Datasheet

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SLUS542F − OCTOBER 2003 − REVISED JULY 2009
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DETAILED PIN DESCRIPTIONS (continued)
VIN (pin 16 − UCC2891 and UCC2893 only)
The UCC2891 and UCC2893 controllers are equipped with a high voltage, P-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
approximately 15-mA 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 13.5 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. When using the internal JFET for startup, the external load
on VDD must be limited to less than 4 mA.
For more information on biasing the UCC2891 family, refer to the Setup Guide and Additional Application
Information Sections of this datasheet.
LINEOV (pin 16 − UCC2892 and UCC2894 only)
In the UCC2892 and UCC2894 controllers 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
in Figure 7). 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
+
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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