BD9845FV Datasheet, PDF (11/16 Pages) Rohm – Single/Dual-output High-frequency Step-down Switching Regulator(Controller type)

BD9845FV
●Example of application circuit
VREF
VREF
CT
INV
VREF
SS
FB
DT
VCC
STB
VCC
VREF
OSC
-
+ ERR
+
UVLO
-
+ PWM
+
VCC
OCP2
-
OCP
+
VCC-5V
VCC-5V
VCC
5VREG
TSD
VCC
DRV
VCC-5V
OCP+
OCP-
GND
C5V
OUT
Technical Note
Vin
Vo
(Step-down)
Fig.37
1) Setting of output unit coil (L) and capacitor (Co)
Set the coil and capacitor as follows in step-down application:
<Setting of L-value>
When load current gets heavy, the current flowing through the coil gets continuous, and the relation below is established:
Vin: Input voltage
Tsw
L=

IL
(Vin-Vo)×Vo
Vin
Tsw: 1/(switching frequency)
Delta IL: Ripple current of coil
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.
<Setting of output capacitor Co>
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,
Vo ≒ IL×ESR
ESR: Equivalent serial resistance of output capacitor Co
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.
<Switching element>
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.
Isw (peak) = Io + IL/2
Select a switching element of permissible current having a sufficient margin over peak current calculated by the equation.
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11/15
2009.05 - Rev.A