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FAN7346 Datasheet(PDF) 11 Page - Fairchild Semiconductor

Part No. FAN7346
Description  4-Channel LED Current Balance Controller
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
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FAN7346 Datasheet(HTML) 11 Page - Fairchild Semiconductor

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© 2011 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN7346 • 1.0.0
11
Functional Description
The FAN7346 is a high-efficiency 4-channel LED driver.
It can drive a 4-channel LED string with four external
balance switches. It provides an integrated feedback for
a DC-DC controller on the secondary side or on the
flyback / LLC controller of the primary side.
Due to the high withstanding voltage of the sensing pin
for drain voltage of external switch, the FAN7346 can be
used in high-voltage LED string operation without any
clamping circuit. Both MOSFET and transistor can be
used as external balance switch, so the FAN7346 can
drive a high-current LED string.
For four-string operation, the FAN7346 supports four
individual PWM signal input pins. The FAN7346 can be
operated by parallel connection mode.
For a primary-side direct-power control system, the
FAN7346 supports an advanced power sequence and
soft-start timing. LED driving voltage can be set before
LED ignition for a reliable LED backlight ignition.
The FAN7346 supports various and programmable
protections; Short-LED Protection (SLP), Open-LED
Protection (OLP), Over-Current Protection (OCP), and
Over-Voltage Regulation (OVR). Except OCP, all
protection are auto-recovery / auto-restart. Except OVR,
all protections are channel-individual protection. In case
of SLP, OLP and OCP; even if some strings are in
protection condition, other strings continues to operating
for higher system reliability.
Figure 27 shows the work-flow of startup sequence.
1. Startup
When VCC voltage is higher than UVLO threshold
voltage, internal 5V regulation output is operated. At the
same time, feedback starts to control OVR voltage as 1V.
If OVR voltage is lower than 1V, feedback is pulled up. If
OVR voltage is higher than 1V, feedback is pulled down.
After enable is applied, the FAN7346 begins soft-start
procedure. The FAN7346 checks if OVR voltage is
higher than 0.9V. If OVR voltage is lower than 0.9V, soft-
start function is not started. If OVR voltage is higher than
0.9V, the FAN7346 starts LED current balance and soft-
start. The LED current of each string is increased
gradually for 10ms, which is fixed soft-start timing. During
soft-start timing, SLP and OLP are disabled.
Figure 27. Startup Sequence
2. Feedback
After soft-start, the FAN7346 starts feedback drain-
voltage of external balance switch.
Drain voltage of each channel is sensed on the CHx pin.
CHx means any of the CH1 / CH2 / CH3 / CH4 pins.
Minimum drain voltage is selected between four CHx
voltages. The FAN7346 feeds back drain voltage to
control minimum drain voltage at 1V.
If minimum drain voltage is higher than 1V, FB is pulled
LOW. If minimum drain voltage is lower than 1V, FB is
pulled HIGH.
Feedback response can be controlled by adding a
resistor and capacitor to the CMP pin.
3. Analog Dimming
In the FAN7346, LED current can be modified by
changing ADIM voltage.
LED current is controlled by the FBx pin voltage. FBx
means any of the FB1 / FB2 / FB3 / FB4 pins. External
current balance switch is operating in the saturation
region to control LED current. Sensed voltage on the FBx
pin is compared with internal reference voltage and the
controller supplies a gate / base signal for the external
current balance switch.
Internal reference voltage is decided by ADIM voltage.
The formula for LED current calculation is:
S E N S E
A D I M
L E D
R
1 0
V
I
(1)
ADIM voltage is clamped internally from 0.5V to 4V. If
ADIM voltage is lower than 0.5V, it is clamped as 0.5V. If
ADIM voltage is higher than 4V, it is clamped as 4V.


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