Wide Input Voltage 3.0 A Step Down Regulator
A8498
7
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
COMPONENT SELECTION
L1
The inductor must be rated to handle the total load current. The
value should be chosen to keep the ripple current to a reasonable
value. The ripple current, I
RIPPLE
, can be calculated by:
I
RIPPLE
= V
L(OFF)
× t
OFF
/ L (5)
V
L(OFF)
= V
OUT
+ V
f
+ I
L(AV)
× R
L
(6)
Example:
Given V
OUT
= 5 V, V
f
= 0.55 V, V
IN
= 42 V, I
LOAD
= 0.5 A, power
inductor with L = 180 µH and R
L
= 0.5 Ω Rdc at 55°C, t
OFF
=
7 µs, and R
DS(on)
= 0.5 Ω.
Substituting into equation 6:
V
L(OFF)
= 5 V + 0.55 V+ 0.5 A × 0.5 Ω = 5.8 V
Substituting into equation 5:
I
RIPPLE
= 5.8 V × 7 µs / 180 µH = 225 mA
The switching frequency, f
SW
, can then be estimated by:
f
SW
= 1 / ( t
ON
+ t
OFF
) (7)
t
ON
= I
RIPPLE
× L / V
L(ON)
(8)
V
L(ON)
= V
IN
– I
L(AV)
× R
DS(on)
–
I
L(AV)
× R
L
– V
OUT
(9)
Substituting into equation 9:
V
L(ON)
= 42 V – 0.5 A × 0.5 Ω – 0.5 A × 0.5 Ω – 5 V = 36.5 V
Substituting into equation 8:
t
ON
= 225 mA × 180 µH / 36.5 V = 1.11 µs
Substituting into equation 7:
f
SW
= 1 / (7 µs +1.11 µs) = 123 kHz
Higher inductor values can be chosen to lower the ripple cur-
rent. This may be an option if it is required to increase the total
maximum current available above that drawn from the switching
regulator. The maximum total current available, I
LOAD(MAX)
, is:
I
LOAD(MAX)
= I
CL
(min) – I
RIPPLE
/ 2 (10)
where I
CL
(min) is 3.5 A, from the Electrical Chracteristics table.
D1
The Schottky catch diode should be rated to handle 1.2 times the
maximum load current. The voltage rating should be higher than
the maximum input voltage expected during all operating condi-
tions. The duty cycle for high input voltages can be very close to
100%.
COUT
The main consideration in selecting an output capacitor is volt-
age ripple on the output. For electrolytic output capacitors, a
low-ESR type is recommended.
The peak-to-peak output voltage ripple is simply I
RIPPLE
× ESR.
Note that increasing the inductor value can decrease the ripple
current. The ESR should be in the range from 50 to 500 mΩ.
