BD9873CP-V5,BD9874CP-V5

Technical Note

7/12

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2009.07 - Rev.A

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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

Schottky Diode D1

Select a Schottky diode having an inter-terminal capacity as small as possible (reverse recovery time as short as possible)

and a forward voltage VF as low as possible. (Noise can be reduced and efficiency improved by reduction of switching

noise and switching loss, as well as reduction of voltage drop loss of forward voltage.)

Diode should be selected on the basis of maximum current rating in forward direction, voltage rating in reverse direction,

and power dissipation of diode.

・

・

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 C4

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

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

The output capacitor is one of the important parts for system stability, and when some capacitor is selected, expected

characteristics cannot be provided, depending on ambient temperature, output voltage setting condition, etc.

Fully confirm ESR, temperature characteristics, DC, and bias characteristics before evaluation.

Intput Capacitor C1,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

⊿

L1

(VCC－VOUT)

VOUT

VCC

fosc

1

×

×

ESR≦⊿VL/⊿IL

⊿

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

IRMS＝IOUT×√(1－VOUT / VCC)×VOUT / VCC