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MC34025DWR2G

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
MC34025, MC33025
http://onsemi.com
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
error amp. Usually this is connected to V
ref
, 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 R
T
The value of R
T
sets the charge current through timing Capacitor, C
T
.
6 C
T
In conjunction with R
T
, 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 C
T
. 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 V
C
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 V
CC
This pin is the positive supply of the control IC.
16 V
ref
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
V
ref
C
T
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
V
ref
C
T
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.
MC34025, MC33025
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11
C
T
V
ref
R
DT
R
T
4
Oscillator
Figure 24. Dead Time Addition
5
6
Additional dead time can be added by the addition of a dead
time resistor from V
ref
to C
T
. See text on oscillator section for
more information.
5.0 V
0 V
C
T
R
T
Oscillator
Figure 25. External Clock Synchronization
4
5
6
The sync pulse fed into the clock pin must be at least 3.9 V. R
T
and C
T
need to be set 10% slower than the sync frequency. This
circuit is also used in voltage mode operation for master/slave
operation. The clock signal would be coming from the master
which is set at the desired operating frequency, while the slave
is set 10% slower.
Figure 26. Resistive Current Sensing
I
Sense
9
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:
A
i
+
R
Sense
turns ratio
Figure 27. Primary Side Current Sensing
R
w
I
Sense
9
Figure 28. Primary or Secondary Side
Current Sensing
0
I
Sense
R
w
9
A
i
+
R
w
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:
MC34025, MC33025
http://onsemi.com
12
3
7
6
5
4
Figure 29A. Slope Compensation (Noise Sensitive)
Oscillator
C
T
C
1
R
1
R
2
Current Sense
Information
1.25 V
This method of slope compensation is easy to implement, however, it 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
.
7
3
7
3
Figure 29B. Slope Compensation (Noise Immune)
R
w
Output
R
M
C
M
R
f
C
f
1.25 V
Ramp
Input
Current Sense
Transformer
Current Sense
Resistor
R
f
C
f
C
M
1.25 V
R
M
Ramp
Input
Output
Figure 29. Keeps Fig numbering sequence correct
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 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
.
Figure 30. Current Mode Master/Slave Operation Over Short Distances
V
ref
6
5
4
Oscillator
C
T
R
T
6
5
4
Oscillator
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

MC34025DWR2G

Mfr. #:
Buy MC34025DWR2G
Manufacturer:
ON Semiconductor
Description:
Switching Controllers Dual Output Voltage or Current Mode PWM
Lifecycle:
New from this manufacturer.
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