TC2575VAT datasheet(8/25 Pages) TELCOM

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TC2575

1.0A Step-Down Switching Regulator

TC2575-1 3/13/00

Procedure (Fixed Output Voltage Version) (Continued)

In order to simplify the switching regulator design, a step-by-step design procedure and examples are provided.

Procedure

Example

5. Output Capacitor Selection (COUT)

A. Since the TC2575 is a forward–mode switching regulator

with voltage mode control, its open loop 2-pole-2-zero

frequency characteristic has the dominant pole–pair dete-

rmined by the output capacitor and inductor values.

For stable operation and an acceptable ripple voltage,

(approximately 1% of the output voltage) a value

between 100

µF and 470µF is recommended.

B. Due to the fact that the higher voltage electrolytic

capacitors generally have lower ESR (Equivalent Series

Resistance) numbers, the output capacitor’s voltage

rating should be at least 1.5 times greater than the output

voltage. For a 5.0V regulator, a rating at least 8.0V is

appropriate, and a 10Vor 16V rating is recommended.

5. Output Capacitor Selection (COUT )

A. COUT = 100µF to 470µF standard aluminium

electrolytic.

B. Capacitor voltage rating = 16V.

Procedure (Adjustable Output Version: TC2575-ADJ)

Procedure

Example

Given Parameters:

VOUT = Regulated Output Voltage

VIN (max) = Maximum DC Input Voltage

ILOAD (max) = Maximum Load Current

1. Programming Output Voltage

To select the right programming resistor R1 and R2

value (see Figure 2) use the following formula:

VOUT = VREF

(1.0 + R2 ) where V

REF = 1.23V

Resistor R1 can be between 1.0k

Ω and 5.0kΩ. (For best

temperature coefficient and stability with time, use 1% metal

film resitors).

R2 = R1

( VOUT – 1)

VREF

2. Input Capacitor Selection (CIN )

To prevent large voltage transients from appearing at the

input and for stable operation of the converter, an aluminium

or tantalum electrolytic bypass capacitor is needed between

the input pin +VIN and ground pin GND. This capacitor

should be located close to the IC using short leads. This

capacitor should have a low ESR (Equivalent Series

Resistance) value.

For additional information see input capacitor section in the

“EXTERNAL COMPONENTS” section of this data sheet.

Given Parameters:

VOUT = 8.0V

VIN (max) = 12V

ILOAD (max) = 1.0V

2. Input Capacitor Selection (CIN )

A 100

µF, aluminium electrolytic capacitor located near the

input and ground pin provides sufficient bypassing.

1. Programming Output Voltage (selecting R1 and R2)

Select R1 and R2

VOUT = 1.23

(1.0 + R2 ) Select R1 = 1.8kΩ

R2 = R1

( VOUT – 1.0) = 1.08k ( 8.0V – 1)

VREF

1.23V

R1 = 9.91k

Ω, choose a 9.88kΩ metal film resistor.

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