11 / 18 page
C2472, C2473 and C2474
Datasheet
RDFC Controllers for Offline Applications
Product data
Page 11 of 18
DS-1423-0709C
© Cambridge Semiconductor Ltd 2007
26-Sep-2007
The effective thresholds for current through the primary switch for both power reduction (OCPL) and
overload (OCPH) are programmed by the current-sense resistors connected to the CS pin as shown in
Figure 10.
Figure 10: Current Sense Diagram
The internal current source (ICSBIAS) develops an offset voltage across the series resistor (R2 in Figure 10) so
setting OCPL current threshold. Switch current in excess of overload (OCPH) is detected using a fixed
threshold voltage but the contribution from the offset voltage across R2 has to be taken into account.
IOCPL threshold current = (VOCPL + ICSBIAS.R2)/Rcs
IOCPH threshold current = (VOCPH + ICSBIAS.R2)/Rcs
CSBIAS
OCPL
CSBIAS
OCPH
OCPH
OCPL
OCPL
OCPH
I
I
I
I
I
V
I
V
R
2
OCPL
OCPH
OCPL
OCPH
I
I
V
V
Rcs
Note: IOCPL, IOCPH, VOCPH ICSBIAS are all positive magnitude in these formulae
OCPH
threshold (-ve)
GND
OCPH
OCPL
Current
sense
comparators
I
CSBIAS
Rcs
R2
CS
GND
Primary
switch
current
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