UBA2071ATS datasheet(14/35 Pages) NXP

14 / 35 page

UBA2071_A_1

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 Vth(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 Idch(CVFB). As a result, the switching frequency increases

and the output voltage of the transformer will decrease5. As soon as the voltage at the

VFB pin drops below Vth(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 VCVFB(max) when the

minimum switching frequency fsw(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 Vref(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.

Presuming that the load impedance is in inductive region.

Fig 10. Frequency function of CVFB voltage

Voltage on CVFB-pin

frequency

fsw(min)

fsw(max)

VCVFB(max)

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UBA2071ATS Datasheet(PDF) 14 Page - NXP Semiconductors