M51995AP

Manufacturer Part NumberM51995AP
DescriptionSwitching regulator control
ManufacturerMITSUBISHI
M51995AP datasheet
 


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(2)Oscillator operation when intermittent action
and OSC control circuit operates.
When over current signal is applied to CLM+ or CLM-
terminal,and the current limiting circuit,intermittent action and
OSC control circuit starts to operate.In this case T-OFF terminal
voltage depends on VF terminal voltage,so the oscillation
frequency decreases and dead-time spreads.
The rise rate of oscillation waveform is given as
V
-
~
T
ON
(V/s) ................................................(5)
R
X C
ON
F
The fall rate of oscillation waveform is given as
~
V
- V
V
-
VF
VFO
T
ON
+
(V/s)
R
X C
16 X R
X C
OFF
F
ON
F
~
where V
-
4.5V
T
ON
V
;V
terminal voltage
VF
F
~
0.4V
V
VFO
V
-V
=0 if V
-V
<0
VF
FO
VF
VFO
V
-V
=V
-
if V
-V
>V
VF
VFO
T
OFF
VF
VFO
So when V
>3.5V,the operation is just same as that in the
VF
no current limiting operation state.
The maximum on-duration is just same as that in the no-
operation state of intermittent and oscillation control circuit
and is given as follows;
(V
- V
)
~
X R
X C
OSCH
OSCL
OFF
F
(s)
V
-
T
ON
The minimum off-duration is approximately given as;
(V
- V
)
X C
~
OSCH
OSCL
F
V
- V
V
-
VF
VFO
T
ON
+
(s)
...............(8)
R
X C
16 X R
X C
OFF
F
ON
F
The oscillation period is given by the summation of Equation(7)
and (8).
As shown in Fig.7,the internal circuit kills the first output pulse in
the output waveform.The output waveform will appear from the
second pulse cycle because the duration of first cycle takes C
charging time longer comparing with that at the stable operating
state.
Usually the applied voltage to VF terminal must be proportional
the output voltage of the regulator.
So when the over current occurs and the output voltage of the
regulator becomes low,the off-duration becomes wide.
There are two methods to get the control voltage,which
depends on the output voltage,on primary side.For the fly back
type regulator application,the induced voltage on the third or
bias winding is dependent on output voltage.On the other
hand,for the feed forward type regulator application,it can be
used that the output voltage depends on the product of induced
voltage and "on-duty",as the current of choke coil will continue
at over load condition,it means the "continuous current"
condition.
Fig.8 shows one of the examples for VF terminal application for
the feed forward type regulator.
V
~
V
~
...............(6)
Fig.6 OSC.waveform with operation of intermittent
~
-
3.5V
T
OFF
V
V
...............(7)
Fig.7 Relation between OSC. and output waveform
F
Fig.8 Feedback loop with low pass filter from output
choke coil will continue at over load condition,it means the
"continuous current" condition.
Fig.8 shows one of the examples for VF terminal application
for the feed forward type regulator.
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14
M51995AP/FP
SWITCHING REGULATOR CONTROL
OSCH
4.4V
OSCL
2.0V
V
OH
V
OL
and OSC.control circuit operation
START FROM 0V
OSCH
OSCL
0
V
OH
NO GENERATE
PULSE
V
OL
0
OPERATION START
circuit operation at start up
M51995
V
OUT
R
VFFB
VF
C
VFFB
to VF terminal
FIRST
PULSE