A4447
High Voltage Step Down Regulator
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 reason-
able value. The ripple current, I
RIPPLE
, can be calculated by:
I
RIPPLE
=
V
L(OFF)
× t
OFF
/
L
V
L(OFF)
= V
OUT
+ V
f
+
I
L(AV)
×
R
L
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)
= 1
Ω.
Substituting into equation 4:
V
L(OFF)
= 5 V + 0.55 V+ 0.5 A × 0.5
Ω
= 5.8 V
Substituting into equation 3:
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
)
t
ON
=
I
RIPPLE
×
L / V
L(ON)
V
L(ON)
= V
IN
–
I
L(AV)
× R
DS(on)
–
I
L(AV)
×
R
L
–
V
OUT
Substituting into equation 7:
V
L(ON)
= 42 V – 0.5 A × 1
Ω
– 0.5 A × 0.5
Ω
– 5 V = 36 V
Substituting into equation 6:
t
ON
= 225 mA × 180
μH
/ 36 V = 1.12
μs
(5)
(6)
(7)
(3)
(4)
Substituting into equation 7:
f
SW
= 1 / (7
μs
+1.12
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
(8)
where I
CL
(min) is 2.2 A, from the Electrical Characteristics 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
conditions. The duty cycle for high input voltages can be very
close to 100%.
COUT.
The main consideration in selecting an output capacitor
is voltage 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 minimum voltage rating of the capacitor is
10 V. However, because ESR decreases with voltage, the most
cost-effective choice may be rated higher in voltage. It is recom-
mended that the ESR be less than 100 mΩ.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
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