Datasheet

t
ON t
DM
T
SW
I
PP
I
S
x N
S
/N
P
UCC28700, UCC28701
UCC28702, UCC28703
www.ti.com SLUSB41 – JULY 2012
Primary-Side Current Regulation
Timing information at the VS pin and current information at the CS pin allow accurate regulation of the secondary
average current. The control law dictates that as power is increased in CV regulation and approaching CC
regulation the primary-peak current is at I
PP(max)
. Referring to Figure 16 below, the primary-peak current, turns
ratio, secondary demagnetization time (t
DM
), and switching period (T
SW
) determine the secondary average output
current. Ignoring leakage inductance effects, the average output current is given by Equation 6. When the
average output current reaches the regulation reference in the current control block, the controller operates in
frequency modulation mode to control the output current at any output voltage at or below the voltage regulation
target as long as the auxiliary winding can keep VDD above the UVLO turn-off threshold.
Figure 16. Transformer Currents
(6)
Figure 17. Typical Target Output V-I Characteristic
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