A4401 데이터 시트보기 (PDF) - Allegro MicroSystems
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A4401 Datasheet PDF : 17 Pages
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A4401
Automotive Quasi-Resonant Flyback Control IC
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 μA.
Q1 MOSFET Selection
In general, the higher the RDS 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 RDS, it is important to consider
its value at minimum battery voltage, as it tends to
increase with low VGS values. Below a battery volt-
age, VBAT, of 10 V, the actual drive amplitude, VGS, is
VBAT minus 500 mV (worst case). MOSFET suppliers
usually quote the variation of RDS with VGS ampli-
tude. Another factor that influences the “real” RDS is
the operating temperature. At a temperature of 140°C,
this figure is increased by approximately 1.8. Again,
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:
how long the driver takes to remove the Miller (gate
to drain) charge. The Miller charge, QGD , 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
IDRIVE = 40 ns
= 163 mA
(8)
Then, the time, tLOSS, taken to shift the Miller charge
can be found:
tLOSS =
QGD
IDRIVE
.
(9)
The turn-off switching loss can now be estimated:
PTURNOFF =
IPK ×
VDS
2
× tLOSS × fSW(min)
,
(10)
where fSW(min) is specified in the Electrical Charac-
teristics table and the peak current, IPK , 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. VDS is calculated in the next section.
+V
MOSFET turned-off commanded
Static Losses The rms current, IRMS, that flows in the
0
MOSFET is identical to the current that flows in the
sense resistor previously calculated. Therefore:
+I, +V
PSTATIC =
¨
©
©
IPK
ª
×
⎛
⎜⎜
⎝
D(max)
3
⎟⎟⎠⎞½·¹¸¸
²
×
RDS
.
(7)
Switching Turn-Off Losses Assume that the turn-off
threshold, VTH, is similar to where the Miller “pla-
teau” effect takes place. Figure 2 illustrates how an
VDS
Loss Region
tLOSS
approximation can be made in terms of the turn-off
Time
losses. The duration of the tloss region is determined by Figure 2. MOSFET, Q1, loss approximations
Allegro MicroSystems, Inc.
7
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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