MAX1996AETI+ Maxim Integrated Products, MAX1996AETI+ Datasheet - Page 11

• Manufacturer Part Number: MAX1996AETI+
• Description: Display Drivers CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX1996AETI+

• Lead Free Status / Rohs Status: Details

Note that switching transitions on all four power
MOSFETs occur under ZVS conditions, which reduces
transient power losses and EMI.
The equivalent circuit of the resonant tank is shown in
Figure 3. The resonant frequency is determined by the
RLC resonant tank elements: C
the series capacitance on the primary side of the trans-
former. C
ondary. L
inductance. RB is an idealized resistance that models
the CCFL load in normal operation.
The MAX1996A uses a current loop and a voltage loop
to control the energy applied to the CCFL. The current
loop is the dominant control in setting the lamp bright-
ness. The rectified lamp current is measured with a
sense resistor in series with the CCFL. The voltage
across this resistor is applied to the IFB input to regulate
the average lamp current. The voltage loop controls the
voltage across the lamp and is active during the begin-
ning of DPWM on-cycles and the open-lamp fault condi-
tion. It limits the energy applied to the resonant network
once the transformer secondary voltage is above the
threshold of 500mV average measured at V
Both voltage and current circuits use transconduc-
tance-error amplifiers to compensate the loops. The
voltage-error amplifier creates an error current based
upon the voltage difference between V
nal reference level (typically 500mV) (Figure 4). The
error current is then used to charge and discharge a
capacitor at the CCV output to create an error voltage
V
based on the voltage difference between IFB and the
dimming control signal. This signal is set by either the
Figure 3. Equivalent Circuit
CCV
. The current loop produces a similar signal at CCI
SOURCE
SOURCE
AC
AC
P
L
Current and Voltage-Control Loops
is the parallel cap on the transformer’s sec-
is the transformer secondary leakage
C
S
/(NxN)
______________________________________________________________________________________
C
S
1:N
LI
S
, C
Range, CCFL Backlight Controller
P
, L
High-Efficiency, Wide Brightness
C
C
P
P
FB
L
, and RB. C
and the inter-
FB
RB
.
S
is
SMBus interface or the analog interface (see the
Dimming Range section). This error voltage is called
V
of the regulator so long as V
control signal is compared with an internal ramp signal
to set the high-side switch on time (t
When DPWM is employed, the two control loops work
together to limit the transformer voltage and to allow a
wide dimming range with good line rejection. During the
DPWM off-cycle, V
loop error amplifier output is high impedance. V
set to 0.6V to create a soft-start at the beginning of each
DPWM on-cycle in order to avoid overshoot on the trans-
former’s secondary. When the transconductance amplifi-
er in the current loop is high impedance, it acts like a
sample-and-hold circuit to keep V
during the off-cycles. This action allows the current-con-
trol loop to regulate the average lamp current.
See the Current-Sense Resistor and the Voltage-Sense
Capacitors sections for information regarding setting
the current- and voltage-loop thresholds.
Operation during startup differs from the steady-state
condition described in the Current and Voltage-Control
Loops section. Upon power-up, V
increasing the duty cycle, which provides soft-start.
During this time, V
is limited to 150mV above V
voltage reaches the strike voltage, the lamp current
begins to increase. When the lamp current reaches the
regulation point, V
steady state. With MINDAC = V
and the current loop remains in control regulating the
lamp current.
The MAX1996A has a feed-forward control circuit,
which influences both control loops. Feed-forward con-
trol instantly adjusts the t
voltage. This feature provides immunity to changes in
input voltage at all brightness levels and makes com-
pensation over wide input ranges easier. The feed-for-
ward circuit improves line regulation for short DPWM
on-times and makes startup transients less dependent
on input voltage.
Feed-forward control is implemented by varying the
internal voltage ramp rate. This has the effect of varying
t
about the same signal levels at V
the required voltage change across the compensation
capacitors is minimal, the controller’s response to
change in V
ON
CCI
as a function of input voltage while maintaining
. In normal operation, the current loop is in control
BATT
is essentially instantaneous.
CCV
CCV
CCI
, which is the faster control loop,
exceeds V
is set to 1.2V and the current-
ON
Feed-Forward Control
CCI
CCI
time to changes in input
CC
is less than V
. Once the secondary
CCI
CCV
, DPWM is disabled
ON
CCI
CCI
and V
).
and it reaches
from changing
slowly rises,
CCV
Startup
CCV
. Since
VFB
. The
11
is