TL2575 datasheet(17/31 Pages) TI

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BUCK REGULATOR DESIGN PROCEDURE

TL2575, TL2575HV

1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS

SLVS638B – MAY 2006 – REVISED JANUARY 2007

PROCEDURE (Fixed Output)

EXAMPLE (Fixed Output)

Known:

VOUT = 3.3 V, 5 V, 12 V, or 15 V

VOUT = 5 V

VIN(Max) = Maximum input voltage

VIN(Max) = 20 V

ILOAD(Max) = Maximum load current

ILOAD(Max) = 1 A

1. Inductor Selection (L1)

A. From Figure 15 through Figure 18, select the appropriate inductor A. From Figure 16 (TL2575-05), the intersection of 20-V line and

code based on the intersection of VIN(Max) and ILOAD(Max).

1-A line gives an inductor code of L330.

B. From Table 2, choose the appropriate inductor based on the

B. L330

→ L1 = 330 µH

inductor code. Parts from three well-known inductor manufacturers

Choose from:

are given. The inductor chosen should be rated for operation at

34042 (Schott)

52-kHz and have a current rating of at least 1.15

× I

LOAD(Max) to

PE-52627 (Pulse Engineering)

allow for the ripple current. The actual peak current in L1 (in normal

operation) can be calculated as follows:

RL1952 (Renco)

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

Where ton = VOUT/VIN× (1/fosc)

2. Output Capacitor Selection (COUT)

A. The TL2575 control loop has a two-pole two-zero frequency

A. COUT = 100-µF to 470-µF, standard aluminum electrolytic

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

L1. To meet stability requirements while maintaining an acceptable

output ripple voltage (Vripple ≈ 0.01 × VOUT), the recommended range

for a standard aluminum electrolytic COUT is between 100 µF and

470

µF.

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

B. Although a COUT rated at 8 V is sufficient for VOUT = 5 V, a

low output ripple voltage is desired, choose capacitors with a

higher-voltage capacitor is chosen for its typically lower ESR (and

higher-voltage ratings than the minimum required, due to their

hence lower output ripple voltage)

→ Capacitor voltage

typically lower ESRs.

rating = 20 V.

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

A. In normal operation, the catch diode requires a current rating of

A. Pick a diode with 3-A rating.

at least 1.2

× I

LOAD(Max). For the most robust design, D1 should be

rated to handle a current equal to the TL2575 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 30-V rated Schottky diode (1N5821, MBR330, 31QD03, or

1.25

× V

IN(Max).

SR303) or 100-V rated Fast Recovery diode (31DF1, MURD310, or

HER302).

4. Input Capacitor (CIN)

An aluminum electrolytic or tantalum capacitor is needed for input

CIN = 100 µF, 25 V, aluminum electrolytic

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

possible.

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