MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 9

• Manufacturer Part Number: MAX8722
• Description: Low-Cost CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

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PIN
10
11
12
13
14
15
16
17
1
2
3
4
5
6
7
8
9
NAME
DPWM
COMP
SHDN
SYNC
BATT
CNTL
FREQ
BST2
BST1
TFLT
ISEC
GH2
ILIM
VFB
LX2
LX1
IFB
_______________________________________________________________________________________
Supply Input. BATT is the input to the internal 5.4V linear regulator that powers the device. Bypass BATT to
GND with a 0.1µF ceramic capacitor.
Shutdown Control Input. The device shuts down when SHDN is pulled to GND.
Primary Current-Limit Adjustment Input. Connect a resistive voltage-divider between V
the primary current limit. The current-limit threshold is 1/5 of the voltage at ILIM. Connect it to V
pullup resistor to select the default current-limit threshold of 0.2V.
Fault Timer Adjustment Pin. Connect a capacitor from TFLT to GND to set the timeout periods for open-
lamp and secondary overcurrent faults.
Brightness Control Input. Varying V
between 10% (minimum) and 100% (maximum). The brightness remains at maximum for V
than 2V.
Dual-Function DPWM Signal Pin. The DPWM pin can be used either as the DPWM signal output or as a
low-frequency sync input. See the Digital PWM Dimming Control and Digital PWM Frequency Setting
sections.
DPWM High-Frequency Sync Input. The DPWM chopping frequency can be synchronized to an external
high-frequency signal by connecting FREQ to V
chopping frequency is 1/128 of the frequency of the external signal.
DPWM Frequency Dual-Mode Adjustment Pin. Connect a resistor from FREQ to GND to set the DPWM
frequency. Connect FREQ to V
f
Transconductance Error-Amplifier Output. A compensation capacitor connected between COMP and GND
sets the rise and fall time of the lamp current in DPWM operation.
Lamp-Current Feedback Input. The average voltage on IFB is regulated to 0.8V by controlling the on-time
of high-side switches. If V
MAX8722 activates the fault latch.
Transformer Secondary Voltage Feedback Input. A capacitive voltage-divider between the high-voltage
terminal of the CCFL tube and GND sets the maximum average lamp voltage during lamp strike and open-
lamp conditions. When the average voltage on VFB exceeds the internal overvoltage threshold, the
controller turns on an internal current sink discharging the COMP capacitor.
Transformer Secondary Current Feedback Input. A current-sense resistor connected between the low-
voltage end of the transformer secondary and ground sets the maximum secondary current during faults.
When the average voltage on ISEC exceeds the internal overcurrent threshold, the controller turns on an
internal current sink discharging the COMP capacitor.
High-Side MOSFET NH2 Gate-Driver Output
GH2 Gate-Driver Return. LX2 is the input to the current-limit and zero-crossing comparators. The device
senses the voltage across the low-side MOSFET NL2 to detect primary current zero-crossing and primary
overcurrent.
GH2 Gate-Driver Supply Input. Connect a 0.1µF capacitor from LX2 to BST2 and a diode from V
to form a bootstrap circuit.
GH1 Gate-Driver Supply Input. Connect a 0.1µF capacitor from LX1 to BST1 and a diode from V
to form a bootstrap circuit.
GH1 Gate-Driver Return. LX1 is the input to the current-limit and zero-crossing comparators. The device
senses the voltage across the low-side MOSFET NL1 to detect primary current zero-crossing and primary
overcurrent.
DPWM
= 209Hz x 169k / R
Low-Cost CCFL Backlight Controller
IFB
FREQ
falls below 0.6V for a period longer than the timeout period set by TFLT, the
CC
to set DPWM frequency using SYNC.
CNTL
between 0 and 2V varies the DPWM duty cycle (brightness)
CC
FUNCTION
and SYNC to the external signal source. The DPWM
Pin Description
CC
and GND to set
CNTL
CC
greater
DD
DD
with a
to BST2
to BST1
9