A4401 Datasheet, PDF (10/17 Pages) Allegro MicroSystems – This device provides all the necessary control functions to provide the power rails for driving a vacuum fluorescent display (VFD) using minimal external components.

A4401
Automotive Quasi-Resonant Flyback Control IC
• Magnetics sizing can be determined using techniques
such as area product or geometric product, or by fol-
lowing manufacturers guidelines, usually in the form
of nomograms.
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_BAT = 13.5 V (note that this voltage can be
adjusted for any value). At V_BAT voltages of less than
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:
n ₌ ^VOUT ,
13.5
(19)
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_BAT is at a minimum and duty cycle, D, a
maximum:
D(max) ₌
V_OUT
.
(V_BAT(min) × n ) + V_OUT
(20)
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_PRI
=
2
H
× f_SW(min)
×
× P_OUT
( V_BAT(min) ×
D(max)) ^²
.
(21)
Then, maximum peak current can be found:
I_PEAK(max)
=
V_BAT(min) × D(max)
f_SW(min) × L_PRI
.
(22)
The worst case operating flux density, B_OP, can be
found from the ferrite core manufacturers datasheet,
by taking the saturation flux density, B_SAT , at elevated
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:
N_P
=
V_BAT(min) × D(max)
f_SW(min) × B_OP × A_e
.
(23)
where A_e is the magnetics cross-sectional area in m².
The number of secondary turns can be derived through
the turns ratio found previously:
N_{S1 =} n × N_P .
(24)
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:
N_S2
=
N_S1×
V_OUT2
V_OUT1
,
(25)
where:
N_S2 is the quantity of turns in the additional wind-
ings,
V_OUT1 is the output voltage from main winding, and
V_OUT2 is the output voltage from any additional
windings.
The total air gap, l_g, can be found. First, an approxi-
mate air gap, l_g(approx), is found before flux fringing
is taken into account. Given:
μ_{O = 4 × 10} -6 _,
(26)
Allegro MicroSystems, Inc.
10
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com