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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 FO signal
is used to turn off input power or the total system. FO
signal is only triggered in OLP condition.
V
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 (1.4 V ~ 4 V)
of the ADIM voltage x4. At 1 μ s delay after the OCP is
triggered, the IC turns off both the boost FET and
dimming FET and restarts the gate signal every t AR
automatically. t AR can be calculated as :
FO
V SENSE
0.2V
8192/f
seconds
1.
2.
When V ADIM =0.3 V (V ADIM x4=1.2 V).
OCP threshold level is set to 1.4 V.
(7)
If OLPi is triggerd,
Triggered
V SENSE >0.2V
V SENSE <0.2V
Dimming
Time
Dimming
Dimming
off
off
OLPi
OLP Released
Dimming is pulled
to 100% full duty
over 5μs
Figure 29. Open-LED Protection
3.
OCP is triggered at feedback voltage level = 1.4 V.
V SENSE =1.4V
V SENSE
V ADIM =0.3V
In LED open load condition, OVP is triggered ahead of
OLP.
Over-Voltage Protection (OVP)
Over-voltage protection is triggered when the voltage of
GATE
Figure 32. OCP Waveforms at V ADIM =0.3 V
the external output voltage trip point meets 3 V. After
triggering OVP, the dimming switch and boost switch are
turned off. The protection signal is recovered when the
output voltage divider is below 2.9 V.
1.
2.
3.
When V ADIM =0.8 V (V ADIM x4=3.2 V).
OCP threshold level is set to 3.2 V.
OCP is triggered at V SENSE = 3.2 V.
V LED
(Open)
V SENSE =3.2V
V SENSE
R OVP1
V ADIM =0.8V
OVP
3V
R OVP2
GATE
Figure 33. OCP Waveforms at V ADIM =0.8 V
V OVP
3.0V
2.9V
Figure 30. Over-Voltage Trip Point
1.
2.
3.
When V ADIM =1.2 V (V ADIM x4=4.8 V).
OCP threshold level is set to 4.0 V.
OCP is triggered at V SENSE = 4.0 V.
Boost
Gate
Time
V SENSE
V SENSE =4.0V
Figure 31.
OVP Trigger and Release
V ADIM =1.2V
GATE
? 2013 Fairchild Semiconductor Corporation
Figure 34.
OCP Waveforms at V ADIM =1.2 V
www.f airchildsemi.com
FAN7340 ? 1.0.1
13
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