BD9870FPS

Technical Note

5/11

www.rohm.com

2009.05 - Rev.A

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

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Application component selection and settings

Inductor L1

A large inductor series impedance will result in deterioration of efficiency. OCP operation greater than 1.6A may cause

inductor overheating, possibly leading to overload or output short.

Iout（MAX）: maximum load current

If you flow more than maximum current rating, coil will become overload, and cause magnetic saturation, and those

account for

efficiency deterioration. Select from enough current rating of coil which doesn’t over peak current.

L1：inductor value, VCC：maximum input voltage, VOUT：output voltage,

⊿

IL：coil ripple current value, fosc：oscillation frequency

If you make a point of efficiency, we will recommend C10-H5R(mitsumi).The efficiency will improve about 1-2%.

Schottky Diode D1

A Schottky diode with extremely low forward voltage should be used. Selection should be based on the following

guidelines regarding maximum forward current, reverse voltage, and power dissipation:

・

・

The reverse voltage rating is higher than the VIN value.

・

Power dissipation for the selected diode must be within the rated level.

The power dissipation of the diode is expressed by the following formula:

Pdi=Iout(MAX)×Vf×(1-VOUT/VCC)

Iout（MAX）: maximum load current, Vf: forward voltage, VOUT: output voltage, VCC: input voltage

Output Capacitor C1

A suitable output capacitor should satisfy the following formula for ESR:

ESR≦⊿VL/⊿IL

⊿

VL : permissible ripple voltage, ⊿IL : coil ripple current

Another factor that must be considered is the permissible ripple current. Select a capacitor with sufficient margin, governed

by the following formula:

IRMS =⊿IL/2√3

IRMS: effective value of ripple current to the output capacitor, ⊿IL : coil ripple current

Use ceramic capacitor over B characteristic of temperature. Except that, it is possible to cause abnormal movement of IC.

It’s depends on ambient temperature or output voltage setting

Also it is possible to use Al electronic capacitor, but use it by enough confirmation.

Input Capacitor C2

The input capacitor is the source of current flow to the coil via the built-in Pch FET when the FET is ON. When selecting

the input capacitor sufficient margin must be provided to accommodate capacitor voltage and permissible ripple current.

The expression below defines the effective value of the ripple current to the input capacitor. It should be used in

determining the suitability of the capacitor in providing sufficient margin for the permissible ripple current.

IRMS : effective value of the ripple current to the input capacitor

IOUT : output load current, VOUT: output voltage, VCC: input voltage

Capacitor C3

This capacitor is utilized to stabilize the frequency characteristics, but is seldom used. However, if the phase margin is

insufficient and oscillation is likely, connecting this capacitor may improve frequency stability.

Resistor R1,R2

These resistors determine the output voltage:

VOUT＝1.0V×(1 + R1/R2)

Select resistors less than 10kΩ.

×

⊿

L1

(VCC－VOUT)

VOUT

VCC

fosc

1

×

IRMS＝IOUT×√ (1－VOUT / VCC)×VOUT / VCC