UCC2891, UCC2892

UCC2893, UCC2894

SLUS542A − OCTOBER 2003 − REVISED JULY 2004

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DETAILED PIN DESCRIPTIONS (continued)

OUT (pin 13)

This high-current output drives an external N-channel MOSFET. Each controller in the UCC2891 family uses

active high drive signals for the main switch of the converter.

Due to the high speed and high-drive current capability of these outputs (AUX, OUT) the parasitic inductance

of the external circuit components connected to these pins should be carefully minimized. A potential way of

avoiding unnecessary parasitic inductances in the gate drive circuit is to place the controller in close proximity

to the MOSFETs and by ensuring that the outputs (AUX, OUT) and the gates of the MOSFET devices are

connected by wide, overlapping traces.

VDD (pin 14)

The VDD rail is the primary bias for the internal, high-current gate drivers, the internal 5-V bias regulator and

for parts of the undervoltage lockout circuit. To reduce switching noise on the bias rail, a good quality ceramic

properties of the external MOSFET transistors used in the power stage.

In addition to the low-impedance, high-frequency filtering, the controller’s bias rail requires a larger value energy

to operate the UCC2891 family (including gate drive power requirements) during start up. In steady state

operation the controller must be powered from a bootstrap winding off the power transformer or by an auxiliary

bias supply. In case of an independent auxiliary bias supply, the energy storage is provided by the output

capacitance of the bias supply.

LINEUV (pin 15)

This input monitors the incoming power source to provide an accurate undervoltage lockout function with user

programmable hysteresis for the power supply controlled by the UCC2891 family. The unique property of the

UCC2891 family is to use only one pin to implement these functions without sacrificing on performance. The

input voltage of the power supply is scaled to the precise 1.27-V threshold of the undervoltage lockout

resistor connected to pin 1 of the controller. The actual current level is given as:

I

2.5 V

R

DEL

0.05

For more information on how to program the line monitoring function refer to the Setup Guide of this datasheet.

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