bd9845fv ROHM Co. Ltd., bd9845fv Datasheet - Page 11

• Manufacturer Part Number: bd9845fv
• Description: Single-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.
L
ΔVo ≒ ΔIL×ESR
Isw (peak) = Io + ΔIL/2
VREF
INV
FB
DT
CT
SS
=
VREF
Tsw
ΔIL
VREF
×
-
+
+
ERR
VREF
OSC
VCC
(Vin-Vo)×Vo
VCC
Vin
-
+
+
PWM
OCP2
Fig.37
ESR: Equivalent serial resistance of output capacitor Co
STB
OCP
+
-
VCC
VCC-5V
OCP+
UVLO
11/15
OCP-
VCC-5V
TSD
DRV
VCC
VCC-5V
5VREG
GND
VCC
Vin: Input voltage
Tsw: 1/(switching frequency)
Delta IL: Ripple current of coil
C5V
OUT
(Step-down)
Vo
Vin