UCC28220, UCC28221

SLUS544C − SEPTEMBER 2003 − REVISED OCTOBER 2007

9

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APPLICATION INFORMATION

LINE Over Voltage and Under Voltage

Three pins are provided to turn-off the output drivers and reset the soft-start capacitor when the converter input

voltage is outside a prescribed range. The under-voltage set point and under-voltage hysteresis are accurately

set via external resistors. The over-voltage set point is also accurately set via a resistor ratio, but the hysteresis

is fixed by the same resistor that sets the u*nder-voltage hysteresis.

Figure 1 and 2 show detailed functional diagram and operation of the under voltage lockout (UVLO) and

over-voltage lockout (OVLO) features. The equations for setting the thresholds defined in Figure 2 are:

V1

+ 1.26

R1

(R2

) R3)

) 1.26

V2

+ 1.26

(R1

) Rx)

Rx

, where Rx

+ R4 (R2 ) R3)

V4

+ 1.26

(R1

) R2 ) R3)

R3

V3

+ V4 * 1.26

R1

R4

The UVLO hysteresis and the OVLO hysteresis can then be calculated as V2 − V1 and V4 − V3, respectively.

By examining the design equations it becomes apparent that the value of R4 sets the amount of hysteresis at

both thresholds. By realizing this fact, the designer can then set the value of R4 based on the most critical

hysteresis specification either at high line or at low line. In most designs the value of R4 will be determined by

the desired amount of hysteresis around the UVLO threshold. As an example consider a telecom power supply

with the following input UVLO and OVLO design specifications:

Then,

and

(1)

(2)

(3)

(4)