M51995AP

Manufacturer Part NumberM51995AP
DescriptionSwitching regulator control
ManufacturerMITSUBISHI
M51995AP datasheet
 
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Page 16/27:

Intermittent action circuit section

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It is rather recommended to use not "CLM+" but "CLM-"
terminal,as the influence from the gate drive current of MOS-FIT
can be eliminated and wide voltage rating of + 4V to -4V is
guaranteed for absolute maximum rating.
There happen some noise voltage on R
of power transistor due to the snubber circuit and stray
capacitor of the transformer windings.
To eliminate the abnormal operation by the noise voltage,the
low pass filter,which consists of R
and C
NF
in Fig.12.
It is recommended to use 10 to 100 for R
range of R
is not influenced by the flow-out current of some
NF
200µA from C
terminal and C
is designed to have the
LM
NF
enough value to absorb the noise voltage.
+
M51995A
M51995A
V
OUT
V
OUT
R
NF
CLM+
GND
C
R
NF
CLM
GND
CLM-
(a)In case of CLM+
(b)In case of CLM-
Fig.12 How to connect current limit circuit
Intermittent action and oscillation control
section
When the internal current limiting circuit states to operate
and also the VF level decreases to lower than the certain level
of some 3V,the dead-time spreads and intermittent action and
OSC control circuit(which is one of the timer-type-protection
circuit)starts to operate.
The intermittent action and OSC control circuit is the one to
generate the control signal for oscillator and intermittent action
circuit.
Fig.13 shows the timing-chart of this circuit.When the output of
intermittent action and oscillation control is at "high" level,the
waveform of oscillator depends on the VF terminal voltage and
the intermittent action circuit begins to operate.
OSC WAVEFORM
OF CF TERMINAL
CURRENT LIMIT
SIGNAL
OUTPUT OF
CURRENT LIMIT
LATCH
OUTPUT OF
INTERMITTENT
ACTION and OSC.
CONTROL CIRCUIT
(a) With current limit signal
OSC WAVEFORM
OF CF TERMINAL
CURRENT LIMIT
during the switching
CLM
SIGNAL
OUTPUT OF CURRENT
LIMIT LATCH
is used as shown
NF
OUTPUT OF INTERMITTENT
because such
NF
ACTION and OSC.
CONTROL CIRCUIT
Fig.13 Timing chart of intermittent and OSC.control circuit
+

Intermittent action circuit section

Intermittent action circuit will start to operate when the output
signal from the intermittent action and oscillation control circuit
are "high" and also VF terminal voltage is lower than V
about 3V.
Fig.14 shows the block diagram of intermittent action
circuit.Transistor Q is on state when VF terminal voltage is
C
R
NF
CLM
higher than V
near to GND potential.
When VF terminal voltage is lower than V
R
NF
"off" and the CT has the possibility to be charged up.
Under this condition,if the intermittent action and oscillation
control signal become "high" the switch SWA will close only in
this "high" duration and C
120µA through SW
will rise.The output pulse can be generated only this duration.
When the CT terminal voltage reaches to 8V,the control logic
circuit makes the SW
I
TIMEOFF
The IC operation will be ceased in the falling duration.
On the other hand,when CT terminal voltage decreases to lower
than 2V,the IC operation will be reset to original state,as the
control logic circuit makes the SW
Therefore the parts in power circuit including secondary rectifier
diodes are protected from the overheat by the over current.
Fig.14 Block diagram of intermittent action circuit
(
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/ 27 )
M51995AP/FP
SWITCHING REGULATOR CONTROL
(b) Without current limit signal
of about 3V,so the CT terminal voltage is
THTIME
THTIME
is charged up by the current of
T
(SW
is open) and CT terminal potential
A
B
"off" and SW
"on",in order to flow in the
A
B
of 15µA to CT terminal.
"on" and SW
A
I
TIMEON
(~120µA)
V
(~ 3V)
A
THTIME
CT
SWA
SWB
Q
B
VF
I
TIMEOFF
(~15µA)
CT
GND
GND
GND
of
THTIME
,Q becomes
"off".
B
CONTROL
LOGIC