Datasheet

 
SLUS544E − SEPTEMBER 2003 − REVISED MARCH 2009
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
Ŧ
(
R2 ) R3
)
V4 + 1.26
(
R1 ) R2 ) R3
)
R3
V3 + V4 * 1.26
ǒ
R1
R4
Ǔ
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:
• V1 = 32.0 V
• V2 = 34.0 V
• V3 = 83.0 V
• V4 = 84.7 V
Then,
• R1 = 976 kΩ
• R2 = 24.9 kΩ
• R3 = 15.0 kΩ
and
• R4 = 604 kΩ
(1)
(2)
(3)
(4)