SC4808B Semtech Corporation, SC4808B Datasheet - Page 18

• Manufacturer Part Number: SC4808B
• Description: Sc4808b High-performance Dual Ended Pwm Controller | Semtech
• Manufacturer: Semtech Corporation

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 SC4808 achieves this by using an internal slope com-
pensation circuit. The oscillator ramp is internally buffered
and an internal 25kOhms resistor in conjunction with an
external resistor at the CS pin will program the level of slope
compensation.
RSlope Comp
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 R
will determine the percentage of the slope compensation.
As the value for R
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.
If the R
dominant signal and more voltage mode of operation is
POWER MANAGEMENT
Application Information (Cont.)
2005 Semtech Corp.
RSlope Comp value will determine the Mode of operation (Voltage or Current)
slope comp
D
Cfilter
82p
Rsense
10k
R
is increased, the internal ramp becomes the
Current Transformer
N = 100
slope comp
Rosc
15k
Cosc
200p
REF
is reduced, the current informa-
5
4
3
2
1
CS
REF
FB
RC
SYNC
25k
SC4808
2.2u,16V
C
GND
6
7
slope comp
18
achieved. As it can be calculated from the second formula
below, a 100% voltage mode operation can be achieved by
choosing R
100% current mode of operation is required, R
reduced to zero and the contribution from the internal ramp
is completely eliminated.
or
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.
%
R
external
Slope
_
Comp
R
slope comp
int
ernal
V
to be greater than 6.25K ohms. Also if a
Ramp
1
0
2 .
0
2 .
R
%
slope
%
slope
slope
R
_
slope
Comp
_
V
SC4808A/B
CS
_
Comp
_
Comp
Comp
R
sense
www.semtech.com
R
sense
R
slope comp
int
ernal
is