UBA2071ATS datasheet(13/35 Pages) NXP

13 / 35 page

UBA2071_A_1

Product data sheet

Rev. 01 — 23 June 2008

13 of 35

NXP Semiconductors

UBA2071; UBA2071A

Half bridge control IC for CCFL backlighting

The lamps start switching again on period GL and GH and the frequency is swept back to

the regulation frequency freg. The duration of the PWM frequency sweep is inverse

proportional to the capacitor connected to the CSWP pin.

8.9 Overvoltage control

The overvoltage control circuit is intended to prevent the transformer output voltage from

exceeding its maximum rating. It can also be used to regulate the output voltage to the

required lamp ignition voltage.

Under normal circumstances the capacitor at the CVFB pin is charged by a constant bias

current Ich(CVFB), thus the voltage on the CVFB pin will increase resulting in a decrease of

switching frequency. If the IC is in current regulation this bias current will ﬂow away via a

tracking circuit which makes the voltage on the CVFB pin following the voltage on the

CIFB pin, see Figure 11.

Fig 9.

Initial ignition frequency sweep and PWM dimming frequency sweep signals

fsw

min

max

ignition

freg

fign

Vsec

VCVFB

VCIFB

VCSWP

max

VPWMD

Ilamp

nom

PWM-enable delay

014aaa104

steady-state

initial

ignition

sweep

VCVFB

VCIFB

VCSWP

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