LM2575

8

MOTOROLA ANALOG IC DEVICE DATA

PIN FUNCTION DESCRIPTION

Pin

Symbol

Description (Refer to Figure 1)

1

Vin

This pin is the positive input supply for the LM2575 step–down switching regulator. In order to minimize

voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass

capacitor must be present (Cin in Figure 1).

2

Output

This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is typically 1.0 V.

It should be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to

minimize coupling to sensitive circuitry.

3

Gnd

Circuit ground pin. See the information about the printed circuit board layout.

4

Feedback

This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the

internal resistor divider network R2, R1 and applied to the non–inverting input of the internal error amplifier.

In the Adjustable version of the LM2575 switching regulator this pin is the direct input of the error amplifier

and the resistor network R2, R1 is connected externally to allow programming of the output voltage.

5

ON/OFF

It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total

input supply current to approximately 80

µA. The input threshold voltage is typically 1.4 V. Applying a

voltage above this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than

1.4 V or if this pin is connected to ground, the regulator will be in the “on” condition.

DESIGN PROCEDURE

Buck Converter Basics

The LM2575 is a “Buck” or Step–Down Converter which is

the most elementary forward–mode converter. Its basic

schematic can be seen in Figure 15.

The operation of this regulator topology has two distinct

time periods. The first one occurs when the series switch is

on, the input voltage is connected to the input of the inductor.

The output of the inductor is the output voltage, and the

rectifier (or catch diode) is reverse biased. During this period,

since there is a constant voltage source connected across

the inductor, the inductor current begins to linearly ramp

upwards, as described by the following equation:

I

L(on) +

V

in

–Vout ton

L

During this “on” period, energy is stored within the core

material in the form of magnetic flux. If the inductor is properly

designed, there is sufficient energy stored to carry the

requirements of the load during the “off” period.

Figure 15. Basic Buck Converter

D1

Vin

Vout

RLoad

L

Cout

Power

Switch

The next period is the “off” period of the power switch.

When the power switch turns off, the voltage across the

inductor reverses its polarity and is clamped at one diode

voltage drop below ground by catch dioded. Current now

flows through the catch diode thus maintaining the load

current loop. This removes the stored energy from the

inductor. The inductor current during this time is:

I

L(off) +

Vout –VD toff

L

This period ends when the power switch is once again

turned on. Regulation of the converter is accomplished by

varying the duty cycle of the power switch. It is possible to

describe the duty cycle as follows:

d

+

ton

T

, where T is the period of switching.

For the buck converter with ideal components, the duty

cycle can also be described as:

d

+

Vout

V

in

Figure 16 shows the buck converter idealized waveforms

of the catch diode voltage and the inductor current.

Power

Switch

Figure 16. Buck Converter Idealized Waveforms

Power

Switch

Off

Power

Switch

Off

Power

Switch

On

Power

Switch

On

Von(SW)

VD(FWD)

Time

Time

ILoad(AV)

Imin

Ipk

Diode

Diode

Power

Switch