Datasheet

UBA2071_A_1 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 01 — 23 June 2008 9 of 35
NXP Semiconductors
UBA2071; UBA2071A
Half bridge control IC for CCFL backlighting
8.5 Non-overlap
During each transition between the two states GL high/GH low and GL low/GH high, GL
and GH will both be low for a fixed time t
no
(non-overlap time) to allow the half bridge point
to be charged or discharged by the load current (presuming the load always has an
inductive behavior), and thus enabling zero voltage switching, see Figure 5.
8.6 Low-side and high-side drivers
The low-side and high-side drivers are identical. The output of each driver is connected to
the equivalent gate of an external power MOSFET. The high-side driver is supplied by the
bootstrap capacitor, which is charged from the V
DD
voltage via an internal diode when the
low-side power MOSFETs is on. The low-side driver is directly supplied by the V
DD
voltage.
8.7 DC blocking capacitor charging
When the IC is off, either because V
VDD
is too low, it is disabled via the EN pin, or it
stopped after the time-out period during a fault condition, the low-side power switch
(Q
LS
in Figure 3) is turned on by making GL high. This ensures that the supply buffer
capacitor of the floating supply (C
FS
in Figure 3) is fully charged at start-up. As a side
effect the DC blocking capacitor (C
block
in Figure 3) will be completely discharged at
start-up. To prevent large inrush currents during the first switching cycles the UBA2071
and UBA2071A can first charge the DC blocking capacitor at start-up.
Fig 5. Oscillator, driver and half bridge voltages
t
V
CF
V
CFH
0
t
V
GH
− V
SH
V
DD
0
t
V
GL
V
DD
0
t
V
HB
V
HV
0
t
no
014aaa102