LM1575, LM2575-N, LM2575HV
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SNVS106E –MAY 1999–REVISED APRIL 2013
PROCEDURE (Adjustable Output Voltage Versions)
EXAMPLE (Adjustable Output Voltage Versions)
Given:
Given:
VOUT = Regulated Output Voltage
VIN(Max) = Maximum Input Voltage
ILOAD(Max) = Maximum Load Current
F = Switching Frequency (Fixed at 52 kHz)
VOUT = 10V
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 25 and Figure 26)
Use the following formula to select the appropriate resistor values.
(1)
(3)
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)
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)
(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 Figure 31.
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.
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 Table 2. 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 INDUCTOR SELECTION.
3. Output Capacitor Selection (COUT
)
3. Output Capacitor Selection (COUT
)
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:
(7)
However, for acceptable output ripple voltage select
C
OUT ≥ 220 μF
(6)
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
capacitor rate for a higher voltage than would normally be needed.
(Continued)
(Continued)
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