M51995 Mitsubishi, M51995 Datasheet - Page 22

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

INPUT
SMOOTHING
CAPACITOR
INPUT
SMOOTHING
CAPACITOR
To design the R
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
100 .
The R
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
detecting-sensitivity" and to consist the low pass filter.
(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.
(2)Over current limiting curve
C
C
FIN
FIN
NF1
Fig.32 Peripheral circuit diagram of CLM- terminal
Fig.31 Peripheral circuit diagram of CLM+ terminal
CLM
NF2
and R
have the functions both to adjust the "current-
should be the non-inductive resistor.
M51995A
M51995A
NF2
Vcc
NF1
Vcc
GND
,(R
and R
CLM+
NF1
COLLECTOR
COLLECTOR
EMITTER
/R
NF2
NF2
EMITTER
V
GND
,it is required to consider the
)is recommended to be less than
V
CLM+
OUT
OUT
Cvcc
Cvcc
R1
R1
C
C
NF
NF
INCLM
NF
R
R
and the R
NF2
R
NF2
+ or I
R
NF1
NF1
R
R
CLM
NFCLM
CLM
NF1
-),
(
22
/ 27 )
The demerit of the pulse by pulse current limiting system is that
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.
Fig.33 Primary and secondary current waveforms
Fig.34 Over current limiting curve on feed forward
CLM
(b) Primary and secondary current
I1
I2
SWITCHING REGULATOR CONTROL
I1
under the current limiting operation
condition on feed forward system
system
(a) Feed forward system
R
CLM
OUTPUT CURRENT
M51995AP/FP
I2
PDCLM
I
I
P2
P1
from CLM
PDCLM