FAN73402 Datasheet, PDF (13/16 Pages) Fairchild Semiconductor – LED Backlight Driving Boost Switch

t_.
8192 / f seconds
(6)
In system operation, OLP is triggered in only direct-short
condition. Direct short means that some point of the LED
string is shorted to set ground. In direct-short condition,
the boost controller cannot control the LED current and a
large current flows into the LED string directly from input
power. To prevent this abnormal condition, the FAULT
signal is used to turn off input power or the total system.
FAULT signal is only triggered in OLP condition.
LED Over-Current Protection (OCP)
The primary purpose of the over-current protection
function is to protect the internal dimming MOSFET from
excessive current. The OCP is triggered when the
feedback voltage meets the clamping level (1.4V ~ 4V) of
the ADIM voltage x4. At 1μs delay after the OCP is
triggered, the IC turns off both the boost FET and
dimming FET and restarts the gate signal every tAR
automatically. t_AR can be calculated as:
t 128 / f seconds
(7)
When V_ADIM=0.3V (V_ADIMx4=1.2V).
OCP threshold level is set to 1.4V.
OCP is triggered at feedback voltage level = 1.4V.
V_SENSE
V_SENSE=1.4V
Figure 29. Open-LED Protection
Note:
6. In LED-open load condition, OVP is triggered ahead
of OLP.
Over-Voltage Protection (OVP)
Over-voltage protection is triggered when the voltage of
the external output voltage reaches 3V. After triggering
OVP, the dimming switch and boost switch are turned off.
The protection signal is recovered when the output
voltage divider is below 2.9V.
V_ADIM=0.3V
GATE
Figure 32. OCP Waveforms at V_ADIM=0.3V
When V_ADIM=0.8V (V_ADIMx4=3.2V).
OCP threshold level is set to 3.2V.
OCP is triggered at V_SENSE = 3.2V.
Figure 30. Over-Voltage Trip Point
Figure 33. OCP Waveforms at V_ADIM=0.8V
When V_ADIM=1.2V (V_ADIMx4=4.8V).
OCP threshold level is set to 4.0V.
OCP is triggered at V_SENSE = 4.0V.
Figure 31. OVP Trigger and Release
© 2012 Fairchild Semiconductor Corporation
FAN73402 • 1.0.0
Figure 34. OCP Waveforms at V_ADIM=1.2V
www.fairchildsemi.com
13