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BD8381EFV-M

LED Drivers for Automotive Light

ROHM Semiconductor 

BD8381EFV-M

Technical Note

●5V voltage reference (VREG)

5V (Typ.) is generated from the VCC input voltage when the enable pin is set high. This voltage is used to power internal

circuitry, as well as the voltage source for device pins that need to be fixed to a logical HIGH.

UVLO protection is integrated into the VREG pin. The voltage regulation circuitry operates uninterrupted for output

voltages higher than 4.5 V (Typ.), but if output voltage drops to 4.3 V (Typ.) or lower, UVLO engages and turns the IC off.

Connect a capacitor (Creg = 10µF Typ.) to the VREG terminal for phase compensation. Operation may become unstable if

Creg is not connected.

●About the method of setting the output current

ILED=min[THM / 5 V , 0.2V] / RISET [A]

As for min[THM / 5 V, 0.2V], small one is selected from among THM and VFB=0.2V.

Please input within the range of 0.25-5.0V when controlling the output current with THM. Please connect with VREG when

not using THM. There is a possibility that the LED GND short detection malfunctions when THM≦0.25V.

As for PWM brightness, the control by the PWM signal from the outside and brightness with the CR timer are possible.

(GND short protection detection timer (SCP) works at the same time as turning on EN when PWM brightness from the

outside is used. Therefore, there is a possibility of mis-detecting SCP for the time from the EN turning on to the PWM

turning on > GND short protection detection timer.)

VTH

Rcr1

Rcr2

Ccr

20 kΩ

10 kΩ

100000 pF

OUTL

ILED

1.44

FPWM=

(RCR1+2RCR2)CCR

TON_PWM=

RCR2

(RCR1+2RCR2)

×100

Fig. 12

●About time from EN turning on to PWM turning on and the start from PWM low Duty

※The GND short protection detecting function (hereafter,

SCP) starts with EN=Low→Hi, and after the time of the

①SCP timer detection starts

timer set with the external capacitor connected with CT, it

becomes latch off. (Above figure ① and ②)

EN

The charge with SS begins synchronizing with turning on

②Time of SCP timer

EN. The PWM latch off function is built into when there is

not PWM turning on, and when the PWM latch off is

③Time until turning on PWM

detected, (② of SS and the SCP counter) is reset. (The PWM

time of the timer at latch OFF is calculated by oscillatory

frequency ×32770 counts of DC/DC. ) Therefore, the

following relations exist at time until PWM is turned on,

④Time of PWM latch timer ⑤Time until switching starts after inputting PWM

time of PWM latch timer and SCP detection time after EN

is turned on at external brightness.

COUP

(However, after ③ is turned on, ③ <④ is deleted from

the sequence because ④ doesn't operate. )

Each sequence

OUTL

②＜④＜③⇒SCP is detected and No LED light.

④＜②＜③⇒LED lighting

SS

④＜③＜②⇒LED lighting

●About Dirating of the LED current that uses THM

It is an ability to set the Dirating curve of the LED current to the temperature as one of the functions to use THM. As for LED,

because deterioration at the high temperature is fast, the maximum allowance LED currents and the curve of temperatures

is given to the data sheet of LED. The voltage with a negative temperature characteristic in THM the Thermistor resistance

is used is input, and the LED current is controlled when the LED current is controlled according to the temperature

characteristic. Moreover, external Tr is used, and two input composition is also possible.

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© 2011 ROHM Co., Ltd. All rights reserved.

5/20

2011.04 - Rev.A

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