MC33023 Motorola, MC33023 Datasheet - Page 11

• Manufacturer Part Number: MC33023
• Description: High Speed Single-Ended PWM Controller
• Manufacturer: Motorola

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:
slope necessary. By choosing R M and C M with a larger time constant than the switching frequency, you can assume that its charge is linear. First choose C M , then
R M 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 I M can be calculated
by I M = C M S e . Then R M can be calculated by R M = V CC /I M.
MOTOROLA ANALOG IC DEVICE DATA
The addition of an RC filter will eliminate instability caused by the
When only one output is used, this method of slope compensation can be used and it is relatively noise immune. Resistor R M and capacitor C M provide the added
Output
Figure 23. Resistive Current Sensing
R M
C M
Current Sense
Transformer
R w
C f
R f
A
i
+
9
turns ratio
R Sense
Ramp
Input
7
3
Figure 25A. Slope Compensation (Noise Sensitive)
is noise sensitive. Capacitor C 1 provides AC coupling. The oscillator
signal is added to the current signal by a voltage divider consisting of
resistors R 1 and R 2 .
Figure 25B. Slope Compensation (Noise Immune)
1.25 V
Current Sense
This method of slope compensation is easy to implement, however, it
Information
I Sense
C T
MC34023 MC33023
R 2
R 1
Figure 25.
C 1
4
5
6
7
3
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:
The addition of an RC filter will eliminate instability caused by the
1.25 V
Current Sense
Figure 24. Primary Side Current Sensing
Oscillator
Resistor
R f
C M
A
i
Output
+
9
R M
C f
turns ratio
R
w
Ramp
Input
7
3
R w
1.25 V
I Sense
11