BD9778F 查看數據表（PDF） - ROHM Semiconductor

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BD9778F

Flexible Step-down Switching Regulators with Built-in Power MOSFET

ROHM Semiconductor 

BD9778F, BD9778HFP, BD9001F, BD9781HFP

Description of external components

Technical Note

VIN

VIN

C

Cin

RT

SW

INV

L

Di

VO

+

CO

R1

RT

CT

SS

FB

R2

GND

CC

RC

CSS

Fig.30

Design procedure

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.

VO=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.

Calculation example

When Vo = 5 V and R2 = 10 kΩ,

5=1 x (1+R1/10kΩ)

R1=40kΩ

2. Selection of coil (L)

The value of the coil can be obtained by the formula shown

below:

When VIN = 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

L=(VIN-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.

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.

L=47µH

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.

∆IL=(VIN-VO) x VO/(L x f x VIN)

∆Vpp=∆IL x ESR+(∆IL x Vo)/(2 x Co x f x VIN)

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.

3.5m x (ILimit-Io(Max))

CMax=

Vo

ILimit:2A(BD9778F/HFP,BD9001F), 4A(BD9781HFP)

If this capacitance is not optimum, faulty startup may result.

(Ţ3.5m is soft start time(min.))

VIN=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

∆IL=0.31A

When ILimit: 2 A, Io (Max) = 1 A, and Vo = 5V,

CMax=3.5m x (2-1)/5

=700µ

CMax=700µF

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9/16

2010.02 - Rev. B

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