BD9207FPS データシートの表示（PDF） - ROHM Semiconductor

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BD9207FPS

LED Driver series for lighting Step-down type with 1.5A output

ROHM Semiconductor 

BD9207FPS

Technical Note

●Selection and setting of application parts

Coil L1

The application efficiency will decrease by the larger serial resistance element of coil. Moreover, because OCP operates

by 1.6A(min) or more, so please notice the heat generated by overload output or short.

Please set current ratings of coil with IOUT(MAX)＋⊿IL or more. Iout（MAX）: maximum load current

If current flows more than ratings current, then coil generates heat, magnetic saturation, and causes oscillation or low

efficiency.

Please select the peak current with enough margins not to exceed current rating of coil.

(VCC－VOUT)

⊿IL. =

L1

VOUT

1

× VCC × fosc

L1: Coil value,VCC: Input voltage,VOUT: Output voltage, ⊿IL: Ripple current, fosc: Oscillation frequency

When value the efficiency , we recommend C10-H5R(mitsumi) . Efficiency will be improved about 1~2%.

Schottky diode D1

Please select Schottky diode with lowest forward voltage VF.

(We can suppress the dissipation of forward voltage descent, and raise the efficiency. ）

Diode selection is based on maximum forward current rating, reverse voltage rating and permissible power dissipation of

diode.

・The maximum current ratings are assumed more than the value of maximum load current+ coil ripple current（⊿IL）.

・Reverse voltage rating is set to be VIN or more.

・Please choose the smallest one of reverse current IR.

Especially, IR grows at high temperature, and causes thermo runaway.

・Select the diode which permissible power dissipation within ratings.

Diode power dissipation Pdi is shown below.

Pdi=Iout(MAX)×Vf×(1-VOUT/VCC)

Iout（MAX）: Load current. Vf: Forward voltage, VOUT: Output voltage, VCC: Input voltage

Output capacitor C1

Select the output capacitor with ESR met the following formula.

ESR≦⊿VL/⊿IL

⊿VL : Permissible ripple voltage, ⊿IL : Ripple current

Moreover, permissible ripple current effective value of output capacity is calculated by the next formula, and use this

value to select the capacitor with enough margins.

IRMS =⊿IL/2√3

IRMS: Permissible ripple current effective value of output capacity, ⊿IL : Ripple current

Ceramic capacitor must use the one more than B characteristic.

Besides above mentioned, IC might not be normally operated according to the different setting condition of ambient

temperature and output voltage.

Moreover, even aluminum electrolytic capacitor can be used, please confirm its operation enough before using it.

Input capacitor C2

Input capacitor built in Pch MOS FET ON is the current supply of coil by FET.

So, please close to IC as near as possible to connect it. Enough margins is taken on setting input capacitor in capacitor

withstand voltage and permissible ripple current value. According to the ripple current effective value of input capacitor

shown below, select the capacitor with enough permissible ripple current value.

IRMS＝IOUT×√ (1－VOUT / VCC)×VOUT / VCC

IRMS : Permissible ripple current value of input capacitor

IOUT : Load current. VOUT: Output voltage, VCC: Input voltage

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5/11

2009.07 - Rev.A

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