FAN7319 Datasheet, PDF (14/23 Pages) Fairchild Semiconductor – LCD Backlight Inverter Drive IC

Functional Description
UVLO
The under-voltage lockout (UVLO) circuit guarantees
stable operation of the IC control circuit by stopping and
starting it as a function of VIN. The UVLO circuit turns on
the control circuit when VIN exceeds 9V. When VIN is
lower than 8V, the IC startup current is less than 120µA.
ENA
Applying voltage higher than 1.4V to the ENA pin
enables the IC. If VENA is higher than 2.5V, the IC enters
DCR mode. If VENA is lower than 2.1V, IC enters normal
mode. Applying voltage lower than 0.7V to the ENA pin
disables the IC.
Main Oscillator
The internal 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
2V. 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 main
frequency can be programmed by adjusting the RT
values. The main frequency can be calculated as:
[ ] f_OSC
=
0.000189
500
• RT[kΩ]
+
t_delay
kHz
(1)
where tdelay is:
20 < RT[kΩ] < 30 : 0.00075
30 < RT[kΩ] < 40 : 0.00070
40 < RT[kΩ] < 60 : 0.00055
60 < RT[kΩ] < 70 : 0.00050
Analog Dimming
For analog dimming, reference voltage of the internal
error amplifier can be controlled by the ADIM pin. Error
amplifier control average voltage of OLP input voltage.
The ADIM polarity of ADIM is positive.
The peak OLP voltage can be calculated as:
[ ] V_OmLaPx
= V_ADIM
π
2
V
(4)
The lamp intensity is proportional to VADIM. As VADIM
increases, the lamp intensity increases; but ADIM voltage
is clamped by 1.5V internally. Even if ADIM voltage is
higher than 1.5V, internal error amplifier reference is set to
1.5V. Figure 30 shows an example of the relationship
between ADIM voltage and output current.
Figure 30. ADIM Voltage vs. Output Current
Figure 31 shows the lamp current waveform vs. VADIM in
analog dimming mode.
I_char
=
2V
R_t
/
20
12pF
I_dis
=
2V
R_t
1.3V
V_{AD IM}
0.5V
CMP
i ^{max 0}
Lamp
iLamp 0
Figure 29. Main Oscillator Circuit
In striking mode, the timing resistor for striking (RT1) is
connected to ground internally. The total timing resistor is
the parallel connecting value of RT and RT1. The
calculation of the striking frequency is similar with
Equation (1). RT is only different with normal frequency.
RT value for striking frequency is calculated as:
RT_str
=
RT
RT
• RT1
+ RT1
(2)
© 2010 Fairchild Semiconductor Corporation
FAN7319 • 1.0.1
14
Figure 31. Analog Dimming Waveforms
www.fairchildsemi.com