UBA2072T datasheet(13/28 Pages) PHILIPS

13 / 28 page

UBA2072

Preliminary data sheet

Rev. 08.0 — February 2007

13 of 28

NXP Semiconductors

UBA2072

Low Voltage Full-bridge control IC for CCFL backlighting

An internal latch makes the OV fault signal continuesly high even if the voltage at the

VFB-pin only exceeds VVFB(ovref) during part of the output period. So the peak of the

voltage on the VFB-pin determines if an over voltage fault condition is seen.In order to

avoid that OV fault condition at the nominal switching frequency (with the lamps operating

normally), the voltage ripple on the VFB-pin must not be too large.

The voltage at CVFB is limited by the oscillator circuit to VCVFB(range) when the minimum

switching frequency Fs(min) is reached. This ensures an immediate frequency increase

capability at over voltage detection.

7.8.2

Over current detection

When the absolute value3 of the voltage across the current sense resistor (connected to

the IFB-pin) exceeds the OC reference level VIFB(ocref), over-current is detected. As result

PWM dimming is disabled and the fault timer is started.

7.8.3

Arcing detection

If arcing occurs, for instance due to a bad lamp connection, it causes repetitive short

current spikes that can be seen as voltage spikes at the IFB input4. The arcing detection

circuit is directly connected to the IFB-pin, so it can only see spikes with a positive polarity.

Usually that will be sufﬁcient. It can detect spikes with amplitude above VIFB(arcref) and a

duration longer then TSPIKE(min). Each spike will trigger an internal one-shot, which signals

to the control circuits that arcing has been detected. If this happens PWM dimming is

disabled, and the fault timer is started.

7.8.4

Ignition Failure (IF)

When the current control loop comes close to its regulation point, the lamps are presumed

to be on (ignited). This is when the average double side rectiﬁed IFB-pin voltage is above

VIFB(lampon). If the lamps are not on when the ignition sweep is ﬁnished (switching

frequency has reached FS(min)), then an ignition failure is detected, PWM dimming is

disabled and the fault timer is started.

7.8.5

The nonFAULT-pin

The nonFAULT-pin provides bidirectional signalling of the fault status between the IC and

any external circuit. When no fault is detected, the voltage on the pin is pulled high by an

internal current source.

An external circuit can signal to the IC that a fault has been detected by pulling down the

pin. The IC will detect the current drawn from the pin and start the fault timer. To prevent

interference with the PWM dimming, the IC will only look at the nonFAULT pin during the

period that the lamp current regulation loop is closed (VCSWP=VCIFB).

When the IC detects a fault internal (as in Section 7.8.1 to Section 7.8.4), it signals this via

the nonFAULT pin by pulling the pin down. In this case the IC can not see anymore if

there’s an external detected fault, but that’s no problem, because the faulttimer is then

already running.

The OC comparator is behind the double side rectiﬁer at the IFB-pin

Provided that the current sensing circuit is simple sense resistor only.

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