A4401KL-T Allegro Microsystems Inc, A4401KL-T Datasheet - Page 10

• Manufacturer Part Number: A4401KL-T
• Description: IC QUASI FLYBACK CONVERTER 8SOIC
• Manufacturer: Allegro Microsystems Inc

Specifications of A4401KL-T

• Display Type: Vacuum Fluorescent (VF)
• Current - Supply: 2.3mA
• Voltage - Supply: 7 V ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Interface: -
• Configuration: -
• Digits Or Characters: -

A4401
• Magnetics sizing can be determined using techniques
To simplify the design process, the resonant switch-
ing transition is ignored and the calculations are based
on the switcher operating at the boundary condition
of continuous/discontinuous conduction. This is a
reasonable assumption as the resonant period forms a
small percentage of the overall period at minimum line
voltage and maximum load (where the magnetics are
designed because of worst case conditions).
The objective of the design is to achieve 0 V switch-
ing when V
adjusted for any value). At V
13.5 V, 0 V switching is still achieved. In addition, the
LX voltage is prevented from swinging negative as
the MOSFET, Q1, is commanded on as soon as 0 V is
reached.
To ensure that the reflected output voltage forces the
LX node to 0 V, the following condition must be met:
where n is the step-up turns ratio from primary to con-
trolled output.
Worst case conditions in terms of core saturation
occur when V
maximum:
The primary magnetizing inductance can be deter-
mined from the following formula, which is derived
by equating the input energy to the output energy
times the efficiency:
L
such as area product or geometric product, or by fol-
lowing manufacturers guidelines, usually in the form
of nomograms.
PRI
=
2 × f
D(max) =
SW
BAT
(min) × P
BAT
= 13.5 V (note that this voltage can be
is at a minimum and duty cycle, D, a
(V
BAT
OUT
n =
(min) × n ) + V
× ( V
V
V
13.5
OUT
OUT
BAT
BAT
Automotive Quasi-Resonant Flyback Control IC
(min) × D(max))
voltages of less than
,
OUT
.
²
.
(19)
(20)
(21)
Then, maximum peak current can be found:
The worst case operating flux density, B
found from the ferrite core manufacturers datasheet,
by taking the saturation flux density, B
temperatures and subtracting a margin, approximately
15% , to allow for operation in current limit mode or
during startup.
After an appropriate magnetic core set has been
selected, the number of turns required on the primary
winding can be found:
where A
The number of secondary turns can be derived through
the turns ratio found previously:
If there is more than one output, the additional second-
ary windings are simply scaled from the main second-
ary as shown in the following formula:
where:
The total air gap, l
mate air gap, l
is taken into account. Given:
N
ings,
V
V
windings.
S2
OUT1
OUT2
is the quantity of turns in the additional wind-
e
I
is the output voltage from main winding, and
is the output voltage from any additional
PEAK
is the magnetics cross-sectional area in m
(max) =
g
N
(approx), is found before flux fringing
P
=
N
g
, can be found. First, an approxi-
S2
N
V
f
SW
V
S1
BAT
μ
=
BAT
O
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
(min) × B
f
=
N
SW
(min) × D(max)
= 4 × 10
S1
(min) × D(max)
n × N
(min) × L
×
V
V
OUT2
OUT1
P
OP
-6
.
× A
PRI
,
e
,
SAT
.
OP
.
, at elevated
, can be
(22)
(23)
(24)
(25)
(26)
2
.
10