SNVS106E – MAY 1999 – REVISED APRIL 2013
PROCEDURE (Adjustable Output Voltage Versions)
Given:
V
OUT
= Regulated Output Voltage
V
IN
(Max) = Maximum Input Voltage
I
LOAD
(Max) = Maximum Load Current
F = Switching Frequency
(Fixed at 52 kHz)
Given:
EXAMPLE (Adjustable Output Voltage Versions)
V
OUT
= 10V
V
IN
(Max) = 25V
I
LOAD
(Max) = 1A
F = 52 kHz
1. Programming Output Voltage
(Selecting R1 and R2, as shown
1. Programming Output Voltage
(Selecting R1 and R2)
in
and
Use the following formula to select the appropriate resistor values.
(1)
R
1
can be between 1k and 5k.
(For best temperature coefficient and
R2 = 1k (8.13
−
1) = 7.13k, closest 1% value is 7.15k
stability with time, use 1% metal film resistors)
(3)
(2)
2. Inductor Selection (L1)
A.
Calculate the inductor Volt • microsecond constant,
E • T (V •
μs),
from the following formula:
2. Inductor Selection (L1)
A.
Calculate E • T (V •
μs)
(5)
(4)
B.
E • T = 115 V •
μs
B.
Use the E • T value from the previous formula and match it with
the E • T number on the vertical axis of the
Inductor Value
Selection Guide
shown in
C.
On the horizontal axis, select the maximum load current.
D.
Identify the inductance region intersected by the E • T value and
the maximum load current value, and note the inductor code for that
region.
E.
Identify the inductor value from the inductor code, and select an
appropriate inductor from the table shown in
Part numbers
are listed for three inductor manufacturers. The inductor chosen
must be rated for operation at the LM2575 switching frequency (52
kHz) and for a current rating of 1.15 × I
LOAD
. For additional inductor
information, see
3. Output Capacitor Selection (C
OUT
)
3. Output Capacitor Selection (C
OUT
)
A.
The value of the output capacitor together with the inductor
A.
defines the dominate pole-pair of the switching regulator loop. For
stable operation, the capacitor must satisfy the following
requirement:
However, for acceptable output ripple voltage select
C.
I
LOAD
(Max) = 1A
D.
Inductance Region = H470
E.
Inductor Value = 470
μH
Choose from
AIE
part #430-0634,
Pulse
Engineering
part #PE-53118, or
Renco
part #RL-1961.
(7)
(6)
The above formula yields capacitor values between 10
μF
and 2000
μF
that will satisfy the loop requirements for stable operation. But to
achieve an acceptable output ripple voltage, (approximately 1% of
the output voltage) and transient response, the output capacitor may
need to be several times larger than the above formula yields.
B.
The capacitor's voltage rating should be at last 1.5 times greater
than the output voltage. For a 10V regulator, a rating of at least 15V
or more is recommended.
Higher voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select a
capacitor rate for a higher voltage than would normally be needed.
(Continued)
C
OUT
≥
220
μF
C
OUT
= 220
μF
electrolytic capacitor
(Continued)
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15
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