M51995 Mitsubishi, M51995 Datasheet - Page 14

• Manufacturer Part Number: M51995
• Description: SWITCHING REGULATOR CONTROL
• Manufacturer: Mitsubishi

(2)Oscillator operation when intermittent action
The rise rate of oscillation waveform is given as
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 fall rate of oscillation waveform is given as
~
where V
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.
So when V
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;
The minimum off-duration is approximately given as;
~
~
R
and OSC control circuit operates.
V
V
R
(V
V
ON
R
VF
T
OFF
VF
OFF
OSCH
(V
-
X C
- V
V
V
V
ON
V
- V
OSCH
X C
VF
T
VFO
VF
VF
X C
VFO
-
VF
F
VFO
;V
-V
-V
- V
ON
(V/s) ................................................(5)
F
>3.5V,the operation is just same as that in the
F
V
F
FO
VFO
~
- V
OSCL
terminal voltage
T
~
+
=0 if V
+
-
OSCL
=V
0.4V
ON
16 X R
4.5V
)
16 X R
T
-
X R
)
OFF
V
VF
V
X C
T
T
-V
OFF
-
ON
if V
-
ON
ON
VFO
F
ON
X C
X C
VF
X C
<0
-V
F
F
F
VFO
(V/s)
(s)
(s)
>V
...............(8)
...............(6)
...............(7)
T
-
OFF
~
3.5V
F
(
14
/ 27 )
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.
Fig.6 OSC.waveform with operation of intermittent
Fig.7 Relation between OSC. and output waveform
V
V
V
Fig.8 Feedback loop with low pass filter from output
V
~
~
OSCH
OSCL
OSCH
OSCL
V
V
V
2.0V
V
4.4V
OH
OL
OH
OL
0
0
and OSC.control circuit operation
circuit operation at start up
to VF terminal
OPERATION START
SWITCHING REGULATOR CONTROL
NO GENERATE
PULSE
R
C
VFFB
VFFB
START FROM 0V
V
VF
M51995
OUT
M51995AP/FP
FIRST
PULSE