Datasheet

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
SLUS610 − JULY 2004
21
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APPLICATION INFORMATION
Setting the Undervoltage and Overvoltage Thresholds
The UVLO and OVLO pins can be used to set the undervoltage (V
UV
) and overvoltage (V
OV
) thresholds of the
hot swap circuit. When the input supply is below V
UV
or above V
OV
, the GATE pin is held low, disconnecting
power from the load, and deasserting the PG
output. When input voltage is within the UV/OV window, the GATE
drive is enabled, assuming all other input conditions are valid for turn-on.
Threshold hysteresis is provided via two internal sources which are switched to either pin whenever the
corresponding input level exceeds the internal 1.4-V reference. The additional bias shifts the pin voltage in
proportion to the external resistance connected to it. This small voltage shift at the device pin is gained up by
the external divider to input supply levels.
UDG−20119
R8
R2
R1 R7
GND
−48V
GND
−48V
−VIN
RTN
TPS2393A*
UVLO
OVLO
1
14
11
8
−VIN
RTN
TPS2393A*
UVLO
OVLO
1
14
11
8
R1
200 kΩ
1%
R2
4.99 kΩ
1%
R3
3.92 kΩ
1%
(a)
(b)
V
OV_L
+
R1 ) R2 ) R3
R3
V
REF
* I
SRC_UV
R1
V
UV_L
+
R1 ) R2 ) R3
R2 ) R3
V
REF
V
OV_L
+
R7 ) R8
R8
V
TH_OV
V
UV_L
+
R1 ) R2
R2
V
TH_UV
*Additional details omitted for clarity.
Figure 25. Programming the Undervoltage and Overvoltage Thresholds
The UV and OV thresholds can be individually programmed with a three-resistor divider connected to it as
shown in the typical application diagram, and again in Figure 27a. When the desired trip voltages and
undervoltage hysteresis have been established for the protected board, the resistor values needed can be
determined from the following equations. Generally, the process is simplest by first selecting the top leg of the
divider (R1 in the diagram) needed to obtain the threshold hysteresis. This value is calculated from equation (9).
R1 +
V
HYS_UV
10 mA
where:
D V
HYS_UV
is the undervoltage hysteresis value
(9)