UCC28180
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SLUSBQ5A –NOVEMBER 2013–REVISED NOVEMBER 2013
VCC Under-Voltage LockOut (UVLO)
UVLO
VCC
Auxiliary Supply
+
S
R
Q
Q
VCCON 11.5V
VCCOFF 9.5V
CDECOUPLE
GND
UVLO
+
Figure 25. UVLO
During startup, Under-Voltage LockOut (UVLO) keeps the device in the off state until VCC rises above the 11.5-
V enable threshold, VCCON. With a typical 1.7 V of hysteresis on UVLO to increase noise immunity, the device
turns off when VCC drops to the 9.5-V disable threshold, VCCOFF
.
If, during a brief AC-line dropout, the VCC voltage falls below the level necessary to bias the internal FAULT
circuitry, the UVLO condition enables a special rapid discharge circuit which continues to discharge the VCOMP
capacitors through a low impedance despite a complete lack of VCC. This helps to avoid an excessive current
surge should the AC-line return while there is still substantial voltage stored on the VCOMP capacitors. Typically,
these capacitors can be discharged to less than 1 V within 150 ms of loss of VCC.
Output Over-Voltage Protection (OVP)
There are two levels of OVP: When VSENSE exceeds 107% (VOVP_L) of the reference voltage, a 4-kΩ resistor
connects VCOMP to ground to rapidly discharge VCOMP. If VSENSE exceeds 109% (VOVP_H) of the reference
voltage, GATE output is disabled until VSENSE drops below 102% of the reference voltage.
Open Loop Protection/Standby (OLP/Standby)
If the output voltage feedback components were to fail and disconnect (open loop) the signal from the VSENSE
input, then it is likely that the voltage error amp would increase the GATE output to maximum duty cycle. To
prevent this, an internal pull-down forces VSENSE low. If the output voltage falls below 16.5% of its rated
voltage, causing VSENSE to fall below 0.82 V, the device is put in standby, a state where the PWM switching is
halted and the device is still on but draws standby current below 2.95 mA. This shutdown feature also gives the
designer the option of pulling VSENSE low with an external switch (standby function).
ISENSE Open-Pin Protection (ISOP)
If the current feedback components were to fail and disconnect (open loop) the signal to the ISENSE input, then
it is likely that the PWM stage would increase the GATE output to maximum duty cycle. To prevent this, an
internal pull-up source drives ISENSE above 0.085 V so that a detector forces a state where the PWM switching
is halted and the device is still on but draws standby current below 2.95 mA. This shutdown feature avoids
continual operation in OVP and severely distorted input current.
ICOMP Open-Pin Protection (ICOMPP)
If the ICOMP pin shorts to ground, then the GATE output increases to maximum duty cycle. To prevent this,
once ICOMP pin voltage falls below 0.2 V, the PWM switching is halted and the device is still on but draws
standby current below 2.95 mA .
FAULT Protection
VCC UVLO, OLP/Standby, ISOP and ICOMPP funtions constitute the fault protection feature in the UCC28180.
Under fault protection, VCOMP pin is pulled low and the device is in standby.
Copyright © 2013, Texas Instruments Incorporated
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