bd9842fv ROHM Co. Ltd., bd9842fv Datasheet - Page 11

• Manufacturer Part Number: bd9842fv
• Description: Dual-output High Voltage High-frequency Step-down Switching Regulator Controller Type
• Manufacturer: ROHM Co. Ltd.

●Example of application circuit
<Setting of output capacitor Co>
<Switching element>
<Setting of L-value>
1) Setting of output unit coil (L) and capacitor (Co)
Set the coil and capacitor as follows in step-down application:
When load current gets heavy, the current flowing through the coil gets continuous, and the relation below is established:
Normally set Delta IL below 30% of the maximum output current (Iomax).
When L-value is made greater, ripple current (Delta IL) becomes smaller. In general, the greater the L-value is, the smaller
the permissible current of coil gets, and when the current exceeds permissible current, the coil is saturated and L-value
changes. Contact the coil manufacturer and check permissible current.
Select an output capacitor Co by ESR (equivalent serial resistance) property of capacitor.
Output ripple voltage (Delta Vo) is almost ESR of output capacitor, therefore,
The relation above is established.
Ripple component by output capacitor is small enough to be neglected in comparison with ripple component by ESR in many
cases. As for Co value, it is recommended to use a sufficiently large capacitor with a capacity that satisfies ESR condition.
Determine a switching element by peak current. Peak current Isw <peak> flowing through the switching element is equal to
peak current flowing through the coil, therefore the equation below is established.
Select a switching element of permissible current having a sufficient margin over peak current calculated by the equation.
（Slave IC） （Master IC）
Use in slave IC mode,
CT = CT
L
ΔVo ≒ ΔIL×ESR
Isw (peak) = Io + ΔIL/2
To master IC CT pin
=
Tsw
ΔIL
VREF
CT
20
1
DT2
DT1
19
2
×
SS2
SS1
18
3
(Vin-Vo)×Vo
INV2
INV1
17
BD9842FV
4
Vin
FB2
FB1
（Slave IC） （Master IC）
16
5
Use in slave IC mode,
STB = VREF
GND
Fig.37
STB
ESR: Equivalent serial resistance of output capacitor Co
15
6
2.5V
OCP1- OCP1+
OCP2- OCP2+
14
7
11/15
13
8
VCC
C5V
12
9
OUT1
OUT2
11
10
Vin: Input voltage
Tsw: 1/(switching frequency)
Delta IL: Ripple current of coil
Lo
Lo
Co
Co
Vo1
Vin
Vo2
(Step-down)
(Step-down)