Datasheet

UBA2071_A_1 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 01 — 23 June 2008 11 of 35
NXP Semiconductors
UBA2071; UBA2071A
Half bridge control IC for CCFL backlighting
Remark: Due to the nature of the charging sequence, V
Cblock
will automatically be
charged to 0.5 × V
HV
(when given enough time). If the IC is kept in DC blocking capacitor
charging mode longer than necessary, V
Cblock
will remain 0.5 × V
HV
. Therefor the time
constant made by R1 and C1 in example A in Figure 6, is not critical. However, since
V
Cblock
cannot become more than 0.5 × V
HV
, it is important to take a lower target value for
V
Cblock
than 0.5 × V
HV
(with enough margin) in examples B and C of Figure 6, otherwise
the IC may get stuck in DC blocking capacitor charging mode.
8.8 Lamp (re-)ignition
The IC starts at its maximum switching frequency f
sw(max)
. The lamp current and the lamp
voltage control loops are enabled. The frequency is swept down towards the minimum
frequency f
sw(min)
, see Figure 9. During this initial ignition frequency sweep the lamp
voltage will increase as the frequency comes closer to the resonant frequency of the
unloaded resonance circuit. Once the ignition voltage V
ign
is reached the lamps will ignite
and the lamp voltage will drop
4
to the voltage of the loaded resonance curve, see
Figure 8.
Fig 7. DC blocking capacitor charging mode
014aaa252
0
0
V
SH
(L)(EN)
(H)1(EN)
(H)2(EN)
EN
GH
GL
disabled
DC blocking capacitor charging
initial ignition sweep
and normal mode
0.5 T
T7.5 T
7.5 T
3 / f
sw(min)
3 / f
sw(min)
1 / f
sw(max)
4. For CCFLs only.