PROCEDURE (Adjustable Output Voltage Versions)
Given:
EXAMPLE (Adjustable Output Voltage Versions)
Given:
VOUT = Regulated Output Voltage
VOUT = 10V
VIN(Max) = Maximum Input Voltage
ILOAD(Max) = Maximum Load Current
F = Switching Frequency (Fixed at 52 kHz)
VIN(Max) = 25V
ILOAD(Max) = 1A
F = 52 kHz
1. Programming Output Voltage (Selecting R1 and R2, as shown 1.Programming Output Voltage (Selecting R1 and R2)
in Figure 2 )
Use the following formula to select the appropriate resistor values.
R1 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)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant,
E • T (V • μs), from the following formula:
A. Calculate E • T (V • μs)
B. E • T = 115 V • μs
C. ILOAD(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.
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 Figure 7.
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 Figure 9. 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 × ILOAD. For additional inductor
information, see the inductor section in the application hints section
of this data sheet.
3. Output Capacitor Selection (COUT
)
3. Output Capacitor Selection (COUT
)
A. The value of the output capacitor together with the inductor de- A.
fines the dominate pole-pair of the switching regulator loop. For
stable operation, the capacitor must satisfy the following require-
ment:
However, for acceptable output ripple voltage select
COUT ≥ 220 μF
COUT = 220 μF electrolytic capacitor
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)
15
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