MC34025 MC33025

10

MOTOROLA ANALOG IC DEVICE DATA

PIN FUNCTION DESCRIPTION

Pin No.

Fi

Di

i

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. Because this part is a

push–pull output, each output runs at one–half the frequency set at this pin.

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

Output A

This is a high current totem pole output.

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 B

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.

Output Voltage

Feedback Input

2

1

3

7

6

5

4

1.25 V

Oscillator

Vref

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.