M51995AP

Manufacturer Part NumberM51995AP
DescriptionSwitching regulator control
ManufacturerMITSUBISHI
M51995AP datasheet
 


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Current limiting section

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PWM comparator and PWM latch section
Fig.9 shows the PWM comparator and latch section. The on-
duration of output waveform coincides with the rising duration of
CF terminal waveform,when the infinitive resistor is connected
between F/B terminal and GND.
When the F/B terminal has finite impedance and current flows
out from F/B terminal,"A" point potential shown in Fig.9 depends
on this current.So the "A" point potential is close to GND level
when the flow-out current becomes large.
"A" point potential is compared with the CF terminal oscillator
waveform and PWM comparator,and the latch circuit is set
when the potential of oscillator waveform is higher than "A"
point potential.
On the other hand,this latch circuit is reset by high level signal
during the dead-time of oscillation(falling duration of oscillation
waveform).So the "B" point potential or output waveform of latch
circuit is the one shown in Fig.10.
The final output waveform or "C" point potential is got by
combining the "B" point signal and dead-time signal
logically.(please refer to Fig.10)
~
7.1V
5.8V
POINT A
-
+
6S
1S
PWM
F/B
COMP
M51995A
CF
Fig.9 PWM comparator and latch circuit
OSC WAVEFORM
POINT A
WAVEFORM
OF O.S.C. &
POINT A
POINT B
POINT C
Fig.10 Waveforms of PWM comparator input point A,
latch circuit points B and C

Current limiting section

When the current-limit signal is applied before the crossing
instant of "A" pint potential and CF terminal voltage shown in
Fig.9,this signal makes the output "off" and the off state will
continue until next cycle.Fig.11 shows the timing relation among
them.
The current limiting circuit has two input terminals,one has the
detector-sensitivity of +200mV to the GND terminal and the
other has -200mV.The circuit will be latched if the input signal is
over the limit of either terminal.
If the current limiting circuit is set,no waveform is generated at
output terminal however this state is reset during the
succeeding dead-time.
So this current limiting circuit is able to have the function in
every cycle,and is named "pulse-by-pulse current limit".
POINT B
TO
LATCH
OUTPUT
POINT C
FROM
OSC
OSC WAVEFORM
OF CF TERMINAL
WAVEFORM OF
CLM- TERMINAL
(
/ 27 )
15
M51995AP/FP
SWITCHING REGULATOR CONTROL
OSC WAVEFORM
OF CF TERMINAL
~
V
THCLM
WAVEFORM OF
CLM+ TERMINAL
CURRENT LIMIT
SIGNAL TO SET
LATCH
WAVEFORM OF
V
TERMINAL
OUT
(a) +current limit
~
V
-200mV
THCLM
CURRENT LIMIT
SIGNAL TO SET
LATCH
WAVEFORM OF
V
TERMINAL
OUT
(b) -current limit
Fig.11 Operating waveforms of current limiting circuit
200mV