SC4808C Ver la hoja de datos (PDF) - Semtech Corporation
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SC4808C Datasheet PDF : 24 Pages
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SC4808C
POWER MANAGEMENT
Application Information (Cont.)
The instability can be corrected by modification of the peak
current information slope. One of the methods to alter the
peak current information is to add a positive going ramp to
the output of the current sensing circuitry.
The SC4808C achieves this by using an internal slope com-
pensation circuit. The oscillator ramp is internally buffered
and an internal 25kOhm resistor in conjunction with an
external resistor at the CS pin will program the level of slope
compensation.
Current Transformer
N = 100
If the Rslope comp is increased, the internal ramp becomes the
dominant signal and more voltage mode of operation is
achieved. As it can be calculated from the second formula
below, a 100% voltage mode operation can be achieved by
choosing Rslope comp to be greater than 6.25kOhm. Also if a
100% current mode of operation is required, Rslope comp is
reduced to zero and the contribution from the internal ramp
is completely eliminated.
( ( ) ) %Slope _ Comp=
VRamp ×
Rslope _ Comp + Rsense
Rslope_ Comp + Rsense + Rint ernal
VCS
10k
R
D
RSlope Comp
Rsense
Cfilter
82p
REF
C
2.2u,16V
Rosc
15k
Cosc
200p
5 REF
4 FB
3
25k
CS
2 RC
1 SYNC
GND 6
7
SC4808
RSlope Comp value will determine the Mode of operation (Voltage or Current)
The Peak current information is sensed and the result is
realistically summed to the buffered oscillator ramp, as
shown above. The value of the external resistor Rslope comp
will determine the percentage of the slope compensation.
As the value for Rslope comp is reduced, the current informa-
tion becomes more dominant and the mode of operation
becomes more current mode. At the same time the slope
of the current information is modified to provide the slope
compensation.
or
Rexternal
≅
Rint ernal
•
0.2 × (%slope _ Comp)
1− (0.2 × %slope _ Comp)
Next page illustrates how the buffered oscillator ramp is
used to modify the sensed inductor current.
It should be noted that in order for the slope compensa-
tion to be effective, the current sensed signal slope should
be at least 50% less steeper than the oscillator positive
ramp slope. The slope will include the magnetizing current
of the transformer and the inductor output current in iso-
lated applications. In non-isolated applications, the slope
will only include the inductor output current.
2006 Semtech Corp.
11
www.semtech.com
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