M51995 Mitsubishi, M51995 Datasheet - Page 16

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

M51995A
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.
CLM+
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
in Fig.12.
It is recommended to use 10 to 100 for R
range of R
200µA from C
enough value to absorb the noise voltage.
V
GND
OUT
OSC WAVEFORM
OF CF TERMINAL
OUTPUT OF
CURRENT LIMIT
LATCH
OUTPUT OF
INTERMITTENT
ACTION and OSC.
CONTROL CIRCUIT
CURRENT LIMIT
SIGNAL
(a)In case of CLM+
Fig.12 How to connect current limit circuit
NF
is not influenced by the flow-out current of some
LM
C
terminal and C
R
NF
+
NF
R
CLM
(a) With current limit signal
M51995A
NF
CLM-
V
GND
NF
OUT
is designed to have the
and C
(b)In case of CLM-
CLM
NF
NF
during the switching
is used as shown
because such
C
R
NF
+
NF
R
CLM
(
16
/ 27 )
OUTPUT OF INTERMITTENT
ACTION and OSC.
CONTROL CIRCUIT
OUTPUT OF CURRENT
LIMIT LATCH
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
higher than V
near to GND potential.
When VF terminal voltage is lower than V
"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
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.
Intermittent action circuit section
TIMEOFF
OSC WAVEFORM
OF CF TERMINAL
CURRENT LIMIT
SIGNAL
Fig.13 Timing chart of intermittent and OSC.control circuit
Fig.14 Block diagram of intermittent action circuit
VF
V
of 15µA to CT terminal.
THTIME
THTIME
SWITCHING REGULATOR CONTROL
(~ 3V)
(b) Without current limit signal
A
of about 3V,so the CT terminal voltage is
(SW
A
"off" and SW
B
T
is charged up by the current of
is open) and CT terminal potential
Q
M51995AP/FP
A
CT
"on" and SW
B
CT
"on",in order to flow in the
SWA
SWB
THTIME
I
I
TIMEON
(~120µA)
TIMEOFF
(~15µA)
B
A
,Q becomes
B
CONTROL
LOGIC
"off".
THTIME
of
GND
GND
GND