UCC2897
SLUS591A − NOVEMBER 2003 − REVISED MARCH 2004
DETAILED PIN DESCRIPTIONS (continued)
VDD (pin 17)
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
capacitor (C
HF
) must be placed very closely between the VDD pin and PGND (pin 11) to provide adequate
filtering. The recommended C
HF
value is 1-µF for most applications but its value might be affected by the
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
storage capacitor (C
BIAS
) connected parallel to C
HF
. The energy storage capacitor must provide the hold up time
to operate the UCC2897 (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 18)
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 UCC2897. The unique property of the
UCC2897 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 comparator
by an external resistor divider (R
IN1
, R
IN2
). Once the line monitor’s input threshold is exceeded, an internal
current source gets connected to the LINEUV pin. The current generator is programmed by the R
DEL
resistor
connected to pin 1 of the controller. The actual current level is given as:
I
HYST
+
2.5 V
R
DEL
0.05
(7)
As this current flows through R
IN2
of the input divider, the undervoltage lockout hysteresis is a function of I
HYST
and R
IN2
allowing accurate programming of the hysteresis of the line monitoring circuit.
For more information on how to program the line monitoring function refer to the
Setup Guide
of this datasheet.
LINEOV (pin 19)
In the UCC2897 controller the high-voltage start-up device is not utilized thus pin 16 is used for a different
function. This input monitors the incoming power source to provide an accurate overvoltage protection with user
programmable hysteresis for the power supply controlled by the controller. The circuit implementation of the
overvoltage protection function is identical to the technique used for monitoring the input power rail for
undervoltage lockout. This allows implementing an accurate threshold and hysteresis using only one pin. The
input voltage of the power supply is scaled to the precise 1.27-V threshold of the overvoltage protection
comparator by an external resistor divider (R
IN3
, R
IN4
). Once the line monitor’s input threshold is exceeded, an
internal current source gets connected to the LINEOV pin. The current generator is programmed by the R
DEL
resistor connected to pin 1 of the controller. The actual current level is given as:
I
HYST
+
2.5 V
R
DEL
0.05
(8)
As this current flows through R
IN4
of the input divider, the overvoltage protection hysteresis is a function of I
HYST
and R
IN4
allowing accurate programming of the hysteresis of the line monitoring circuit.
For more information on how to program the overvoltage protection, refer to the
Setup Guide
of this datasheet.
www.ti.com
11
TI [ TEXAS INSTRUMENTS ]
