m51996ap Renesas Electronics Corporation., m51996ap Datasheet - Page 27

• Manufacturer Part Number: m51996ap
• Description: Switching Regulator Control
• Manufacturer: Renesas Electronics Corporation.

M51996AP/AFP
Current Limiting Circuit
1. Peripheral circuit of CLM terminal
2. Over current limiting curve
REJ03D0836-0201 Rev.2.01 Nov 14, 2007
Page 27 of 35
Figure 23 shows the example circuit diagrams around the CLM terminal. It is required to connect the low pass
filter, in order to reduce the spike current component, as the main current or drain current contains the spike current
especially during the turn-on duration of MOS FET.
1,000 pF to 22,000 pF is recommended for C
detecting-sensitivity” and to consist the low pass filter.
To design the R
which value is in the range of 90 to 270 A.
In order to be not influenced from these resistor paralleled value of R
be less than 100 .
The R
(1) In case of feed forward system
Figure 24 Primary and Secondary Current Waveforms Under The Current Limiting Operation
Figure 24 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 figure 25.
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
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 prevent that the SOFT terminal is used to lower the frequency when the curve starts to become vertical.
CLM
should be the non-inductive resistor.
CLM
NF1
PDCLM
I
and R
1
Input
smoothing
capacitor
from CLM terminal to output terminal of type M51996A. The typical T
Figure 23 Peripheral Circuit Diagram of CLM Terminal
(a) Feed forward system
R
NF2
C
CLM
FIN
, it is required to consider the influence of CLM terminal source current (I
+
Condition on Feed Forward System
M51996A
V
GND
CC
I
2
NF
Collector
Emitter
and the R
V
CLM+
OUT
NF1
+
R1
C
and R
VCC
C
NF
I
I
(b) Primary and secondary current
1
2
NF2
R
NF1
NF2
R
have the function both to adjust the “current-
NF1
and R
R
CLM
NF2
, (R
NF1
R
NF2
) is recommended to
PDCLM
I
I
P1
P2
is 100 ns.
INCLM
),