Datasheet

UBA2071_A_1 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 01 — 23 June 2008 19 of 35
NXP Semiconductors
UBA2071; UBA2071A
Half bridge control IC for CCFL backlighting
• OverCurrent Protection (OCP),
• bad contact or ARCing (ARC),
• Ignition Failure (IF),
• open or shorted current feedback (IFB pin open/short),
• open or shorted voltage feedback (VFB pin open/short)
• Hard Switching (HS).
There are also two pins, the NONFAULT pin and the COMM pin, via which a fault can be
signalled to the IC, by an external circuit.
The fault protection functions are explained in the following sections.
8.14.1 Voltage feedback open or short protection
If the VFB pin is left open or shorted to SGND, the voltage at the VFB pin will drop below
the VFB open/short protection threshold voltage V
th(osp)VFB
, due to the internal bias
resistor R
i(VFB)
. If the voltage at the VFB pin is below V
th(osp)VFB
continuously or only
during a part of each switching cycle the PWM dimming is disabled and the fault timer is
started.
To protect the inverter transformer(s) against overvoltage if the voltage feedback loop is
broken, the frequency stays at f
sw(max)
or is increased by discharging the capacitor at the
CVFB pin (by I
dch(CVFB)
). For this function the voltage at the VFB pin has to be below
V
th(osp)VFB
during more than 50 % of each switching cycle.
Fig 15. Simplified control schematic
enable
low_supply
NONFAULT-pin_fault_signal
IFB_open/short
over_voltage_extra
over_voltage
VFB_open/short
COMM-pin_fault_signal
hard_switching_detected
over_current
f_max
arcing_detected
lamp_current_detected
f_min
ic_off
FAULT-
TIMER
FILTER
stop
R
S
Q
lamp_on
ignition_
failure
R
S
Q
DELAY enable_PWM_
dimming
014aaa111