FAN7346 Datasheet, PDF (12/19 Pages) Fairchild Semiconductor – 4-Channel LED Current Balance Controller

4. PWM Dimming
The FAN7346 has four PWM dimming signal pins. Each
PWM dimming pin controls an LED string.
If PWM dimming voltage is higher than 2V, an external
balance switch is turned on and conducts constant
current operation. If PWM dimming voltage is lower than
0.8V, an external balance switch is turned off and blocks
LED current.
During PWM dimming off, open-LED protection and
short-LED protection is disabled.
CHx drain voltage is sampled during PWM dimming on
and held during PWM dimming off to maintain drain
voltage regardless of dimming on/off signal.
5. Short-LED Protection (SLP)
To sense a short-LED condition, the FAN7346 uses the
drain voltage of external balance switch. If some LEDs
are shorted, its LED forward voltage is lower than other
LED strings, so the drain voltage of the external balance
switch is higher than other drain voltage.
SLP threshold voltage can be programmed by SLPR
(Short-LED Protection Reference) voltage. Internal SLP
threshold voltage is calculated as:
V
10 * V
SLP TH _
SLP R
(2)
Minimum SLP threshold voltage is 0V and maximum SLP
threshold voltage is 45V.
SLP is a channel-individual protection. If any string is in
SLP condition, the SLP strings is turned off and other
string operate normally.
If the sensed-drain voltage (CHx voltage) is higher than
the programmed threshold voltage for 20µs, CHx goes into
short-LED protection, which forces the corresponding
channel to turn off. To increase reliability, the SLP channel
is restarted in the next PWM dimming signal rising timing.
After restarting, the FAN7346 checks the drain voltage. If
the drain voltage is higher than the SLP threshold voltage
for 20µs, the switch is turned off. If drain voltage is lower,
the channel is restored to normal operation. Shorted LED
channel operation is minimum duty PWM dimming with a
20µs dimming on period. During 100% full duty PWM
dimming, auto restart is not activated.
Figure 28 shows the SLP operation. As soon as
SLPR*10 (blue line) is lower than CHx voltage, CHx is
forced into minimum duty dimming operation.
Output
Normal
LED Current
CHx Voltage
SLPR
SLP
Figure 28. Short-LED Protection Operation
6. Open LED Protection (OLP)
To sensing open-LED condition, the FAN7346 use the
drain voltage of an external balance switch. If an LED
string is open, the drain voltage of external balance
switch is grounded, and the FAN7346 can detect open-
LED condition. OLP threshold voltage is 0.3V.
If CHx voltage is lower than 0.3V for 20µs, its drain-
voltage feedback is pulled up to 5V. This means the open
LED string is eliminated from minimum drain voltage
feedback loop. Without OLP, if minimum drain voltage is
0V, drain voltage feedback forces the FB signal to
increase output power. This can cause SLP or thermal
stress in other channels.
OLP is an auto-recovery protection. As soon as drain
voltage is higher than 0.3V, OLP is finished and the drain
voltage feedback system is restored.
Figure 29 shows the OLP operation. Before open CH2,
CH2 is the headroom channel. (CH2 drain voltage is
minimum voltage – 1V.) As soon as open CH2, its drain
voltage is 0V, OLP is activated, and CH2 is removed
from headroom feedback control loop. CH1 is selected
as minimum drain voltage channel, so its drain voltage is
controlled as 1V. Reconnecting CH2 string, restores
normal operation.
Output
CH2 LED Current
CH1 Drain Voltage
CH2 Drain Voltage
CH2 Open
Figure 29. Open-LED Protection Operation
© 2011 Fairchild Semiconductor Corporation
FAN7346 • 1.0.0
12
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