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| 型号： | A4401KLTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 7/17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
I DRIVE = = 163 mA (8)
Q GD
t LOSS =
(9)
V DS
P TURNOFF = I PK ×
× t LOSS × f SW (min)
If there is a special requirement for larger output
capacitors, and the onboard soft start is insufficient,
an external soft start can be introduced. This can be
implemented by “pulling” down the amplifier output
(COMP pin) and then “releasing” it gradually over the
duration of the new soft start period. The pull-down
circuit has to be capable of sinking at least 30 μ A.
Q1 MOSFET Selection
In general, the higher the R DS and the smaller the
package, the lower the cost of the MOSFET, Q1. On
the basis of selecting a MOSFET to minimize cost, it
is important to understand the power losses associated
with it.
When selecting the R DS , it is important to consider
its value at minimum battery voltage, as it tends to
increase with low V GS values. Below a battery volt-
age, V BAT , of 10 V, the actual drive amplitude, V GS , is
V BAT minus 500 mV (worst case). MOSFET suppliers
usually quote the variation of R DS with V GS ampli-
tude. Another factor that influences the “real” R DS is
the operating temperature. At a temperature of 140°C,
this figure is increased by approximately 1.8. Again,
how long the driver takes to remove the Miller (gate
to drain) charge. The Miller charge, Q GD , is quoted in
the MOSFET datasheet.
The driver capability can be found from data in the
Electrical Characteristics table, as follows. At turn-off,
the driver can shift a charge of (7 V – 0.5 V) × 1 nF, in
40 ns. The drive current required to do this is:
6.5 V × 1 nF
40 ns
Then, the time, t LOSS , taken to shift the Miller charge
can be found:
.
I DRIVE
The turn-off switching loss can now be estimated:
, (10)
2
where f SW (min) is specified in the Electrical Charac-
teristics table and the peak current, I PK , is identical to
the current that flows in the sense resistor, the calcula-
tion of which is shown in the Current Sense Resistor
Selection section. V DS is calculated in the next section.
manufacturers will provide this information.
Worst case losses will occur at minimum battery volt-
age and maximum load. These can be considered in
terms of static losses, switching turn-off losses, and
switching turn-on losses:
Static Losses The rms current, I RMS , that flows in the
MOSFET is identical to the current that flows in the
sense resistor previously calculated. Therefore:
+V
0
+I, +V
MOSFET turned-off commanded
P STATIC =
? D (max) ? ?
I PK × ? 3 ?
? ?
2
× R DS
.
(7)
Switching Turn-Off Losses Assume that the turn-off
threshold, V TH , is similar to where the Miller “pla-
teau” effect takes place. Figure 2 illustrates how an
V DS
Loss Region
t LOSS
approximation can be made in terms of the turn-off
losses. The duration of the t loss region is determined by
Figure 2. MOSFET, Q1, loss approximations
Time
Allegro MicroSystems, LLC
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
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