MC34023 MC33023

10

MOTOROLA ANALOG IC DEVICE DATA

PIN FUNCTION DESCRIPTION

Pin

DIP/SOIC

Function

Description

1

Error Amp

Inverting

Input

This pin is usually used for feedback from the output of the power supply.

2

Error Amp

Noninverting

Input

This pin is used to provide a reference in which an error signal can be produced on the output of the

error amp. Usually this is connected to Vref, however an external reference can also be used.

3

Error Amp

Output

This pin is provided for compensating the error amp for poles and zeros encountered in the power

supply system, mostly the output LC filter.

4

Clock

This is a bidirectional pin used for synchronization.

5

RT

The value of RT sets the charge current through timing Capacitor, CT.

6

CT

In conjunction with RT, the timing Capacitor sets the switching frequency.

7

Ramp Input

For voltage mode operation this pin is connected to CT. For current mode operation this pin is

connected through a filter to the current sensing element.

8

Soft–Start

A capacitor at this pin sets the Soft–Start time.

9

Current Limit/

Shutdown

This pin has two functions. First, it provides cycle–by–cycle current limiting. Second, if the current is

excessive, this pin will reinitiate a Soft–Start cycle.

10

Ground

This pin is the ground for the control circuitry.

11

Current Limit

Reference

Input

This pin voltage sets the threshold for cycle–by–cycle current limiting.

12

Power Ground

This is a separate power ground return that is connected back to the power source. It is used to reduce

the effects of switching transient noise on the control circuitry.

13

VC

This is a separate power source connection for the outputs that is connected back to the power source

input. With a separate power source connection, it can reduce the effects of switching transient noise

on the control circuitry.

14

Output

This is a high current totem pole output.

15

VCC

This pin is the positive supply of the control IC.

16

Vref

This is a 5.1 V reference. It is usually connected to the noninverting input of the error amplifier.

Output Voltage

Feedback Input

2

1

3

7

6

5

4

1.25 V

Oscillator

Vref

CT

Figure 21. Voltage Mode Operation

In voltage mode operation, the control range on the output of the Error

Amplifier from 0% to 90% duty cycle is from 2.25 V to 4.05 V.

Vref

Output Voltage

Feedback Input

2

1

3

7

6

5

4

1.25 V

Oscillator

CT

From Current

Sense Element

Figure 22. Current Mode Operation

In current mode control, an RC filter should be placed at the ramp input

to filter the leading edge spike caused by turn–on of a power MOSFET.