参数资料
| 型号： | MAX8709BETI+T |
| 厂商： | Maxim Integrated |
| 文件页数： | 14/24页 |
| 文件大小： | 0K |
| 描述： | IC CNTRLR CCFL HI EFF 28-TQFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 类型： | CCFL 控制器 |
| 频率： | 25 ~ 300 kHz |
| 电流 - 电源： | 1.5mA |
| 电流 - 输出： | 500mA |
| 电源电压： | 4.6 V ~ 28 V |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 供应商设备封装： | 28-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
High-Efficiency CCFL Backlight
Controller with SMBus Interface
and discharges a capacitor connected between CCV
and ground to generate an error voltage V CCV . The
lower of V CCI and V CCV takes control and is compared
with an internal ramp signal to set the high-side
MOSFET switch on-time (t ON ).
Lamp Startup
A CCFL is a gas discharge lamp that is normally driven
in the avalanche mode. To start ionization in a nonion-
ized lamp, the applied voltage (striking voltage) must
be increased to the level required for the start of
avalanche. The striking voltage can be several times
the typical operating voltage.
Because of the resonant topology, the striking voltage
is guaranteed regardless of the temperature. Before the
lamp is ionized, the lamp impedance is infinite. The
transformer secondary leakage inductance and the
high-voltage parallel capacitor determine the unloaded
resonant frequency. Since the unloaded resonant cir-
cuit has a high Q, it is easy to generate high voltages
across the lamp.
Operation during startup differs from the steady-state
condition described in the Current and Voltage Control
Loops section. Upon power-up, V CCI slowly rises,
increasing the duty cycle, which provides soft-start.
During this time, V CCV is limited to 150mV above V CCI .
Once the secondary voltage reaches the strike voltage,
the lamp current begins to increase. When the lamp
current reaches the regulation point, V CCI exceeds
V CCV and it reaches steady state.
Feed-Forward Control and
Dropout Operation
The MAX8709B is designed to maintain tight control of
the transformer secondary under all transient condi-
tions including dropout. The feed-forward control
instantaneously adjusts the t ON time for changes in
input voltage (V BATT ). This feature provides immunity to
input voltage variations and simplifies loop compensa-
tion 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
PWM’s internal voltage ramp rate for higher V BATT . This
has the effect of varying t ON as a function of the input
voltage while maintaining about the same signal levels
at V CCI and V CCV . Since the required voltage change
across the compensation capacitors is minimal, the
controller ’s response to input voltage changes is
essentially instantaneous.
To maximize run time, it may be desirable to allow the
circuit to operate in dropout if the backlight’s perfor-
mance is not critical. When V BATT is very low, the con-
troller loses current regulation and runs at maximum
duty cycle. Under these circumstances, a transient
overvoltage condition could occur when the AC
adapter is suddenly applied to power the circuit. The
feed-forward circuitry minimizes variations in lamp volt-
age due to such input voltage steps. The regulator also
clamps the voltage on V CCI . These two features togeth-
er ensure that overvoltage transients do not appear on
the transformer when leaving dropout.
The V CCI clamp is unique in that it limits V CCI to the
peak voltage of the PWM ramp. As the circuit reaches
dropout, V CCI approaches the PWM ramp’s peak in
order to reach maximum t ON . If V BATT decreases fur-
ther, the control loop loses regulation and V CCI tries to
reach its positive supply rail. The clamp on V CCI pre-
vents this from happening and V CCI rides just above
the PWM ramp’s peak. If V BATT continues to decrease,
the feed-forward control reduces the amplitude of the
PWM ramp and the clamp pulls V CCI down. When
V BATT suddenly steps out of dropout, V CCI is still low
and maintains the drive on the transformer at the old
dropout level. The control loop then slowly corrects and
increases V CCI to bring the circuit back into regulation.
DPWM Dimming Control
The MAX8709B controls the brightness of the CCFL by
“chopping” the lamp current on and off using an internal
DPWM signal. The frequency of the DPWM signal is
210Hz. The brightness code set through the SMBus inter-
face determines the duty cycle of the DPWM signal. A
brightness code of 0b00000 corresponds to a 12.5%
duty cycle for the MAX8709B. A brightness code of
0b11111 corresponds to a 100% DPWM duty cycle. The
duty cycle changes by 3.125% per step. Codes 0b00000
to 0b00011 all produce 12.5% for the MAX8709B.
In DPWM operation, the CCI and CCV control loops work
together to regulate the lamp current, limit the secondary
voltage, and control the rising and falling of the lamp cur-
rent. During the DPWM off-cycle, the output of the volt-
age-loop error amplifier (CCV) is set to 1.15V and the
current-loop error-amplifier output (CCI) is high imped-
ance. The high-impedance output acts like a sample-
and-hold circuit to keep V CCI from changing during the
off-cycles. At the beginning of the DPWM on-cycle, V CCV
linearly rises, gradually increasing t ON , which provides
soft-start. Once V CCV exceeds V CCI , the current-loop
error amplifier takes control and starts to regulate the
lamp current. In the meantime, V CCV continues to rise
and is limited to 150mV above V CCI . At the end of the
DPWM on-cycle, the CCV capacitor discharges linearly,
gradually decreasing t ON and providing soft-stop.
14
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相关代理商/技术参数 |
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|---|---|
| MAX8709ETI | 功能描述:显示驱动器和控制器 RoHS:否 制造商:Panasonic Electronic Components 工作电源电压:2.7 V to 5.5 V 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:QFN-44 封装:Reel |
| MAX8709ETI+ | 功能描述:显示驱动器和控制器 CCFL Backlight Controller w/SMBus RoHS:否 制造商:Panasonic Electronic Components 工作电源电压:2.7 V to 5.5 V 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:QFN-44 封装:Reel |
| MAX8709ETI+T | 功能描述:显示驱动器和控制器 CCFL Backlight Controller w/SMBus RoHS:否 制造商:Panasonic Electronic Components 工作电源电压:2.7 V to 5.5 V 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:QFN-44 封装:Reel |
| MAX8709ETI-T | 功能描述:显示驱动器和控制器 RoHS:否 制造商:Panasonic Electronic Components 工作电源电压:2.7 V to 5.5 V 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:QFN-44 封装:Reel |
| MAX8709EVKIT | 功能描述:显示开发工具 MAX8709 Eval Kit RoHS:否 制造商:4D Systems 产品:4Display Shields 工具用于评估:?OLED-160-G1, ?OLED-160-G2 接口类型:Serial 工作电源电压:5 V |
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