C

7758

OUT

³

(µF)

V

IN(Max)

V

L1(µH)

OUT

·

(

(

R2 = R1

– 1

V

OUT

V

REF

V

= V

OUT

REF

1 +

where V

= 1.23 V

REF

(

(

R2

R1

LM2575

SLVS569F – JANUARY 2005 – REVISED AUGUST 2015

www.ti.com

Typical Application (continued)

PROCEDURE

EXAMPLE

1. Programming Output Voltage (Selecting R1 and R2)

1. Programming Output Voltage (Selecting R1 and R2)

VOUT is defined by:

Select R1 = 1 k

Ω

R2 = 1 (10 / 1.23 – 1) = 7.13 k

Ω

Select R2 = 7.15 k

Ω (closest 1% value)

Choose a value for R1 between 1 k

Ω and 5 kΩ (use 1% metal-film

resistors for best temperature coefficient and stability over time).

2. Inductor Selection (L1)

2. Inductor Selection (L1)

A. Calculate the "set" volts-second (E × T) across L1:

A. Calculate the "set" volts-second (E × T) across L1:

E × T = (VIN – VOUT) × ton

E × T = (25 – 10) × (10 / 25) × (1000 / 52) [V ×

μs]

E × T = (VIN – VOUT) × (VOUT / VIN) × {1000 / fosc(in kHz)} [V × μs]

E × T = 115 V ×

μs

NOTE: Along with ILOAD, the "set" volts-second (E × T) constant

establishes the minimum energy storage requirement for the

inductor.

B. Using Figure 12, select the appropriate inductor code based on B. Using Figure 12, the intersection of 115 V •

μs and 1 A

the intersection of E × T value and ILOAD(Max).

corresponds to an inductor code of H470.

C. The inductor chosen should be rated for operation at 52-kHz and C. H470

→ L1 = 470 μH

have a current rating of at least 1.15 x ILOAD(Max) to allow for the Choose from:

ripple current. The actual peak current in L1 (in normal operation)

34048 (Schott)

can be calculated as follows:

PE-53118 (Pulse Engineering)

IL1(pk) = ILOAD(Max) + (VIN – VOUT) × ton / 2L1

Where ton = VOUT / VIN × (1 / fosc)

RL1961 (Renco)

3. Output Capacitor Selection (COUT)

3. Output Capacitor Selection (COUT)

A. The LM2575 control loop has a two-pole two-zero frequency A. COUT ≥ 7785 × 25 / (10 × 470) [μF]

response. The dominant pole-zero pair is established by COUT and C

OUT ≥ 41.4 μF

L1. To meet stability requirements, COUT must meet the following

To

obtain

an

acceptable

output

voltage

ripple

→

requirement:

COUT = 220 μF electrolytic

However, COUT may need to be several times larger than the

calculated value above in order to achieve an acceptable output

ripple voltage of about 0.01 × VOUT.

B. COUT should have a voltage rating of at least 1.5 × VOUT. But if a

low output ripple voltage is desired, choose capacitors with a higher

voltage ratings than the minimum required due to their typically lower

ESRs.

4. Catch Diode Selection (D1) (see Table 1)

4. Catch Diode Selection (D1) (see Table 1)

A. In normal operation, the catch diode requires a current rating of at A. Pick a diode with a 3-A rating.

least 1.2 × ILOAD(Max). For the most robust design, D1 should be

rated for a current equal to the LM2575 maximum switch peak

current; this represents the worst-case scenario of a continuous

short at VOUT.

B. The diode requires a reverse voltage rating of at least B. Pick a 40-V rated Schottky diode (1N5822, MBR340, 31QD04, or

1.25 × VIN(Max).

SR304) or 100-V rated Fast Recovery diode (31DF1, MURD310, or

HER302)

5. Input Capacitor (CIN)

5. Input Capacitor (CIN)

An aluminum electrolytic or tantalum capacitor is needed for input CIN = 100 μF, 35 V, aluminum electrolytic

bypassing. Locate CIN as close to VIN and GND pins as possible.

12

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