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| 型号： | A4401KLTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 15/17页 |
| 文件大小： | 0K |
| 描述： | IC QUASI FLYBACK CONVERTER 8SOIC |
| 标准包装： | 1 |
| 显示器类型： | 真空荧光 (VF) |
| 电流 - 电源： | 2.3mA |
| 电源电压： | 7 V ~ 40 V |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 8-SOICN |
| 包装： | 标准包装 |
| 产品目录页面： | 1141 (CN2011-ZH PDF) |
| 其它名称： | 620-1217-6 |
A4401
Automotive Quasi-Resonant Flyback Control IC
1
f RES =
(35)
f RES 1
(36)
Asimple low-loss R-C snubber can be deployed to
dampen these oscillations. To do so, follow these sim-
ple steps to determine the component values required:
1. Measure the voltage resonant frequency, f RES , on
the LX node.
2. Add an additional capacitance (C11) between LX
and ground until the resonant frequency is halved,
f RES /2:
,
× × ( L LEAK × C LEAK ) ?
= ,
× × ( L LEAK × C NEW ) ?
Figure 6 illustrates a converter running at full power
with V BAT = 13.5 V. The upper trace is the voltage
across LX and the lower trace is the voltage across the
sense resistor, R3. It can be seen that at turn-on, the
noise is very low, as the resonant action introduces a
very slow voltage slew which reaches 0 V before the
MOSFET is turned on. This action ensures minimal
noise is produced. However, at turn-off there is a high
frequency ringing of 25 MHz which could cause issues
in terms of complying with radiated emissions.
Figure 7 illustrates the effects of adding an R-C snub-
ber (R = 30 Ω and C = 330 pF in series between the
LX pin and GND). It can be seen that, through the
C NEW = C LEAK + C ADD
,
(37)
addition of the snubber, the turn-off ringing has been
considerably damped. This was achieved with almost
where:
C ADD is the additional capacitance added,
C LEAK is the parasitic capacitance, and
L LEAK is the parasitic inductance.
Note that the value of the additional capacitance
should be in the region of a few hundredths of pico-
farads.
3. Now to calculate component values that will result
in f RES /2, first calculate C LEAK , given:
negligible additional power loss.
Conducted Emissions To help reduce the conducted
emissions, an EMI filter may be necessary as shown in
the Functional Block diagram.
MOSFET
Turn-On
C LEAK + C ADD = 4 × C LEAK
,
(38)
C LEAK =
C ADD
3
.
(39)
MOSFET
Turn-Off
4. With C LEAK solved, solve for L LEAK
5. Finally, solve for the characteristic impedance of the
parasitic components:
V LX
V RSENSE
R O = ? ? LEAK
?
? L
C LEAK
? ?
? ?
?
.
(40)
R O can be selected as the damping resistor value (not
shown in functional block diagram). Typically a 1 / 8 W
resistor is adequate.
Figure 7. Voltages with 0 V switching of Q1 with an R-C snubber
Allegro MicroSystems, LLC
115 Northeast Cutoff, Box 15036
15
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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