A4401 Allegro MicroSystems, Inc., A4401 Datasheet - Page 6

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

A4401
In the event of an overload, the current demand signal
produced by the Gm amplifier restricts the output cur-
rent by introducing pulse-by-pulse current limiting.
Regulation Voltage
The feedback resistors, R5 and R6, determine the
voltage of the output rail to which they are connected,
according to the following formula:
where R6 should be approximately 5 kΩ.
The internal 1.2 V reference has a ±2% worst case
tolerance, plus there is an input bias current, I
the feedback node, VA, that has a small influence. This
current flows into the ground referenced resistor, R6,
creating a small voltage offset.
In applications where the main control output (anode
or grid) can run at relatively light loads (relative to the
filament load), it may be necessary to “mix” the feed-
back signal. This involves adding an additional feed-
back from the filament output to the VA input. Note
that this only applies to DC filament outputs.
R3 Current Sense Resistor Selection
To determine the resistance value, the maximum peak
current needs to be determined. First determine the
average input current, I
where P
rent through the sense resistor:
where D is limited to 0.7, or can be precisely found as
described in the Magnetics Design section. Note that
a D of 0.7 is chosen in order to achieve 0 V switching
with a V
OUT
BAT
is the output power. Then, the peak cur-
of 13.5 V.
I
V
I
AV
PK
OUT
=
=
=
D(max)
2 × I
V
% × V
P
AV
REF
OUT
AV
, as follows:
×(R5 + R6)
(max)
R6
IN
(max)
,
Automotive Quasi-Resonant Flyback Control IC
,
,
BIAS
, on
(1)
(2)
(3)
To determine the sense resistor value, assume that the
minimum sense voltage before current limiting occurs
is 600 mV. A reasonable maximum voltage to select
during normal operation would be 500 mV. Then, the
resistor value can be found as follows:
The power losses in the resistor can be found by first
determining the rms current through it:
Then, the losses in sense resistor are:
The power rating of the resistor can be selected based
on the power dissipation. When selecting a resistor
it is worth noting that the maximum power rating
is valid up to 70°C and derates linearly to 0 W at a
temperature of typically between 120°C and 140°C.
Check the resistor manufacturer guidelines.
Note that is imperative that this resistor be a low
inductance type; avoid wire wound. Standard surface
mount devices are usually acceptable.
Soft Start
voltage turn-on threshold is reached, and the EN input
is enabled, the controller is initiated and the MOSFET,
Q1, is turned on for the first switching cycle. Initially,
while the output volts are rising towards the target reg-
ulation point set by the soft start circuit, the MOSFET
will run at current limit.
During a soft start cycle, the reference voltage is
ramped from 0 to 1.2 V in 32 steps over a period of 10
ms under the control of a DAC. This forces the output
of the amplifier to vary between 0.8 and 1.5 V, which
in turn reduces the effects of inrush current and volt-
age overshoot on the outputs.
When power is initially applied, assuming the input
R
I
P
RMS
SENSE
RDS
= I
= I ²
=
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
RMS
500 mV
PK
I
×
PK
× R
⎛
⎜ ⎜
⎝
D(max)
SENSE
3
.
.
⎞
⎟ ⎟
⎠
½
.
(4)
(5)
(6)
6