FAN7311B Datasheet, PDF (7/14 Pages) Fairchild Semiconductor – LCD Backlight Inverter Drive IC

Function Description
UVLO: The under-voltage lockout circuit guarantees sta-
ble operation of the IC’s control circuit by stopping and
starting it as a function of the V_IN value. The UVLO cir-
cuit turns on the control circuit when V_IN exceeds 5V.
When V_IN is lower than 5V, the IC’s standby current is
less than 200µA.
ENA: Applying voltage higher than 2V to the ENA pin
enables operation of the IC. Applying voltage lower than
0.7V to the ENA pin disables operation of the inverter.
Soft-Start: The soft-start function requires that the S_S
pin is connected through a capacitor to GND. A soft-start
circuit ensures a gradual increase in the input and output
power. The capacitor connected to the S_S pin deter-
mines the rate at which the duty ratio rises. It is charged
by a 6µA current source.
Figure 4. Main Oscillator Waveform
Burst Oscillator & Burst Dimming: The timing capaci-
tors (BCTs) are charged by the reference current source,
which is formed by the timing resistor (RT). The timing
resistor’s voltage is regulated at 1.25V. The sawtooth
waveform charges up to 2V. Once this voltage is
reached, the capacitors begin discharging down to 0.5V.
Next, the timing capacitors start charging again and a
new switching cycle begins. The burst dimming fre-
quency can be programmed by adjusting the RT and
BCT values. The burst dimming frequency can be calcu-
lated as shown in Equation 2.
Figure 3. Soft-Start During Initial Operation
Main Oscillator: The timing capacitors (CTs) are
charged by the reference current source, which is
formed by the timing resistor (RT). The timing resistor’s
voltage is regulated at 1.25V. The sawtooth waveform
charges up to 2V. Once this voltage is reached, the
capacitors begin discharging down to 0.5V. Next, the tim-
ing capacitors start charging again and a new switching
cycle begins. The main frequency can be programmed
by adjusting the RT and CT values. The main frequency
can be calculated as shown below.
fburst =
3.75
(2)
96 ⋅ RT ⋅CT
To avoid visible flicker, the burst dimming frequency
should be greater than 120Hz.
By comparing the input of BDIM pin with the 0.5~2V tri-
angular wave of the burst oscillator, the PWM pulses for
burst dimming. The PWM pulse controls EA_OUT’s volt-
age by summing 85µA into the EA_IN pin.
fop =
19
(1)
32 ⋅ RT ⋅CT
© 2007 Fairchild Semiconductor Corporation
FAN7311B Rev. 1.0.0
7
www.fairchildsemi.com