Datasheet

9/16
Technical Note
Description of external components
Design procedure Calculation example
Vo = Output voltage, Vin (Max.) = Maximum input voltage
Io (Max.) = Maximum load current, f = Oscillation frequency
1. Setting or output voltage
Output voltage can be obtained by the formula shown below.
V
O=1 x (1+R1/R2)
Use the formula to select the R1 and R2.
Furthermore, set the R2 to 30 kΩ or less.
Select the current passing through the R1 and R2 to be small
enough for the output current.
2. Selection of coil (L)
The value of the coil can be obtained by the formula shown
below:
L=(V
IN - VO) x VO / (VIN x f x ∆IO)
∆IO: Output ripple current
f = Operating frequency
∆Io should typically be approximately 20 to 30% of Io.
3. Selection of output capacitor (Co)
The output capacitor can be determined according to the
output ripple voltage ∆Vo (p-p) required.
Obtain the required ESR value by the formula shown below
and then select the capacitance.
When V
IN = 13.2 V, Vo = 5 V, Io = 2 A, and f = 100 kHz,
L=(13.2 - 5) x 5/13.2 x 1/100k x 1/(2 x 0.3)
=51.8µH 47µ
V
IN=13.2V, Vo=5V, L=100µH, f=100kHz
∆IL=(13.2-5) x 5/(100 x 10-6 x 100 x 103 x 13.2)
0.31
When Vo = 5 V and R2 = 10 kΩ ,
5=1 x (1+R1/10kΩ)
R1=40kΩ
L=47µH
Fig.30
VIN
RT
SS
SW
INV
FB
GND
CT
Cin Di CO
+
V
OVIN
CC RC
C
L
R1
R2
R
T
CSS
3.5m x (ILimit-Io(Max))
V
o
C
Max=
C
Max=3.5m x (2-1)/5
=700µ
∆IL=0.31
A
C
Max=700µF
If this coil is not set to the optimum value, normal (continuous)
oscillation may not be achieved. Furthermore, set the value of
the coil with an adequate margin so that the peak current passing
through the coil will not exceed the rated current of the coil.
I
Limit:2A(BD9778F/HFP,BD9001F), 4A(BD9781HFP)
If this capacitance is not optimum, faulty startup may result.
(Ţ3.5m is soft start time(min.))
When I Limit : 2 A, Io (Max) = 1 A, and Vo = 5V,
∆IL=(VIN-VO) x VO/(L x f x VIN)
∆Vpp=∆IL x ESR+(∆IL x Vo)/(2 x Co x f x V
IN)
Set the rating of the capacitor with an adequate margin to the
output voltage. Also, set the maximum allowable ripple current
with an adequate margin to ∆IL. Furthermore, the output rise
time should be shorter than the soft start time. Select the output
capacitor having a value smaller than that obtained by the
formula shown below
.
2010.02 - Rev. B
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BD9778F, BD9778HFP, BD9001F, BD9781HFP