MC33023P View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MC33023P

High speed single-ended PWM controller

Motorola => Freescale 

MC34023 MC33023

Figure 23. Resistive Current Sensing

Figure 24. Primary Side Current Sensing

9

ISense

9

Rw

ISense

The addition of an RC filter will eliminate instability caused by the

leading edge spike on the current waveform. This sense signal can also

be used at the ramp input pin for current mode control. For ramp

compensation it is necessary to know the gain of the current feedback

loop. If a transformer is used, the gain can be calculated by:

+ Ai

RSense

turns ratio

The addition of an RC filter will eliminate instability caused by the

leading edge spike on the current waveform. This sense signal can also

be used at the ramp input pin for current mode control. For ramp

compensation it is necessary to know the gain of the current feedback

loop. The gain can be calculated by:

+ Ai

Rw

turns ratio

Figure 25A. Slope Compensation (Noise Sensitive)

4

5

6

CT

C1

Oscillator

Current Sense R1

7

Information R2

3

1.25 V

This method of slope compensation is easy to implement, however, it

is noise sensitive. Capacitor C1 provides AC coupling. The oscillator

signal is added to the current signal by a voltage divider consisting of

resistors R1 and R2.

Figure 25B. Slope Compensation (Noise Immune)

Current Sense

Transformer

Rw

Output

Rf

RM CM Cf

Ramp

Input

7

1.25 V

3

Figure 25.

Current Sense

Resistor

Output

Ramp

Input

RM

7

1.25 V

CM

3

Rf

Cf

When only one output is used, this method of slope compensation can be used and it is relatively noise immune. Resistor RM and capacitor CM provide the added

slope necessary. By choosing RM and CM with a larger time constant than the switching frequency, you can assume that its charge is linear. First choose CM, then

RM can be adjusted to achieve the required slope. The diode provides a reset pulse at the ramp input at the end of every cycle. The charge current IM can be calculated

by IM = CMSe. Then RM can be calculated by RM = VCC/IM.

MOTOROLA ANALOG IC DEVICE DATA

11

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