A4401 Datasheet, PDF (5/17 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
Functional Description
Basic Operation
A peak current-mode control scheme is used to regu-
late one of the converter outputs, which will typically
be the highest output voltage. The regulated output
voltage is potentially divided down and fed into a
Gm stage, where the resulting error signal acts as the
control reference. This reference signal is compared
against the signal that is produced by the inductor
magnetization current flowing through the sense resis-
tor.
As shown in figure 1, at the beginning of a switching
cycle, the external MOSFET, Q1, is turned on. After
the sense resistor signal reaches the control reference
amplitude, the PWM comparator resets the synchronous
rectification (SR) latch and turns off the MOSFET.
When the MOSFET is turned off, the voltage on the
LX node rises until the voltage clamps at the bat-
tery voltage, V_BAT , plus the reflected output voltage,
V_OUT(RFL). The secondary rectification diodes are
forward biased and the energy stored in the coupled
inductor is released to the output circuits. During this
period, the current through the inductor decreases lin-
+V
Coupled inductor goes discontinuous;
resonant ring occurs
V_OUT(RFL)
early. As the current falls to 0 A, a resonance is set up
between the primary magnetizing inductance and any
capacitance appearing between the drain and ground.
A damped voltage ringing occurs, which resonates
around the battery voltage, V_BAT. As the resonant ring
swings negative, the adaptive turn-on circuit moni-
tors to detect the point at which the voltage reaches a
minimum. At this point the MOSFET is commanded
on, thereby minimizing the turn-on losses. Also, the
relatively slow resonant dV/dt helps to reduce EMI.
In most applications, the converter will be operated
with a battery input voltage of 13.5 V. To optimize
the performance of the regulator at this voltage, the
magnetics can be designed to force 0 V across the
MOSFET at turn-on. This minimizes switching losses
and perhaps more importantly reduces EMI caused by
voltage ringing due to the drain to ground capacitor
resonating with the primary inductance. The voltage
resonance at the MOSFET turn-off can be reduced by
a simple low-loss R-C snubber, as described in the
Electromagnetic Interference section.
If a small enough load is applied to the outputs, and
the output of the Gm stage falls below a certain level,
the converter will enter a burst mode of operation.
Burst mode reduces switching losses while maintain-
ing regulation of the outputs.
V_BAT
0
MOSFET
turns off
+I
V_OUT(RFL)
MOSFET
turns on
Current released from
coupled inductor into
output circuit
½ resonant
period
Figure 1. External MOSFET voltage and current
Current builds up in
primary winding of
coupled inductor
During startup, assuming the battery voltage is above
the turn-on threshold and the EN input is enabled, the
controller turns on. A soft start circuit controls the ref-
erence voltage, limiting the amount of current drawn
on the input and the amount of charge transferred to
the output, preventing voltage overshoot. During the
initial phase of the soft start, very little or no voltage is
present on the output. This means that there will be no
resonant phase and the converter will operate in con-
tinuous-conduction mode. The converter effectively
operates in constant-current mode until regulation is
achieved.
Allegro MicroSystems, Inc.
5
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com