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參數(shù)資料
| 型號(hào)： | MAX8729EEI |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類： | 模擬信號(hào)調(diào)理 |
| 英文描述： | Constant-Frequency, Half-Bridge CCFL Inverter Controller |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDSO28 |
| 封裝： | 0.150 INCH, 0.025 INCH PITCH, MO-137AF, QSOP-28 |
| 文件頁數(shù)： | 22/26頁 |
| 文件大小： | 395K |
| 代理商： | MAX8729EEI |
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M
Constant-Frequency, Half-Bridge CCFL
Inverter Controller
22
______________________________________________________________________________________
Setting the Lamp Current
The MAX8729 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 R1 is fed to IFB and is internally full-wave recti-
fied. The MAX8729 controls the desired lamp current
by regulating the average of the rectified IFB voltage.
To set the RMS lamp current, determine R1 as follows:
where I
LAMP(RMS)
is the desired RMS lamp current and
790mV 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 depends on lamp parasitics. The resulting
waveform is an imperfect sinusoid waveform, which has
an RMS value that is not easy to predict. A high-fre-
quency true RMS current meter (such as Yokogawa
2016) should be used to measure the RMS current and
make final adjustments to R1. Insert this meter between
the sense resistor and the lamp’s low-voltage terminal
to measure the actual RMS current.
Setting the Secondary Voltage Limit
The MAX8729 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 of VFB is
proportional to the CCFL voltage. The selection of par-
allel resonant capacitor C3 is described in the
Transformer Design and Resonant-Component
Selection
section. Smaller values for C3 result in higher
efficiency due to lower circulating current. If C3 is too
small, the resonant operation is affected by the panel
parasitic capacitance. Therefore, C3 is usually chosen
to be between 10pF and 18pF. After the value of C3 is
set, select C4 based on the desired maximum RMS
secondary voltage V
LAMP(RMS)_MAX
:
where the 2.34V is the typical value of the VFB peak-
voltage when the lamp is open. To set the maximum
RMS secondary voltage to 1800V with C3 selected to
be 12pF, C4 must be less than or equal to 13nF.
Setting the Secondary Current Limit
The MAX8729 limits the secondary current even if the
IFB sense resistor is shorted or transformer secondary
current finds its way to ground without passing through
R1. ISEC monitors the voltage across the sense resistor
R2 connected between the low-voltage terminal of the
transformer secondary winding and ground. Determine
the value of R2 using the following equation:
where I
SEC(RMS)_MAX
is the desired maximum RMS
transformer secondary current during fault conditions,
and 1.28V is the typical value of the ISEC peak voltage
when the secondary is shorted. To set the maximum
RMS secondary current in the circuit of Figure 1 to
22mA, set R3 = 40.2
Ω
.
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:
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
operation is 800V and the minimum DC input voltage is
10V, the turns ratio should be greater than 178.
The next step in the procedure is to design the resonant
tank so the resonant frequency is close to the switching
frequency set by the HF resistor. The lamp current is clos-
er to sine wave when the switching frequency is close to
the resonant frequency The resonant frequency is deter-
mined by: the primary capacitive voltage-divider C
S1
and
C
S2,
the secondary parallel capacitor C
P,
the transformer
secondary leakage inductance L, and the CCFL lamp.
The simplified CCFL inverter circuit is shown in Figure 7a.
The half-bridge power stage is simplified and represent-
ed as a square-wave AC source. The resonant tank cir-
cuit can be further simplified to Figure 7b by removing the
transformer. C
S
’ is the capacitance of the primary capaci-
tive divider reflected to the secondary and N is the trans-
former turns ratio.
N
V
x V
LAMP RMS
.45
IN MIN
(
≥
(
)
)
R
V
xI
SEC RMS
MAX
2
1 28
.
2
=
(
)_
C
x V
V
x C
LAMP RMS
2 34
.
MAX
4
2
3
=
(
)_
R
x
mV
xI
LAMP RMS
1
790
2 2
=
π
(
)
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| MAX8729EEI+ | 功能描述:顯示驅(qū)動(dòng)器和控制器 Const-f Half-Bridge CCFL Inverter Ctlr RoHS:否 制造商:Panasonic Electronic Components 工作電源電壓:2.7 V to 5.5 V 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-44 封裝:Reel |
| MAX8729EEI+T | 功能描述:顯示驅(qū)動(dòng)器和控制器 Const-f Half-Bridge CCFL Inverter Ctlr RoHS:否 制造商:Panasonic Electronic Components 工作電源電壓:2.7 V to 5.5 V 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-44 封裝:Reel |
| MAX8729EEI-T | 功能描述:顯示驅(qū)動(dòng)器和控制器 RoHS:否 制造商:Panasonic Electronic Components 工作電源電壓:2.7 V to 5.5 V 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-44 封裝:Reel |
| MAX8729EVKIT | 功能描述:電源管理IC開發(fā)工具 MAX8729 Eval Kit RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| MAX872C/D | 功能描述:基準(zhǔn)電壓& 基準(zhǔn)電流 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:Voltage References 拓?fù)浣Y(jié)構(gòu):Shunt References 參考類型:Programmable 輸出電壓:1.24 V to 18 V 初始準(zhǔn)確度:0.25 % 平均溫度系數(shù)（典型值）:100 PPM / C 串聯(lián) VREF - 輸入電壓（最大值）: 串聯(lián) VREF - 輸入電壓（最小值）: 分流電流（最大值）:60 mA 最大工作溫度:+ 125 C 封裝 / 箱體:SOT-23-3L 封裝:Reel |
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