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Jan 21, 2003
TEL: 886-3-5788833
http://www.gmt.com.tw
3
EV5103-10
Global Mixed-mode Technology Inc.
When PWM dimming signal is low (almost equal to 0V):
I
LED
= V
R1
/ R1
V
R1
= V
FB
– V
R2
V
R2
= I
R2
x R2
I
R2
= I
R3
– I
R4
- I
FB
= (V
R3
/ R3) – (V
R4
/ R4) - I
FB
= ((V
BIAS
– V
FB
)/R3) – (V
FB
/R4) - I
FB
I
LED
= (V
FB
– R2 x (((V
BIAS
– V
FB
)/R3) – (V
FB
/R4) - I
FB
)) / R1
I
LED
should be the design full current.
When PWM dimming signal is high (almost equal to V
PWM_H
):
I
LED
= V
R1
/ R1
V
R1
= V
FB
– V
R2
V
R2
= I
R2
x R2
I
R2
= I
R3
– I
R4
- I
FB
= (V
R3
/ R3) – (V
R4
/ R4) - I
FB
= ((V
BIAS
– V
FB
)/R3) – ((V
FB
- V
PWM_H
)/R4) - I
FB
I
LED
= (V
FB
– R2 x (((V
BIAS
– V
FB
)/R3) – ((V
FB
- V
PWM_H
)/R4) - I
FB
)) / R1
I
LED
should be a little negative current to make sure that LED is full dark.
In this demo board, GMT choose R1 = 27
, R2=120 K
, R3=240K
, R4=620K
.
When PWM signal is low:
I
LED
= (1.2V – 120K
x (((3V – 1.2V)/240K
) – (1.2V/620K
) – 30nA)) / 27
= 19.7mA
When PWM signal is high:
I
LED
= (1.2V – 120K
x (((3V – 1.2V)/240K
) – ((1.2V-3V)/620K
) – 30nA)) / 27
= -1.79mA
U2:
G914C is a micro power LDO regulator that make by GMT. It could regulate a +3VDC from a +3V
~ +5.5V power source. The +3VDC provide as V
BIAS
for PWM dimming bias voltage, and provide
as the power of U3.
U3:
PIC12C509A is a 8bit micro controller that make by Microship. It is programmed as a PWM
dimming signal generator, and monitor 3 push button to change PWM duty and frequency for
dimming control.
Note: You need download the hex code to program a blank new PIC12C509A before solder it on demo board. You could
download this hex code from GMT web site http://www.gmt.com.tw
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