FAN7318B Datasheet, PDF (16/24 Pages) Fairchild Semiconductor – LCD Backlight Inverter Drive IC

In full brightness, the maximum rms value of the lamp
current is calculated as:
[ ] i ^max = V
π_A
rms
2 2R ref _max
S1
(4)
The lamp intensity is inversely proportional to VADIM. As
VADIM increases, the lamp intensity decreases and the
rms value of the lamp current is calculated as:
[ ] i ^max = V
π
A
rms
2 ^ref 2R_s
(5)
[ ] V_ref = V_{ref _max} − 0.30V_ADIM A
Figure 38 shows the lamp current waveform vs. VADIM in
an analog dimming mode.
2.0 ^VREF
1.5
1.0
0.5
0
0.5
0.5
1.0
1.5
1.0
1.5
2.0
2.5
ADIM
2.0
2.5
15mA
10mA
5mA
0
-5mA
-10mA
-15mA
Lamp Current
Figure 38. Analog Dimming Waveforms
Burst Dimming
Lamp intensity is controlled with the BDIM signal over a
wide range. FAN7318B provides polarity selection.
When BDIM is inputted, DC voltage or PWM pulse
signal and BCT sets the sawtooth waveform or DC
voltage, respectively. This structure can be implemented
as negative dimming polarity. When BDIM voltage is
lower than BCT voltage, the lamp current is turned on;
0V on BDIM commands full brightness. The duty cycle of
the PWM pulse determines the lamp brightness. The
lamp intensity is inversely proportional to BDIM voltage.
As BDIM voltage increases, the lamp intensity
decreases. Figure 39 shows the lamp current waveform
vs. DIM in negative burst dimming mode.
Figure 39. Negative Burst Dimming Waveform Using
DC Voltage
Burst dimming can be implemented, not only with DC
voltage, but also using PWM pulse as the BDIM signal.
Figure 40 shows how to implement burst dimming using
PWM pulse as BDIM signal.
Figure 40. Negative Burst Dimming Implementation
Circuit Using an External Pulse
Figure 41 shows the lamp current waveform vs. an
external pulse in negative burst dimming mode.
Figure 41. Negative Burst Dimming Waveform Using
an External Pulse
© 2009 Fairchild Semiconductor Corporation
FAN7318B • 1.0.0
16
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