LM1575/LM2575/LM2575HV
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)
1. Programming Output Voltage
(Selecting R1 and R2, as shown
in Figure 2 )
Use the following formula to select the appropriate resistor values.
EXAMPLE (Adjustable Output Voltage Versions)
Given:
V
OUT
= 10V
V
IN
(Max) = 25V
I
LOAD
(Max) = 1A
F = 52 kHz
1.Programming Output Voltage
(Selecting R1 and R2)
R
1
can be between 1k and 5k.
(For best temperature coefficient and
stability with time, use 1% metal film resistors)
R2 = 1k (8.13 − 1) = 7.13k, closest 1% value is 7.15k
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)
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 Selec-
tion 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 the inductor section in the application hints section
of this data sheet.
3. Output Capacitor Selection (C
OUT
)
A.
The value of the output capacitor together with the inductor de-
fines the dominate pole-pair of the switching regulator loop. For
stable operation, the capacitor must satisfy the following require-
ment:
B.
E • T = 115 V •
μs
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.
3. Output Capacitor Selection (C
OUT
)
A.
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 ca-
pacitor rate for a higher voltage than would normally be needed.
(Continued)
(Continued)
However, for acceptable output ripple voltage select
C
OUT
≥
220 μF
C
OUT
= 220 μF
electrolytic capacitor
15
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