Datasheet

V
DRV
V
DS
UCC28700, UCC28701
UCC28702, UCC28703
SLUSB41 – JULY 2012 www.ti.com
Valley-Switching
The UCC28700 utilizes valley-switching to reduce switching losses in the MOSFET, to reduce induced-EMI, and
to minimize the turn-on current spike at the sense resistor. The controller operates in valley-switching in all load
conditions unless the V
DS
ringing has diminished.
Referring to Figure 18 below, the UCC28700 operates in a valley-skipping mode in most load conditions to
maintain an accurate voltage or current regulation point and still switch on the lowest available V
DS
voltage.
Figure 18. Valley-Skipping Mode
Start-Up Operation
Upon application of input voltage to the converter, the start-up resistor connected to VDD from the bulk capacitor
voltage (V
BLK
) charges the VDD capacitor. During charging of the VDD capacitor the device bias supply current is
less than 1.5 µA. When VDD reaches the 21-V UVLO turn-on threshold, the controller is enabled and the
converter starts switching. The initial three cycles are limited to I
PP(min)
. This allows sensing any initial input or
output faults with minimal power delivery. After the initial three cycles at minimum I
PP(min)
, the controller responds
to the condition dictated by the control law. The converter remains in discontinuous mode during charging of the
output capacitor(s), maintaining a constant output current until the output voltage is in regulation.
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Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703