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= V

OUT

REF

1 +

where V

= 1.23 V

REF

(

R2 = R1

– 1

OUT

REF

7758

OUT

(µF)

IN(Max)

L1(µH)

OUT

TL2575, TL2575HV

1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS

SLVS638B – MAY 2006 – REVISED JANUARY 2007

PROCEDURE (Adjustable Output)

EXAMPLE (Adjustable Output)

Known:

VOUT(Nom)

VOUT = 10 V

VIN(Max) = Maximum input voltage

VIN(Max) = 25 V

ILOAD(Max) = Maximum load current

ILOAD(Max) = 1 A

1. Programming Output Voltage (Selecting R1 and R2)

Referring to Fig. 2, 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)

A. Calculate the "set" volts-second (E

•T) across L1:

A. Calculate the "set" volts-second (E

•T) across L1:

•T = (V

IN – VOUT) × ton

•T = (25 – 10) × (10/25) × (1000/52) [V•µs]

•T = (V

IN – VOUT) × (VOUT/VIN) × {1000/fosc(in kHz)} [V•µs]

•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 19, select the appropriate inductor code based on

B. Using Figure 19, the intersection of 115 V

•µs and 1 A

the intersection of E

•T value and I

LOAD(Max).

corresponds to an inductor code of H470.

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

C. H470

→ L1 = 470 µF

inductor code. Parts from three well-known inductor manufacturers

Choose from:

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

34048 (Schott)

52-kHz and have a current rating of at least 1.15 x ILOAD(Max) to

PE-53118 (Pulse Engineering)

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

operation) can be calculated as follows:

RL1961 (Renco)

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

Where ton = VOUT/VIN × (1/fosc)

3. Output Capacitor Selection (COUT)

A. The TL2575 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

COUT ≥ 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 ~0.01

× V

OUT.

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)

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

A. Pick a diode with a 3-A rating.

at least 1.2

× I

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

rated for 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 a 40-V rated Schottky diode (1N5822, MBR340, 31QD04, or

1.25

× V

IN(Max).

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

HER302)

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.

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