A4401L View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
'A4401L' PDF : 17 Pages View PDF
A4401
Automotive Quasi-Resonant Flyback Control IC
A simple low-loss R-C snubber can be deployed to
dampen these oscillations. To do so, follow these sim-
ple steps to determine the component values required:
1. Measure the voltage resonant frequency, fRES , on
the LX node.
2. Add an additional capacitance (C11) between LX
and ground until the resonant frequency is halved,
fRES/2:
fRES = ×
1
,
× (LLEAK × CLEAK)½
(35)
fRES
=
×
1
× (LLEAK × CNEW)½
,
(36)
CNEW = CLEAK + CADD ,
(37)
where:
Figure 6 illustrates a converter running at full power
with VBAT = 13.5 V. The upper trace is the voltage
across LX and the lower trace is the voltage across the
sense resistor, R3. It can be seen that at turn-on, the
noise is very low, as the resonant action introduces a
very slow voltage slew which reaches 0 V before the
MOSFET is turned on. This action ensures minimal
noise is produced. However, at turn-off there is a high
frequency ringing of 25 MHz which could cause issues
in terms of complying with radiated emissions.
Figure 7 illustrates the effects of adding an R-C snub-
ber (R = 30 Ω and C = 330 pF in series between the
LX pin and GND). It can be seen that, through the
addition of the snubber, the turn-off ringing has been
considerably damped. This was achieved with almost
negligible additional power loss.
CADD is the additional capacitance added,
CLEAK is the parasitic capacitance, and
LLEAK is the parasitic inductance.
Note that the value of the additional capacitance
should be in the region of a few hundredths of pico-
farads.
3. Now to calculate component values that will result
in fRES/2, first calculate CLEAK, given:
CLEAK + CADD = 4 × CLEAK ,
(38)
CLEAK
=
CADD
3
.
(39)
4. With CLEAK solved, solve for LLEAK
5. Finally, solve for the characteristic impedance of the
parasitic components:
RO =
⎛
⎜⎜
⎝
LLEAK
CLEAK
⎞½
⎟⎟
⎠
.
(40)
RO can be selected as the damping resistor value (not
shown in functional block diagram). Typically a 1/8 W
resistor is adequate.
Conducted Emissions To help reduce the conducted
emissions, an EMI filter may be necessary as shown in
the Functional Block diagram.
MOSFET
Turn-On
MOSFET
Turn-Off
VLX
VRSENSE
Figure 7. Voltages with 0 V switching of Q1 with an R-C snubber
Allegro MicroSystems, Inc.
15
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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