Datasheet

UCC28700, UCC28701
UCC28702, UCC28703
SLUSB41 – JULY 2012 www.ti.com
VDD Capacitance, C
DD
The capacitance on VDD needs to supply the device operating current until the output of the converter reaches
the target minimum operating voltage in constant-current regulation. At this time the auxiliary winding can sustain
the voltage to the UCC28700. The total output current available to the load and to charge the output capacitors is
the constant-current regulation target. The equation below assumes the output current of the flyback is available
to charge the output capacitance until the minimum output voltage is achieved. There is an estimated 1 mA of
gate-drive current in the equation and 1 V of margin added to VDD.
(25)
VDD Start-Up Resistance, R
STR
Once the VDD capacitance is known, the start-up resistance from V
BULK
to achieve the turn-on time target can be
determined.
(26)
VS Resistor Divider, Line Compensation, and Cable Compensation
The VS divider resistors determine the output voltage regulation point of the flyback converter, also the high-side
divider resistor (R
S1
) determines the line voltage at which the controller enables continuous DRV operation. R
S1
is initially determined based on transformer auxiliary to primary turns ratio and desired input voltage operating
threshold.
(27)
The low-side VS pin resistor is selected based on desired V
O
regulation voltage.
(28)
The UCC28700 can maintain tight constant-current regulation over input line by utilizing the line compensation
feature. The line compensation resistor (R
LC
) value is determined by current flowing in R
S1
and expected gate
drive and MOSFET turn-off delay. Assume a 50-ns internal delay in the UCC28700.
(29)
On the UCC28700 which has adjustable cable compensation, the resistance for the desired compensation level
at the output terminals can be determined using the equation below.
(30)
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Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703