参数资料
| 型号： | MAX8751ETJ+T |
| 厂商： | Maxim Integrated |
| 文件页数： | 22/27页 |
| 文件大小： | 0K |
| 描述： | IC CNTRLR CCFL INV 32-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： | 30 ~ 80 kHz |
| 电流 - 电源： | 3.2mA |
| 电源电压： | 6 V ~ 28 V |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 供应商设备封装： | 32-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
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Fixed-Frequency, Full-Bridge CCFL
Inverter Controller
Setting the Secondary Current Limit
The MAX8751 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:
transformer turns ratio.
Figure 8 shows the frequency response of the resonant
tank’s voltage gain under different load conditions. The
primary series capacitor is 1μF, the secondary parallel
capacitor is 15pF, the transformer turns ratio is 1:93,
and the secondary leakage inductance is 260mH.
Notice that there are two peaks, f S and f P , in the fre-
quency response. The first peak, f S , is the series reso-
nant peak determined by the secondary leakage
R2 =
1 . 26 V
2 x I SEC(RMS)_MAX
inductance (L) and the series capacitor reflected to the
secondary (Cs’):
where I SEC(RMS)_MAX is the desired maximum RMS
transformer secondary current during fault conditions,
f S =
1
2 π LCs '
and 1.26V 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 R2 = 40.2 ? .
Transformer Design and Resonant
The second peak, f P , is the parallel resonant peak
determined by the secondary leakage inductance (L),
the parallel capacitor (C P ), and the series capacitance
reflected to the secondary (C’ S ):
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.
f P =
2 π L
1
Cs ' C P
Cs ' + C P
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:
The actual resonant frequency is between these two
resonant peaks. When the lamp is off, the operating
point of the resonant tank is close to the parallel reso-
N ≥
V LAMP ( RMS )
0 . 90 x V IN ( MIN )
nant peak due to the lamp’s infinite impedance. The cir-
cuit displays the characteristics of a parallel-loaded
resonant converter. While in parallel-loaded resonant
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
7V, the turns ratio should be greater than 120 turns.
The next step to design the resonant tank for CCFL is to
design the resonant frequency of the tank close to the
switching frequency set by the HF resistor. The resonant
frequency is determined by: the primary winding series
capacitor Cs, the secondary parallel capacitor Cp, the
transformer secondary leakage inductance L, and the
CCFL lamp operating resistance R L .
The simplified CCFL inverter circuit is shown in Figure
7(a). The full-bridge power stage is simplified and repre-
sented as a square-wave AC source. The resonant tank
circuit can be further simplified to Figure 7(b) by remov-
ing the transformer. C S ’ is the capacitance of the primary
capacitive divider reflected to the secondary and N is the
operation, the inverter behaves like a voltage source to
generate the necessary striking voltage. Theoretically,
the output voltage of the resonant converter increases
until the lamp is ionized or until it reaches the IC’s sec-
ondary voltage limit. Once the lamp is ionized, the
equivalent-load resistance decreases rapidly and the
operating point moves toward the series-resonant
peak. While in series-resonant operation, the inverter
behaves like a current source.
The leakage inductance of the CCFL transformer is an
important parameter in the resonant tank design. 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
inductance requirements. The series capacitor Cs sets
the minimum operating frequency, which is approximate-
ly two times the series resonant peak frequency. The
series capacitor Cs can be chosen as below:
22
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