FAN73402 Datasheet, PDF (11/16 Pages) Fairchild Semiconductor – LED Backlight Driving Boost Switch

Functional Description
The FAN73402 operates as a constant-current source for
driving high-current LEDs. It uses Current-Mode control
with programmable slope compensation to prevent
subharmonic oscillation.
The IC provides open-LED protection, over-voltage
protection, and over-current protection for improved
system reliability. The IC internally generates a FAULT
OUT signal with a delay in case an abnormal LED string
condition occurs. PWM dimming and analog dimming
functions can be implemented independently. Internal
soft-start prevents inrush current flowing into output
capacitor at startup.
V_CC Under-Voltage Lockout (UVLO)
An internal regulator provides the regulated 5V used to
power the IC. The Under-Voltage Lockout (UVLO) turns
off the IC in the event of the voltage dropping below the
specific threshold level. The UVLO circuit inhibits
powering the IC until a voltage reference is established,
up to predetermined threshold level.
Enable
Applying voltage higher than 1.22V (typical) to the ENA
pin enables the IC. Applying voltage lower than 1.15V
(typical) to the ENA pin disables the IC. If ENA pin
voltage is higher than 1.22V (typical) and V_CC is higher
than 9.0V (typical), the IC starts to supply the 5V
reference voltage from V_CC.
Oscillator (Boost Switching Frequency)
Boost switching frequency is programmed by the value of
the resistor connected from the RT pin to ground. RT pin
voltage is set to 2V. The current through the RT pin
resistor determines the boost switching frequency
according to formula:
f
.
kHz
(1)
Soft-Start Function at Startup
During initial startup, the switching device can be
damaged due to the over-current coming from the input
line by the negative control. This can result in the initial
overshoot of the LED current. Therefore, during initial
startup, the soft-start control gradually increases the duty
cycle so that the output voltage can rise smoothly to
control inrush current and overshoot.
FAN73402 adapts the soft-start function in the boost
converter stage. During soft-start period, boost switch
turn-on duty is limited by the clamped CMP voltage. The
soft-start period is dependent on the boost switching
frequency, which is determined by the RT resistor
(Equation 1). Soft-start period is set to be cumulative
time when the BDIM (PWM dimming) signal is HIGH:
t 600 / f seconds
(2)
Figure 26. Soft-Start Waveforms
LED Current Setting
During the boost converter operating periods, the output
LED current can be set by equation:
I
ADIM V
R
60mΩ
(3)
where ADIM(V) is ADIM pin applied voltage and,
R_SENSE is the sensing resistor value.
Note:
5. An additional 60mΩ comes from an internal wire
bonding resistor. To calculate LED current precisely,
consider the wire bonding resistor.
Analog Dimming and PWM Dimming
Analog dimming (ADIM) is achieved by varying the
voltage level at the ADIM pin. This can be implemented
either with a potentiometer from the VREF pin or from an
external voltage source and a resistor divider circuit. The
ADIM voltage level is adjusted to be the same as the
feedback level (V_SENSE). A V_ADIM range from 0.3V to 3V is
recommended.
PWM dimming (BDIM) helps achieve a fast PWM
dimming response in spite of the shortcomings of the
boost converter. The PWM dimming signal controls three
nodes in the IC: gate signal to the switching FET, gate
signal to the dimming FET, and output connection of the
transconductance amplifier. When the PWM dimming
signal is HIGH, the gates of the switching FET and
dimming FET are enabled. At the same time, the output
of the transconductance op-amp is connected to the
compensation network. This allows the boost converter
to operate normally.
Dynamic Contrast Ratio
The Dynamic Contrast Ratio (DCR) means the maximum
contrast ratio achievable by adjusting the amount of light
(dimming) of the screen using the backlight during an
extremely short period of time. FAN73402 can normally
drive the LED backlight under 0.1% dimming duty cycle
at 200Hz dimming frequency. Even operating at 5µs
dimming MOSFET turn-on time and extremely low
dimming duty, FAN73402 can operate LEDs with normal
peak current level.
© 2012 Fairchild Semiconductor Corporation
FAN73402 • 1.0.0
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