Datasheet

UCC2897A
SLUS829D -- AUGUST 2008 -- REVISED JULY 2009
23
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APPLICATION INFORMATION: SETUP GUIDE
In the UCC2897A controllers, slope compensation is implemented by sourcing a linearly increasing current at
the CS pin. When this current passes through the current sense filter resistor (R
F
), it is converted to a slope
compensation ramp which can be characterized by its

dV
S
∕dt

.The

dV
S
∕dt

of the slope compensation
current is defined by R
SLOPE
according to:
dI
S
dt
=
5 × 2V
t
ON
× R
SLOPE
where
D 2V is the peak--to--peak ramp amplitude of the internal oscillator waveform
D 5 is the multiplication factor of the internal current mirror
The voltage equivalent of the compensation ramp

dV
S
∕dt

can be easily obtained by multiplying with R
F
. After
introducing the application s pecific m and

dV
L
∕dt

values, the equation can be rearranged for R
SLOPE
:
R
SLOPE
=
5 × 2V× R
F
t
ON
× m ×

dV
L
dt

C
BIAS
+V
IN
-- V
IN
C
CLAMP
Q
AUX
Q
MAIN
R
CS
C
IN
FB
VIN
CS
GND
OUT
VDD
AUX
Secondary--Side
Error Amplifier
and Isolation
Synchronous
Rectifier
Control
UCC2897A
Bootstrap
Bias
Load
Connection
Drive
R
OT
Figure 8. Active Clamp Forward Converter
(25)
(26)