Datasheet

UCC2897A
SLUS829D -- AUGUST 2008 -- REVISED JULY 2009
13
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DETAILED PIN DESCRIPTIONS (continued)
VIN
The UCC2897A controller is equipped with a high voltage, N-channel JFET start up device to initiate operation
from the input power source of the converter in applications where the input voltage does not exceed the 110-V
maximum rating of the start up transistor. In these applications, the VIN pin can be connected directly to the
positive terminal of the input power source. The internal JFET start up transistor provides charge current for the
energy storage capacitor (C
BIAS
) connected across the VDD (pin 14) and PGND (pin 11) terminals. Note that
the start up device is turned off immediately when the voltage on the VDD pin exceeds approximately 12.7 V,
the controller’s undervoltage lockout threshold for turn-on. The JFET is also disabled at all times when the
high-current gate drivers are switching to protect against excessive power dissipation and current through the
device. For dependable s tartup, VDD must not be loaded by more than 4 mA.
For more information on biasing the UCC2897A, refer to the Setup Guide and Additional Application Information
Sections of this datasheet.
LINEOV
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
in Figure 7). Once the line monitor’s input threshold is
exceeded, an internal current source gets c onnected 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
=
V
REF
2
×
1
R
DEL
× 0.05
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.
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