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参数资料
| 型号： | MAX8785ETI+T |
| 厂商： | Maxim Integrated |
| 文件页数： | 13/19页 |
| 文件大小： | 0K |
| 描述： | IC CCFL CTRL 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 |
| 电流 - 电源： | 1.5mA |
| 电源电压： | 4.5 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) |
Full-Bridge Controller for
Piezoelectric Transformers
the CCFL strike voltage is reached and the lamp impe-
dance begins to decrease. The operating frequency
continues to decrease as the lamp impedance drops
until the correct operating point is reached, somewhere
between Points C and D. Figure 5 shows the Q of reso-
nant tank is very high. If the operating frequency is
close to resonant frequency, then the high Q gives a
very high efficiency for the converter. However, due to
the high Q, the frequency of operation has to be very
close to the resonant frequency, and this reduces the
range of the switching frequency for the inverter. To
ensure optimal performance, careful selection of exter-
nal components is required.
Variable Frequency Operation
The MAX8785A includes a voltage-controlled oscillator
(VCO) that sets the switching frequency of the
H-bridge. The VCO is controlled by the output of a
transconductance error amplifier that integrates the dif-
ference between the full-wave, rectified lamp-current
feedback signal (IFB) and an internal reference voltage
(800mV typ). As the lamp current-feedback signal
changes with respect to the reference, the error amplifier
sources and sinks current, which appropriately adjusts
COMP, and equivalently, the VCO frequency.
To strike the CCFL, a frequency sweep is initiated by
linearly ramping the COMP capacitor. As the COMP
voltage rises, the VCO sweeps the switching frequency
from the maximum value (set by the R HF resistor) to the
point where the gain of resonant tank is enough to
strike the lamp. The frequency sweep range is 15% of
the maximum switching frequency.
Feed-Forward Control
The MAX8785A has feed-forward control, which main-
tains tight control of the lamp current over the entire
input voltage range. The feed-forward control adjusts
the on-time (t ON ) by varying the slope of internal ramp.
The current into RATE determines the slope of the inter-
nal ramp. Connect a passive network between V IN and
RATE to change the duty ratio with input voltage. The
RATE pin allows the user to adjust the maximum duty
Connect a resistor between BATT and RATE to set D MAX :
R RATE = D MAX (%) × R HF ( k Ω ) × V MIN ( V )
where R HF is the resistor that sets the maximum switching
frequency and V MIN is the minimum input voltage of
operation. The feed-forward network required is depen-
dent upon the PZT used in the application. Further
improvements in the performance at higher input volt-
ages can be achieved by adding a zener diode in
series with the RATE resistor. The MAX8785A has built-
in protection to limit the maximum duty cycle to 45%.
Dimming Control
The MAX8785A has both analog and digital inputs to
control brightness.
Analog Dimming Control
Connect SEL to GND to enable analog control mode.
The voltage at CNTL controls DPWM duty cycle. The
DPWM frequency is externally set by the resistor at LF
(R LF ). In analog control mode, the adjustment range is
10% to 100%. CNTL has a voltage range of 0 to 2V with
256 brightness levels. Figure 6 shows the response of
DPWM duty cycle to the CNTL voltage. When V CNTL is
between 0V and 0.23V, the DPWM duty cycle is fixed at
10%. When V CNTL is 0.23V to 2V, the DPWM duty cycle
changes linearly with CNTL. When the CNTL voltage is
above 2V, the DPWM duty cycle is fixed at 100%.
BRIGHTNESS vs. V CNTL
100
80
60
40
20
ratio (D MAX ) to achieve the optimum performance of
the PZT used in the application.
0
0
0.4
0.8
1.2
1.6
2.0
V CNTL (V)
Figure 6. Brightness vs. V CNTL
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