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Functional Description
The FAN7340 operates as a constant-current source for
driving high-current LEDs. It uses Current-Mode control
with programmable slope compensation to prevent sub-
harmonic oscillation.
The IC provides protections such as 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
GATE
V LED
V CMP
I LED
Soft-Start Period
t
Circuit operation is explained in the following sections.
Figure 26.
Soft-Start Waveforms
V CC Under-Voltage Lockout (UVLO)
An internal regulator provides the regulated 5 V used to
power the IC. The Under-Voltage Lockout (UVLO) turns
off the IC in the event of the voltage dropping below the
LED Current Setting
During the boost converter operating periods, the output
LED current can be set by equation:
specific threshold level. The UVLO circuit inhibits
powering the IC until a voltage reference is established,
up to predetermined threshold level.
?
(3)
Enable
Applying voltage higher than 1.22 V (typical) to the ENA
pin enables the IC. Applying voltage lower than 1.15 V
(typical) to the ENA pin disables the IC. If ENA pin
voltage is higher than 1.22 V (typical) and V CC is higher
than 9.0 V (typical.), the IC starts to supply 5 V re ference
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 2 V. The current through the RT pin
resistor determines boost switching frequency according
to formula:
(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.
FAN7340 adapts the soft-start function in the boost
converter stage. During soft-start period, boost switch
turn-on duty is limited by clamped CMP voltage. The
soft-start period is dependent on boost switching
frequency, which is decided by the RT resistor (Equation
(1)). Soft-start period is set to be cumulative time when
the BDIM (PWM dimming) signal is HIGH:
(2)
? 2013 Fairchild Semiconductor Corporation
FAN7340 ? 1.0.1
11
where ADIM(V) is
ADIM pin applied voltage and, R SENSE is the sensing
resistor value. An additional 60 m Ω comes from an
internal wire bonding resistor. To calculate LED
current precisely, consider the wire bonding resistor.
Analog Dimming and PWM Dimming
Analog dimming 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 0.3 V to 3 V 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
trans-conductance 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 ap-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 instantaneously using the
backlight during the extremely short period of time.
FAN7340 can normally drive the LED backlight under
0.1% dimming duty cycle at 200 Hz dimming frequency.
Even operating at 5 μs-dimming FET turn-on time and
extremely low dimming duty, FAN7340 can operate
LEDs with normal peak current level.
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