩ
3 ^OUT1L
4 ^REGA
5 ^RT
OUT2L ₁₄
STB ₁₃
GND ₁₂
VCC
6
RSS065 ^10kΩ 680pF
NO3
7
(ROHM)
8
0.015μF
FB1
INV1
SCP/SOFT1
FB2
11
220pF 3.3kΩ _RSS065
INV2 ₁₀
NO3
(ROHM)
SCP/SOFT2 ₉
0.015μF
4.7μF
4.7μH
C6-K3L
(MITSUMI)
VOUT2
(2.5V)
RB081
L-20
(ROHM)
10μF
(ceramic）
100kΩ
15kΩ
68pF
47kΩ
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© 2009 ROHM Co., Ltd. All rights reserved.
11/16
2009.05 - Rev.A