MAX1739EEP+ Maxim Integrated Products, MAX1739EEP+ Datasheet - Page 11

• Manufacturer Part Number: MAX1739EEP+
• Description: Display Drivers Wide Brightness Rang e CCFL Backlight Con
• Manufacturer: Maxim Integrated Products

Specifications of MAX1739EEP+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The MAX1739/MAX1839 use two control loops. The cur-
rent control loop regulates the average lamp current. The
voltage control loop limits the maximum average primary-
side transformer voltage. The voltage control loop is
active during the beginning of DPWM on-cycles and in
some fault conditions. Limiting the transformer primary
voltage allows for a lower transformer secondary voltage
rating that can increase reliability and decrease cost of
the transformer. The voltage control loop acts to limit the
transformer voltage any time the current control loop
attempts to steer the transformer voltage above its limit as
set by V
The voltage control loop uses a transconductance
amplifier to create an error current based on the volt-
age between CTFB and the internal reference level
(600mV typ) (Figure 2). The error current is then used
to charge and discharge C
age V
signal based on the voltage between CSAV and its
internal reference level (see the Dimming Range sec-
tion). This error voltage is called V
V
ramp generator to set the buck regulator’s duty cycle.
During DPWM, the two control loops work together to
limit the transformer voltage and to allow wide dimming
range with good line rejection. During the DPWM off-
cycle, V
ance. V
beginning of each DPWM on-cycle in order to avoid
overshoot on the transformer primary. V
high impedance to keep V
off-cycles. This allows the current control loop to regu-
late the average lamp current only during DPWM on-
cycles and not the overall average lamp current.
Upon power-up, V
cycle, which provides soft-start. During this time, V
which is the faster control loop, 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
DAC = VL (DPWM disabled), the current control loop
remains in control and regulates the lamp current.
With MINDAC between REF and GND, DPWM is
enabled and the MAX1739/MAX1839 begin pulsing the
lamp current. During the on-cycle, V
above V
1.2V to provide soft-start at the beginning of the next
on-cycle. Also, V
of the next on-cycle. When V
the buck regulator duty cycle to increase and provides
CCV
CCV
and V
CTFB
CCV
CCV
CCI
CCI
. The current control loop produces a similar
Voltage and Current Control Loops
. After the on-cycle, V
CCI
is set to 1.2V and CCI is set to high imped-
by the CCV-CLAMP . Once the secondary
(see Sense Resistors).
is set to 1.2V to create soft-start at the
is used with the buck regulator’s PWM
CCI
_______________________________________________________________________________________
CCI
CCI
retains its value until the beginning
reaches steady state. With MIN-
slowly rises, increasing the duty
CCI
CCV
CCV
from changing during the
to create an error volt-
CCV
increases, it causes
CCI
CCV
is forced down to
. The lower of
CCI
is at 150mV
is set to
CCFL Backlight Controllers
CCV
,
Wide Brightness Range
soft-start. When V
control loop regains control and regulates the lamp cur-
rent. V
remainder of the on-cycle.
In a lamp-out condition, V
voltage in an attempt to maintain lamp current regula-
tion. As V
voltage reaches its set limit point. At this point, V
stops rising and limits the primary voltage by limiting
the duty cycle. Because V
above V
limit the primary voltage. Without this clamping feature,
the transformer voltage would overshoot to dangerous
levels because V
down from its supply rail. Once the MAX1739/MAX1839
sense less than 1/6 the full-scale current through the
lamp for 2 seconds, it shuts down the Royer oscillator
(see Lamp-Out Detection).
See the Sense Resistors section for information about
setting the voltage and current control loop thresholds.
Both control loops are influenced by the input voltage
feed-forward (V
MAX1839. Feed-forward control instantly adjusts the
buck regulator’s duty cycle when it detects a change in
input voltage. This provides immunity to changes in
input voltage at all brightness levels. This feature
makes compensation over wide input ranges easier,
makes startup transients less dependent on input volt-
age, and improves line regulation for short DPWM on-
times.
The MAX1739/MAX1839 feed-forward control is imple-
mented by varying the amplitude of the buck-switch’s
PWM ramp amplitude. This has the effect of varying the
duty cycle as a function of input voltage while maintain-
ing the same V
forward has the effect of not requiring changes in error-
signal voltage (V
V
voltage minimally to respond to changes in V
controller’s response is essentially instantaneous.
The MAX1739/MAX1839 are designed to maintain tight
control of the transformer primary under all transient
conditions. This includes transients from dropout,
where V
tion and reaches maximum duty cycle. Backlight
designs will want to choose circuit component values to
minimize the transformer turns ratio in order to minimize
primary-side currents and I
BATT
. Since the capacitors only need to change their
CCV
BATT
CCI
CCI
, the voltage control loop is quickly able to
Transient Overvoltage Protection
is limited to 150mV above V
is so low that the controller loses regula-
rises, V
CCI
BATT
CCI
and V
CCV
CCV
and V
) control circuitry of the MAX1739/
CCV
crosses over V
would take more time to slew
CCV
CCV
rises with it until the primary
Feed-Forward Control
CCI
2
. In other words, V
R losses. To achieve this,
CCV
) to respond to changes in
increases the primary
is limited to 150mV
from Dropout
CCI
, the current
CCI
BATT
BATT
for the
, the
feed
CCV
11