W ide Input Voltage, 2.4 MHz , 2.0 A
Asynchronous Buck Regulator
A8582
17
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
100 ns), and VIN(MAX) is the maximum required operational input
voltage to the A8582 (not the peak surge voltage).
<
f
SW
V
OUT
tON(MIN) × VIN(MAX)
(3)
If the A8582 synchronization function is employed, the base
switching frequency should be chosen such that jitter will not
result at the maximum synchronized switching frequency accord-
ing to equation 3, that is, 1.5 × fSW < fSW calculated by equa-
tion 2.
Output Inductor (LO)
The value of the output inductor (LO) is usually calculated to set
a particular peak-to-peak ripple current in the inductor. However,
the inductor physical size and cost will be directly proportional
to the peak current or saturation specification. There are tradeoffs
among: peak-to-peak ripple current, system efficiency, transient
response, and cost. If the peak-to-peak inductor ripple is chosen
to be relatively high, then the inductor value will be low, the sys-
tem efficiency will be reduced, the transient response will be fast,
the inductor physical size will be small, and the cost reduced. If
the peak-to-peak inductor ripple is chosen to be relatively low,
then the inductor value will be high, the system efficiency will be
higher, the transient response will be slow, the inductor physical
size will be larger, and the cost will be increased.
Equation 4 can be used to estimate the inductor value, given a
particular peak-to-peak ripple current (IL ), input voltage (VIN
),
output voltage (VOUT), and switching frequency (fSW). The refer-
ence designs in this datasheet use a peak-to-peak ripple current of
25% of the 2.0 A, DC rating of the A8582, or 0.5 APP
.
L
O–
1
V
OUT
fSW × IL
V
OUT
V
IN
(4)
If the preceding equation yields an inductor value that is not a
standard value, the next higher available value should be used.
After choosing a standard inductor value, equation 5 should be
used to make sure the A8582 slope compensation is adequate.
In this equation VIN(MIN) is the minimum required input voltage,
VOUT is the output voltage, fSW is the switching frequency, and
Vf is the forward voltage of the asynchronous Schottky diode.
0.18 × (V
IN(MIN)
+ V
f )
L
O–
11.3 × V
OUT
+ V
f
V
OUT
+ V
f
fSW
(5)
Ideally, the rated saturation current of the inductor should be
higher than the maximum current capability of the A8582 at
the expected duty cycle. Unfortunately this usually results in
a physically larger, more costly inductor. At a minimum, the
saturation current of the inductor should support the DC rating
of the A8582 (2.0 A), plus ½ of the inductor peak-to-peak ripple
current (usually 0.5 APP
), the capacitive startup current (ICO
),
and some margin for component, frequency, and voltage toler-
ances. For example, an inductor with a 2.7 A rating allows 2.0 A
of load current, 0.25 APEAK of ripple current, 0.25 A of capaci-
tive startup current (ICO
), along with a 20% frequency decrease,
a 20% inductance decrease, and a 10% input voltage increase (at
5.0 VOUT
, 12 VIN
, 2 MHz ).
After an inductor is chosen, it should be tested during output
short circuit conditions. The inductor current should be monitored
using a current probe. A good design should ensure the inductor
or the regulator are not damaged when the output is shorted to
GND at maximum input voltage and the highest expected ambi-
ent temperature
Output Capacitors (COUT)
The output capacitors filter the output voltage to provide an
acceptable level of ripple voltage and they store energy to help
maintain voltage regulation during a load transient. The voltage
rating of the output capacitors must support the output voltage
with sufficient design margin.
The output voltage ripple (VOUT ) is a function of the output
capacitor parameters: ESRCO
, ESLCO
, and CO
, as follows:
VOUT = VESR + VESL + VCO (6)
It is commonly known that, for a constant load on the regula-
tor, the current in the output inductor is equal to the DC output
current plus IL . Therefore, using Kirchoff’s Current law, it can
be shown that the current in the output capacitors is equal to the
ripple current in the output inductor, or IC = IL . Knowing this,
we can determine the first term in equation 6:
VESR = IL × ESRCO (7)