BD9007HFP View Datasheet(PDF) - ROHM Semiconductor

Part Name

Description

Manufacturer

BD9007HFP

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

ROHM Semiconductor 

BD9006F, BD9006HFP, BD9007F, BD9007HFP

●Description of external components

VIN

VIN

＋

C28

CIN

RT

RT

EN/SYNC

GND

L1

Vo

SW

＋

D1

Co

R1

INV

R2

FB

R3 C1

Technical Note

Fig.23

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:

Sample Calculations

When Vo=3.3V, VIN (Typ.)=13.2V

Io(Max.)=1A and f=300kHz

When VO=3.3V and R2=15kΩ

Vo=0.8×(1+R1/R2)

Use the formula to select the R1 and R2.

Furthermore, set the R2 to 30kΩ or less.

Select the current passing through the R1 and R2 to be small

enough for the output current.

3.3=0.8×(1+R1/15kΩ)

R1=46.875kΩ≒47kΩ

R1=47kΩ

2. Selection of coil (L1)

When VIN=13.2V, Vo=3.3V, Io=1A and f=300kHz,

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

L1=(13.2-3.3)×3.3/{13.2×300k×(1×0.3)}

below:

=27.5µH≒33µH

L1=(VIN-Vo)×Vo / (VIN×f×∆Io)

∆Io: Output ripple current

∆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.

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)×Vo/(L×f×VIN)

∆Vpp=∆IL×ESR+(∆IL×Vo)/(2×Co×f×VIN)

L1=33µH

VIN=13.2V, Vo=3.3V, L=33µH, f=300kHz

∆IL=(13.2-3.3)×3.3/(33×10-6×300×103×13.2)

=0.25

∆IL=0.25A

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.

CMAX =

3.0m×(ILIMIT-Io(Max))

Vo

When ILIMIT: 2A, Io(Max)=1A, Vo=3.3V

CMAX =3.0m×(2-1)/3.3

≒910µ

ILIMIT：2A (BD9006F,BD9006HFP, BD9007F,BD9007HFP)

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

CMAX=910µF

(※3.0m is soft start time(min).)

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© 2009 ROHM Co., Ltd. All rights reserved.

10/17

2009.05 - Rev.A

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