FAN73402 Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – LED Backlight Driving Boost Switch

Internal Dimming MOSFET
A dimming MOSFET (200V N-channel MOSFET) is
incorporated in the FAN73402. The power transistor is
produced using Fairchild’s proprietary, planar stripe,
DMOS technology. This advanced technology is tailored
to minimize on-state resistance (R_DS(on)=1.0) to provide
superior switching performance. This device is suited for
high-efficiency SMPS and shows desirable thermal
characteristic during operation. To prevent initial LED
current overshoot at low V_ADIM levels, gate resistance of
the internal dimming FET is designed as 5k.
Feedback Loop Compensation
Stable closed-loop control can be accomplished by
connecting a compensation network between COMP and
GND. The compensation needed to stabilize the
converter can be a Type-I circuit (a simple integrator) or
a Type-II circuit (and integrator with and additional pole-
zero pair). The type of the compensation circuit required
is dependent on the phase of the power stage at the
crossover frequency.
FAN73402 adopts a Type-II compensator circuit.
Figure 27. Slope Compensation
Cycle-by-Cycle Over-Current Protection
In boost topology, the switch can be damaged in
abnormal conditions (inductor short, diode short, output
short). It is always necessary to sense the switch current
to protect against over-current failures. Switch failures
due to excessive current can be prevented by limiting I_d.
Programmed Current Control
FAN73402 uses Current-Mode control. Current-Mode
control loops: an outer feedback loop that senses output
voltage (current) and delivers a DC control voltage to an
inner feedback loop, which senses the peak current of
the inductor and keeps it constant on a pulse-by-pulse
basis. One of the advantages of the Current-Mode
control is line/load regulation, which is corrected
instantaneously against line voltage changes without the
delay of an error amplifier.
Programmable Slope Compensation
When the power converter operates in Continuous
Conduction Mode (CCM), the current programmed
controller is inherently unstable when duty is larger than
50%, regardless of the converter topology. The
FAN73402 uses a Peak-Current-Mode control scheme
with programmable slope compensation and includes an
internal transconductance amplifier to accurately control
the output current over all line and load conditions.
An internal R_slope resistor (5kΩ) connected to sensing
resistor, R_S, and an external resistor, R₁, can control the
slope of V_SC for the slope compensation. Although the
normal operating mode of the power converter is DCM,
the boost converter operates in CCM in the case of rapid
LED current increase. As a result, slope compensation is
an important feature.
The value of an external series resistor (R1) can be
programmed. In normal DCM operation, 5k is
recommended.
Figure 28. Cycle-by-Cycle OCP Circuit
When the voltage drops at R₁ and R_S exceed a threshold
of approximately 0.5V, the power MOSFET over-current
function is triggered after minimum turn-on time or LEB
time (300ns).
The peak voltage level at CS terminal:
V_{_}
45μ R R DT I R
(4)
Choose the boost switch current-sensing resistor (R_CS):
R
0.25
I_{_}
(5)
Open-LED Protection (OLP)
After the first PWM dimming-HIGH signal, the feedback
sensing resistor (R_SENSE) starts sensing the LED current.
If the feedback voltage of the SENSE pin drops below
0.2V, the OLP triggers to generate an error flag signal.
Because OLP can be detected only in PWM dimming-
HIGH; if OLP detecting time is over 5μs, PWM dimming
signal is pulled HIGH internally regardless of external
dimming signal. If OLP signal continues over blanking
time, an error flag signal is triggered.
OLP blanking time is dependent on the boost switch
frequency, per Equation 6. FAULT OUT signal is made
through the FAULT pin, which needs to be connected 5V
reference voltage through a pull-up resistor. In normal
operation, the FAULT pin voltage is pulled down to
ground. In OLP condition, the FAULT pin voltage is
pulled HIGH.
© 2012 Fairchild Semiconductor Corporation
FAN73402 • 1.0.0
12
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