LM2575T-15 datasheet(15/44 Pages) TI1

15 / 44 page

LM1575, LM2575-N, LM2575HV

www.ti.com

SNVS106E – MAY 1999 – REVISED APRIL 2013

PROCEDURE (Adjustable Output Voltage Versions)

EXAMPLE (Adjustable Output Voltage Versions)

Given:

VOUT = Regulated Output Voltage

VOUT = 10V

VIN(Max) = Maximum Input Voltage

VIN(Max) = 25V

ILOAD(Max) = Maximum Load Current

ILOAD(Max) = 1A

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 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

R2 = 1k (8.13

− 1) = 7.13k, closest 1% value is 7.15k

stability with time, use 1% metal film resistors)

(2)

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:

(5)

(4) B. E • T = 115 V • μs

B. Use the E • T value from the previous formula and match it with C. I

LOAD(Max) = 1A

the E • T number on the vertical axis of the Inductor Value

D. Inductance Region = H470

Selection Guide shown in Figure 31.

E. Inductor Value = 470

μH Choose from AIE part #430-0634, Pulse

C. On the horizontal axis, select the maximum load current.

Engineering part #PE-53118, or Renco part #RL-1961.

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)

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

(7)

requirement:

However, for acceptable output ripple voltage select

COUT ≥ 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)

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