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© 2020 ROHM Co., Ltd. All rights reserved.

BD18326NUF-M

28.Sep.2020 Rev.001

www.rohm.com

TSZ22111 • 15 • 001

TSZ02201-0T1T0B300340-1-2

4. PWM Dimming Operation – continued

4.5

About the Deviation of the CRT Pin Voltage Ramp Up/Down Time Due to Reverse Current Characteristics

of Reverse Connection Protection Diodes

If this IC is used to drive LED as shown below, there is a possibility of the deviation of the CRT pin voltage ramp

up/down time from the settings depends on reverse current characteristics of connected reverse current

protection diodes (D2, D3).

Consider a diode which is recommended by ROHM or a diode which is 1 μA (Max) or less of reverse current

characteristics because reverse current Ir of a diode especially increases at high temperature.

between Point A and GND so that the voltage at point A does not rise.

●Mechanism of the deviation of the CRT pin voltage ramp up/down time from the settings.

A) During the PWM dimming mode, Point A on the below figure is in the high impedance (Hi-Z) state.

↓

B) Reverse current Ir of D2 and D3 flow to Point A.

(Power supply voltage is being input into the cathode of D2, so mainly reverse current of D2 flows to C1.)

→Reverse current Ir of D3 is added to the CRT pin charge current and discharge current, so the CRT pin

voltage ramp up/down time deviates from the settings.

↓

C) C1 gets charged, voltage at Point A rises.

↓

D) Point A voltage is the CRT pin voltage of each IC or more.

↓

E) Forward voltage Vf is generated to the diode D3.

↓

F) D3 flows forward current If.

→Forward current If of D3 is added to the CRT pin charge current and discharge current, so the CRT pin

voltage ramp up/down time deviates from the settings.

↓

Repetition of B) to F).

Mechanism of the Deviation of the CRT Pin Voltage Ramp Up/Down Time

due to Reverse Connection Protection Diodes

CRT

VIN

DISC

D1

D2

D3

Ir

If

Vf

Point A

C1

BD18326NUF-M

GND

OUT