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| 型号： | FAN7631SJX |
| 厂商： | Fairchild Semiconductor |
| 文件页数： | 12/19页 |
| 文件大小： | 0K |
| 描述： | IC CTLR PFM RESON CONV 16-SOIC |
| 标准包装： | 2,000 |
| 应用： | 谐振转换器控制器 |
| 电源电压： | 11.1 V ~ 21 V |
| 电流 - 电源： | 10mA |
| 工作温度： | -40°C ~ 130°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 16-SOP |
| 包装： | 带卷 (TR) |
initial switching frequency is not affected by R max and is
solely determined as six times the minimum switching
frequency set by R min as in Equation (1). The maximum
switching frequency is also internally limited at 600kHz.
When V SS reaches V SS_END (4.2V), soft-start ends. Then,
the high threshold of V CT comparator, V TH , is clamped at
V SS_END while V SS keeps increasing until it reaches V SSC
(5V). The soft-start time is given as:
4.2. Capacitive Sensing Method
The MOSFET drain current can be sensed using an
additional capacitor in parallel with the resonant
capacitor, as shown in Figure 34. While the low-side
switch is turned on, the current, I CB , through C B
introduces V SENSE across R SENSE . The I CB is a fraction of
the transformer primary-side current, I p , determined by
the current divider with capacitors C r and C B as:
?
i p p
C B C B
? C C
t SS ? C SS
2.6
3 ? 10 ? 5
(2)
i CB ?
C r B r
i
(3)
Generally, 1/100~1/1000 is adequate for the ratio of
C B /C r . R D is used as a damper for reducing noise
generated by the switching transition. To prevent the
damping resistor from affecting the current divider ratio,
the resistor should be much smaller than the impedance
of C B at the switching frequency, calculated as:
R D ??
1
2 ? f S C B
(4)
Then, V SENSE can be obtained as:
V Sense ?
C B
C r
R sense i p
(5)
Figure 32. Soft-Start Waveforms
4. Current Sensing
FAN7631 employs a negative voltage sensing method to
sense the drain current of the MOSFET. This allows
sensing the current without a leading edge spike caused
by the low-side MOSFET’s driving current. Therefore, the
resistive-sensing method requires only a small RC filter.
The capacitive-sensing method is also available.
4.1. Resistive Sensing Method
The FAN7631 can sense the drain current as a negative
voltage, as shown in Figure 33. An RC filter with a time
constant of 1/30~1/10 of the operating period is typical.
Figure 34. Capacitive Sensing
Figure 33. Resistive Sensing
? 2011 Fairchild Semiconductor Corporation
FAN7631 ? 1.0.2
12
www.fairchildsemi.com
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