Datasheet

UBA2071_A_1 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 01 — 23 June 2008 14 of 35
NXP Semiconductors
UBA2071; UBA2071A
Half bridge control IC for CCFL backlighting
When the voltage on the VFB pin exceeds V
th(ov)(VFB)
, a fault condition is signalled (for
handling of fault conditions, see Section 8.14) and the bias current at the CVFB pin
changed to the discharge current I
dch(CVFB)
. As a result, the switching frequency increases
and the output voltage of the transformer will decrease
5
. As soon as the voltage at the
VFB pin drops below V
th(ov)(VFB)
, the CVFB capacitor is charged again and the output
voltage of the transformer will increase again. Because the charging and discharging of
the CVFB capacitor follows the ripple on the VFB pin voltage, the feedback gain of the
voltage control loop is set by the ripple on the feedback signal.
The voltage at the CVFB pin is limited by the oscillator circuit to V
CVFB(max)
when the
minimum switching frequency f
sw(min)
is reached, see Figure 10. This ensures an
immediate frequency increase when overvoltage is detected.
8.10 Lamp current control
The lamp current control is always active when the IC is on, except if the lamps are off
during PWM dimming. The AC lamp current is sensed by an external resistor connected
to the IFB pin, see Figure 11. The resulting AC voltage on the IFB pin is internally
Double-Sided Rectified (DSR) and compared to a reference level V
ref(creg)
by an
Operational Transconductance Amplifier (OTA).
When the current is being regulated, switch S1 is closed (conducting). The output current
of the OTA is fed into capacitor C1, which is connected to the CIFB pin. So C1 is charged
and discharged according to the voltage on the IFB pin.
5. Presuming that the load impedance is in inductive region.
Fig 10. Frequency function of CVFB voltage
Voltage on CVFB-pin
frequency
f
sw(min)
f
sw(max)
0
V
CVFB(max)
014aaa105