1-967402-1 Datasheet BD9207FPS-E2, 1-967402-1 | P4-P6

Technical Note

4/11

BD9207FPS

www.rohm.com

2009.07 - Rev.A

●Functional explanation of block

・ VREF

The constant voltage to be temperature-compensated is created from the voltage inputted from VCC terminal.

・ OSC

Triangular wave with an oscillating frequency of 900 KHz is created, as an input of PWM comparator, by built-in resistor

and capacitor.

・ Error AMP

It detects, at INC terminal, the voltage of sense resistance terminal created by LEC current, and compares it to the

reference voltage, and amplifies and outputs the differential voltage.

(The reference voltage is 0.2V±3%)

・ PWM COMP

PWM COMP converts the output voltage of error amplifier into the pulse-width modulation waveform and outputs it to

DRIVER.

・ DRIVER

Push-pull FET driver takes in the PWM waveform and directly drives the Pch MOS FET that is built-in in the IC.

・ STBY

The ON/OFF control of output is performed by STBY terminal. The output becomes ON if STBY voltage is High level.

・ Thermal shutdown circuit (TSD)

TSD circuit protects the IC from runaway thermal operation or heat damage.

TSD circuit detects the temperature of chip, and the circuit becomes off if the temperature reaches 175℃. For TSD

detection and release, the hysteresis of 15℃ is set, so the malfunction caused by temperature variation is prevented.

・ Over Current Protection（OCP）

Out is turned off if the voltage difference between VCC – OUT, caused by the current flowing in the Pch MOS EFT built-in

in OCP circuit, is detected and reaches the reference value of OCP. The OCP is not of latch type but of self-return type.

・ Soft start circuit (SS)

Soft start time is set by the built-in capacitor connected to SS circuit. The capacitor is charged when STBY terminal

becomes High level and the IC is started. The soft start time is set at 4ms or so.

Technical Note

5/11

BD9207FPS

www.rohm.com

2009.07 - Rev.A

●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 I

OUT

(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.

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（⊿I

）.

・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.

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 : Permissible ripple current value of input capacitor

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

⊿IL

(VCC－VOUT)

VOUT

VCC

fosc

× ×

IRMS＝IOUT×√

(1－VOUT / VCC)×VOUT / VCC

ESR≦⊿VL/⊿IL

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

Technical Note

6/11

BD9207FPS

www.rohm.com

2009.07 - Rev.A

Resistance R1

LED Current value is set below according to sense resistance R1.

ILED[A]＝VINV[V]/R1[Ω]

＝0.2V/R1[Ω]

Moreover, DCDC Output voltage will be the value below.

VO[V]＝VF（LED Forward voltage）＋VINV[V]

＝VF＋0.2V(typ)

● Recommended circuit example and notes of substrate layout

・ To prevent noise and low efficiency, please set up the capacitor and Schottky diode between VCC – GND as near as

possible to IC.

・ Please connect the resistance and output LC filter capacitor between INV – GND with same GND for the stability of

output voltage.

Coil L1＝10μH ：C6-K3LA（mitsumi）

Diode D1 ：RB050LA-30（ROHM）

Capacitor C1＝10μF(25V) ：Ceramic capacitor GRM31CB31E106KA75L(murata)

C2＝4.7μF(50V) ：Ceramic capacitor GRM32EB31H475KA87L(murata)

Resistance R1＝0.43Ω ：MCR10EZPFR430

（ROHM）

P1-P3 P4-P6 P7-P9 P10-P12

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