参数资料
| 型号： | MAX8722CEEG+ |
| 厂商： | Maxim Integrated |
| 文件页数： | 18/21页 |
| 文件大小： | 0K |
| 描述： | IC CCFL BACKLIGHT CTRL 24-QSOP |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 50 |
| 类型： | CCFL 控制器 |
| 频率： | 30 ~ 80 kHz |
| 电流 - 电源： | 1mA |
| 电流 - 输出： | 300mA |
| 电源电压： | 4.6 V ~ 28 V |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 24-SSOP（0.154"，3.90mm 宽） |
| 供应商设备封装： | 24-QSOP |
| 包装： | 管件 |
Low-Cost CCFL Backlight Controller
Setting the Lamp Current
The MAX8722C senses the lamp current flowing through
resistor R1 (Figure 1) connected between the low-volt-
age terminal of the lamp and ground. The voltage across
R 3 =
1 . 20 V
2 × I SEC ( RMS ) _ MAX
R1 is fed to IFB and is internally full-wave rectified. The
MAX8722C controls the desired lamp current by regulat-
ing the average of the rectified IFB voltage. To set the
RMS lamp current, determine R1 as follows:
where I SEC(RMS) _ MAX is the desired maximum RMS
transformer secondary current during fault conditions,
and 1.20V is the typical value of the ISEC peak voltage
when the secondary is shorted. To set the maximum
R 1 =
π × 780 mV
2 2 × I LAMP(RMS)
RMS secondary current in the circuit of Figure 1 to
21mA, use approximately 40.2 Ω for R3.
where I LAMP(RMS) is the desired RMS lamp current and
780mV is the typical value of the IFB regulation point
specified in the Electrical Characteristics table. To set
the RMS lamp current to 6mA, the value of R1 should
be 148 Ω . The closest standard 1% resistors are 147 Ω
and 150 Ω . The precise shape of the lamp-current
waveform, which is dependent on lamp parasitics, influ-
ences the actual RMS lamp current. Use a true RMS
Transformer Design and Resonant
Component Selection
The transformer is the most important component of the
resonant tank circuit. The first step in designing the
transformer is to determine the turns ratio (N). The ratio
must be high enough to support the CCFL operating
voltage at the minimum supply voltage. N can be calcu-
lated as follows:
current meter connected between the R1/IFB junction
and the low-voltage side of the lamp to make final
adjustments to R1.
N ≥
V LAMP ( RMS )
0 . 9 × V IN ( MIN )
Setting the Secondary Voltage Limit
The MAX8722C limits the transformer secondary voltage
during startup and lamp-out faults. The secondary volt-
age is sensed through the capacitive voltage-divider
formed by C3 and C4 (Figure 1). The voltage on VFB is
proportional to the CCFL voltage. The selection of
the parallel resonant capacitor C3 is described in
the Transformer Design and Resonant Component
Selection section. C3 is usually between 10pF and 22pF.
After the value of C3 is determined, select C4 using the
following equation to set the desired maximum RMS sec-
ondary voltage V LAMP(RMS) _ MAX :
where V LAMP(RMS) is the maximum RMS lamp voltage
in normal operation, and V IN(MIN) is the minimum DC
input voltage. If the maximum RMS lamp voltage in nor-
mal operation is 650V and the minimum DC input volt-
age is 8V, the turns ratio should be greater than 90. The
turns ratio of the transformer used in the circuit of
Figure 1 is 93.
The next step in the design procedure is to determine
the desired operating frequency range. The MAX8722C
is synchronized to the natural resonant frequency of the
resonant tank. The resonant frequency changes with
operating conditions, such as the input voltage, lamp
C 4 =
2 × V LAMP ( RMS ) _ MAX
2 . 3 V
× C 3
impedance, etc.; therefore, the switching frequency
varies over a certain range. To ensure reliable opera-
tion, the resonant frequency range must be within the
operating frequency range specified by the CCFL
where 2.3V is the typical value of the VFB peak voltage
when the lamp is open. To set the maximum RMS sec-
ondary voltage to 1600V using 18pF for C3, use approxi-
mately 15nF for C4.
Setting the Secondary Current Limit
The MAX8722C limits the secondary current even if the
IFB sense resistor (R1) is shorted or transformer sec-
ondary current finds its way to ground without passing
through R1. ISEC monitors the voltage across the sense
resistor R3, connected between the low-voltage terminal
of the transformer secondary winding and ground.
Determine the value of R3 using the following equation:
transformer manufacturer. As discussed in the
Resonant Operation section, the resonant frequency
range is determined by the transformer secondary leak-
age inductance L, the primary series DC blocking
capacitor C2, and the secondary parallel resonant
capacitor C3. Since it is difficult to control the trans-
former leakage inductance, the resonant tank design
should be based on the existing secondary leakage
inductance of the selected CCFL transformer. Leakage-
inductance values can have large tolerance and signifi-
cant variations among different batches, so it is best to
work directly with transformer vendors on leakage-
inductance requirements. The MAX8722C works best
18
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相关代理商/技术参数 |
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| MAX8722CEEG+ | 功能描述:显示驱动器和控制器 CCFL Backlight Controller 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 |
| MAX8722CEEG+T | 功能描述:显示驱动器和控制器 CCFL Backlight Controller 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 |
| MAX8722EEG | 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 |
| MAX8722EEG+ | 功能描述:显示驱动器和控制器 CCFL Backlight Controller 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 |
| MAX8722EEG+T | 功能描述:显示驱动器和控制器 CCFL Backlight Controller 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 |
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