A4401 Allegro MicroSystems, Inc., A4401 Datasheet - Page 7

• Manufacturer Part Number: A4401
• Description: A4401 Automotive Quasi-resonant Flyback Control Ic
• Manufacturer: Allegro MicroSystems, Inc.

A4401
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 R
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
its value at minimum battery voltage, as it tends to
increase with low V
age, V
V
usually quote the variation of R
tude. Another factor that influences the “real” R
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:
Static Losses
MOSFET is identical to the current that flows in the
sense resistor previously calculated. Therefore:
Switching Turn-Off Losses
threshold, V
teau” effect takes place. Figure 2 illustrates how an
approximation can be made in terms of the turn-off
losses. The duration of the t
BAT
minus 500 mV (worst case). MOSFET suppliers
BAT
P
, of 10 V, the actual drive amplitude, V
STATIC
TH
, is similar to where the Miller “pla-
The rms current, I
=
GS
I
PK
DS
values. Below a battery volt-
×
, it is important to consider
⎛
⎜ ⎜
⎝
D(max)
DS
loss
3
Assume that the turn-off
and the smaller the
Automotive Quasi-Resonant Flyback Control IC
region is determined by
DS
⎞
⎟ ⎟
⎠
RMS
½
with V
²
× R
, that flows in the
DS
GS
.
ampli-
DS
GS
, is
is
(7)
how long the driver takes to remove the Miller (gate
to drain) charge. The Miller charge, Q
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:
Then, the time, t
can be found:
The turn-off switching loss can now be estimated:
where f
teristics table and the peak current, I
the current that flows in the sense resistor, the calcula-
tion of which is shown in the Current Sense Resistor
Selection section. V
Figure 2. MOSFET, Q1, loss approximations
P
+I, +V
TURNOFF
SW
+V
0
(min) is specified in the Electrical Charac-
V
DS
=
I
t
LOSS
DRIVE
MOSFET turned-off commanded
LOSS
I
PK
DS
×
=
, taken to shift the Miller charge
=
V
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
I
is calculated in the next section.
6.5 V × 1 nF
2
DRIVE
DS
Q
Loss Region
GD
t
40 ns
LOSS
× t
LOSS
.
× f
= 163 mA
SW
PK
GD
(min)
, is identical to
, is quoted in
,
Time
(10)
(8)
(9)
7