Datasheet

LM2574, LM2574HV
www.ti.com SNVS104C –JUNE 1999–REVISED APRIL 2013
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
Given: Given:
V
OUT
= Regulated Output Voltage V
OUT
= 24V
V
IN
(Max) = Maximum Input Voltage V
IN
(Max) = 40V
I
LOAD
(Max) = Maximum Load Current I
LOAD
(Max) = 0.4A
F = Switching Frequency (Fixed at 52 kHz) F = 52 kHz
1. Programming Output Voltage (Selecting R1 and R2, as shown 1. Programming Output Voltage (Selecting R1 and R2)
in Figure 24)
Use the following formula to select the appropriate resistor values.
R
1
can be between 1k and 5k. (For best temperature coefficient and
R
2
= 1k (19.51−1) = 18.51k, closest 1% value is 18.7k
stability with time, use 1% metal film resistors)
2. Inductor Selection (L1) 2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant, A. Calculate E • T (V • μs)
E • T (V • μs), from the following formula:
B. E • T = 185 V • μs
C. I
LOAD
(Max) = 0.4A
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
D. Inductance Region = 1000
Selection Guide shown in Figure 29.
E. Inductor Value = 1000 μH Choose from Pulse Engineering Part
C. On the horizontal axis, select the maximum load current.
#PE-52631, or Renco Part #RL-1283-1000.
D. Identify the inductance region intersected by the E • T value and
the maximum load current value, and note the inductor value for that
region.
E. Select an appropriate inductor from the table shown in Table 1.
Part numbers are listed for three inductor manufacturers. The
inductor chosen must be rated for operation at the LM2574 switching
frequency (52 kHz) and for a current rating of 1.5 × I
LOAD
. For
additional inductor information, see INDUCTOR SELECTION in
Application Hints of this data sheet.
3. Output Capacitor Selection (C
OUT
) 3. Output Capacitor Selection (C
OUT
)
A. The value of the output capacitor together with the inductor
defines the dominate pole-pair of the switching regulator loop. For
stable operation, the capacitor must satisfy the following
However, for acceptable output ripple voltage select
requirement:
C
OUT
≥ 100 μF
C
OUT
= 100 μF electrolytic capacitor
The above formula yields capacitor values between 5 μF and 1000
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 24V regulator, a rating of at least 35V
is recommended.
Higher voltage electrolytic capacitors generally have lower ESR
numbers, and for this reasion it may be necessary to select a
capacitor rate for a higher voltage than would normally be needed.
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