参数资料
| 型号： | MAX8759ETI+T |
| 厂商： | Maxim Integrated |
| 文件页数： | 27/32页 |
| 文件大小： | 0K |
| 描述： | IC CNTRL CCFL BACKLIGHT 28TQFN |
| 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： | 30 ~ 80 kHz |
| 电流 - 电源： | 2.5mA |
| 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) |
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Low-Cost, SMBus, CCFL Backlight Controller
using the following equation to set the desired maxi-
mum RMS secondary 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 normal oper-
C 5 =
2 × V LAMP ( RMS )_ MAX
2 . 3 V
× C 4
ation is 700V and the minimum DC input voltage is 7.5V,
the turns ratio should be greater than 104. The turns ratio of
the transformer used in the circuit of Figure 1 is 110.
where the 2.3V is the typical value of the VFB peak volt-
age when the lamp is open. To set the maximum RMS
secondary voltage to 1800V when C4 is 10pF, use
10nF for C5.
Setting the Secondary Current Limit
The MAX8759 limits the secondary current even if the
IFB_ sense resistors are shorted or transformer sec-
ondary current finds its way to ground without passing
through the sense resistors. ISEC monitors the peak volt-
age across the sense network (R2 and C6 in Figure 1)
connected between the low-voltage terminal of the trans-
former secondary winding and ground. Using an RC-
sense network instead of a single-sense resistor makes
the secondary current-limit frequency dependent. The UL
safety standard requires the AC peak current in a limited-
current circuit should not exceed 0.7mA for frequencies
below 1kHz. For frequencies above 1kHz, the limit of
0.7mA is multiplied by the value of the frequency in kilo-
hertz but should not exceed 70mA peak when the fre-
quency is equal to or above 100kHz. To meet the UL
current-limit specifications, determine the value of R2
using the current limit at 1kHz and determine the value of
C6 using the current limit at 100kHz:
The next step in the design procedure is to determine the
desired operating frequency range. The MAX8759 is syn-
chronized to the natural resonant frequency of the reso-
nant tank. The resonant frequency changes with
operating conditions, such as the input voltage, lamp
impedance, etc. Therefore, the switching frequency
varies over a certain range. To ensure reliable operation,
the resonant frequency range must be within the operat-
ing frequency range specified by the CCFL transformer
manufacturer. As discussed in the Resonant Operation
section, the resonant frequency range is determined by
transformer secondary leakage inductance L, the primary
series DC blocking capacitors (C S ), and the secondary
parallel resonant capacitor C P . Since it is difficult to con-
trol the transformer leakage inductance, the resonant tank
design should be based on the existing secondary leak-
age inductance of the selected CCFL transformer. The
leakage inductance values can have large tolerance and
significant variations among different batches. It is best to
work directly with transformer vendors on leakage induc-
tance requirements. The MAX8759 works best when the
secondary leakage inductance is between 250mH and
350mH. Series capacitor C S sets the minimum operating
frequency, which is approximately two times the series
C 6 <
1.23 V
R 2 > = 1 . 75 k ?
0 . 7 mA
70 mA
2 π × 100 kHz × 1 . 23 V
= 90 nF
resonant peak frequency. Choose:
N 2
C S ≤
π 2 × f 2 MIN × L
where f MIN is the minimum operating frequency range.
In the circuit of Figure 1, the transformer’s turns ratio is
C S
where 1.23V is the typical value of the ISEC peak volt-
age when the transformer secondary is shorted. The
circuit of Figure 1 uses 3.9k ? for R2 and 68nF for C6.
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 transformer turns ratio.
The ratio must be high enough to support the CCFL
operating voltage at the minimum supply voltage. The
transformer turns ratio N can be calculated as follows:
110 and its secondary leakage inductance is approxi-
mately 300mH. To set the minimum operating frequen-
cy to 30kHz, the total series capacitance needs to be
less than 4.5μF. Therefore, two 2.2μF capacitors (C2
and C3) are used in Figure 1.
Parallel capacitor C P sets the maximum operating fre-
quency, which is also the parallel resonant peak fre-
quency. Choose:
C P ≥
4 π 2 × f 2 MAX × L × C S ? N 2
N ≥
V LAMP ( RMS )
0 . 9 × V IN ( MIN )
In the circuit of Figure 1, to set the maximum operating
frequency to 100kHz, C P needs to be larger than 8.6pF.
A 10pF high-voltage capacitor (C4) is used in Figure 1.
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相关代理商/技术参数 |
参数描述 |
|---|---|
| MAX8759EVKIT | 功能描述:显示开发工具 MAX8759 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 |
| MAX875ACPA | 制造商:Rochester Electronics LLC 功能描述: |
| MAX875ACPA+ | 功能描述:基准电压& 基准电流 2.5/5/10V Precision Voltage Reference RoHS:否 制造商:STMicroelectronics 产品:Voltage References 拓扑结构:Shunt References 参考类型:Programmable 输出电压:1.24 V to 18 V 初始准确度:0.25 % 平均温度系数（典型值）:100 PPM / C 串联 VREF - 输入电压（最大值）: 串联 VREF - 输入电压（最小值）: 分流电流（最大值）:60 mA 最大工作温度:+ 125 C 封装 / 箱体:SOT-23-3L 封装:Reel |
| MAX875ACSA | 制造商:Maxim Integrated Products 功能描述:- Bulk |
| MAX875ACSA+ | 功能描述:基准电压& 基准电流 2.5/5/10V Precision Voltage Reference RoHS:否 制造商:STMicroelectronics 产品:Voltage References 拓扑结构:Shunt References 参考类型:Programmable 输出电压:1.24 V to 18 V 初始准确度:0.25 % 平均温度系数（典型值）:100 PPM / C 串联 VREF - 输入电压（最大值）: 串联 VREF - 输入电压（最小值）: 分流电流（最大值）:60 mA 最大工作温度:+ 125 C 封装 / 箱体:SOT-23-3L 封装:Reel |
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