M51995AP

Manufacturer Part NumberM51995AP
DescriptionSwitching regulator control
ManufacturerMITSUBISHI
M51995AP datasheet
 


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Current limiting circuit

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Current limiting circuit

(1)Peripheral circuit of CLM+,CLM- terminal
Fig.31 and 32 show the example circuit diagrams around the
CLM+ and CLM- terminal.It is required to connect the low pass
filter,as the main current or drain current contains the spike
current especially during the turn-on duration of MOS-FIT.
1,000pF to 22,000pF is recommended for C
and R
have the functions both to adjust the "current-
NF2
detecting-sensitivity" and to consist the low pass filter.
C
FIN
INPUT
SMOOTHING
COLLECTOR
Vcc
CAPACITOR
V
OUT
M51995A
CLM+
GND
EMITTER
Fig.31 Peripheral circuit diagram of CLM+ terminal
C
FIN
INPUT
SMOOTHING
COLLECTOR
Vcc
CAPACITOR
V
OUT
M51995A
EMITTER
GND
CLM+
Fig.32 Peripheral circuit diagram of CLM- terminal
To design the R
and R
,it is required to consider the
NF1
NF2
influence of CLM terminal source current(I
which value is in the range of 90 to 270µA.
In order to be not influenced from these resistor paralleled value
of R
and R
,(R
/R
)is recommended to be less than
NF1
NF2
NF1
NF2
100 .
The R
should be the non-inductive resistor.
CLM
(2)Over current limiting curve
(a)In case of feed forward system
Fig.33 shows the primary and secondary current wave-forms
under the current limiting operation.
At the typical application of pulse by pulse primary current
detecting circuit,the secondary current depends on the primary
current.As the peak value of secondary current is limited to
specified value,the characteristics curve of output voltage
versus output current become to the one as shown in Fig.34.
and the R
NF
NF1
R1
Cvcc
R
NF1
R
NF2
R
C
CLM
NF
R1
Cvcc
R
R
NF2
CLM
C
NF
R
NF1
The demerit of the pulse by pulse current limiting system is that
+ or I
-),
INCLM
NFCLM
the output pulse width can not reduce to less than some value
because of the delay time of low pass filter connected to the
CLM terminal and propagation delay time T
terminal to output terminal of type M51995A.The typical T
is 100ns.
As the frequency becomes higher,the delay time must be
shorter.And as the secondary output voltage becomes
higher,the dynamic range of on-duty must be wider;it means
that it is required to make the on-duration much more narrower.
So this system has the demerit at the higher oscillating
frequency and higher output voltage applications.
To improve these points,the oscillating frequency is set low
using the characteristics of VF terminal.When the current
limiting circuit operates under the over current condition,the
oscillating frequency decreases in accordance with the
decrease of VF terminal voltage,if the VF is lower than
3.5V.And also the dead time becomes longer.
(
/ 27 )
22
M51995AP/FP
SWITCHING REGULATOR CONTROL
I2
CLM
R
CLM
I1
(a) Feed forward system
I1
I2
(b) Primary and secondary current
Fig.33 Primary and secondary current waveforms
under the current limiting operation
condition on feed forward system
OUTPUT CURRENT
Fig.34 Over current limiting curve on feed forward
system
PDCLM
I
P1
I
P2
from CLM
PDCLM