MC34025, MC33025

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10

PIN FUNCTION DESCRIPTION

Pin No.

Function

Description

DIP/SOIC

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

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

6

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

7

Ramp Input

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

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

This pin is the positive supply of the control IC.

16

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

Figure 22. 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

From Current

Sense Element

Figure 23. 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.