Rainbow Electronics MAX8759 - Manual (Page 17)

AC lamp current is sensed with a resistor connected in
series with the low-voltage terminal of the lamp. The
MAX8759 has two lamp-current feedback inputs (IFB1
and IFB2) to support dual-lamp application. The volt-
ages across the sense resistors are fed to the IFB1 and
IFB2 inputs and are internally full-wave rectified. The
transconductance error amplifier selects the higher one
of the two feedback signals and compares the rectified
voltage with an internal threshold to generate an error
current. The error current charges and discharges a
capacitor connected between COMP and ground to
create an error voltage (V
COMP ). V COMP is then com-
pared with an internal ramp signal to set the high-side
MOSFET switch on-time (t
ON ).
Feed-Forward Control
The MAX8759 is designed to maintain tight control of
the lamp current under all transient conditions. The
feed-forward control instantaneously adjusts the on-
time for changes in input voltage (V
BATT ). This feature
provides immunity to input-voltage variations and sim-
plifies loop compensation over wide input-voltage
ranges. The feed-forward control also improves the line
regulation for short DPWM on-times and makes startup
transients less dependent on the input voltage.
Feed-forward control is implemented by increasing the
internal voltage ramp rate for higher V
BATT . This has
the effect of varying t ON as a function of the input volt-
age while maintaining approximately the same signal
levels at V
COMP . Since the required voltage change
across the compensation capacitor is minimal, the con-
troller’s response to input voltage changes is essentially
instantaneous.
Lamp Startup
A CCFL is a gas-discharge lamp that is normally driven in
the avalanche mode. To start ionization in a nonionized
lamp, the applied voltage (striking voltage) must be
increased to the level required for the start of avalanche.
At low temperatures, the striking voltage can be several
times the typical operating voltage.
Because of the MAX8759’s resonant topology, the striking
voltage is guaranteed. Before the lamp is ionized, the
lamp impedance is infinite. The transformer secondary
leakage inductance and the high-voltage parallel capaci-
tor determine the unloaded resonant frequency. Since the
unloaded resonant circuit has a high Q, it can generate
very high voltage across the lamp.
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
______________________________________________________________________________________ 17
AC
SOURCE
CCFL C
P
L C S
1:N
(a)
AC
SOURCE
R L
C P
L
C' S =
(b)
C S
N 2
Figure 5. Equivalent Resonant Tank Circuit
FREQUENCY (kHz)
VOLTAGE GAIN (V/V)
80 60 40 20
1
2
3
4
0
0 100
R
L
INCREASING
Figure 6. Frequency Response of the Resonant Tank