Datasheet
R1
200 kΩ
1 %
R2
4.99 kΩ
1 %
R3
3.92 kΩ
1 %
R9
R2
R1
R8
GND
-48V
GND
-48V
SOURCE
RTN
TPS2394
(1)
UV
OV
1
7
3
4
SOURCE
RTN
TPS2394
(1)
UV
OV
1
7
3
4
V
UV_L
=
R1 + R2 + R3
R2 R3
x V
THUV
V
OV_L
=
R1 + R2 + R3
R3
x V
THOV
-I
HYSUV
x R1
V
UV_L
=
R1 + R2
R2
x V
THUV
V
OV_L
=
R8 + R9
R9
x V
THOV
(a) (b)
HYS_UV
V
R1 =
10 μA
( )
( )
-
é ù
´
ê ú
´ -
ê ú
-
´
ê ú
ë û
UV_L
5
UV_L
OV_L
V
1.4 R1
R2 = 1
V 1.4
V + 10 R1
TPS2394
www.ti.com
SLVSAA9 –AUGUST 2010
Continuing this calculation example, using a 220-µF input capacitor (C
LOAD
), Equation 3 and Equation 4 estimate
the load voltage ramping to approximately –45 V during the soft-start period. If the module should operate down
to –72-V input supply, approximately another 1.4 ms of constant-current charging may be required. Therefore,
Equation 6 and Equation 8 are used to determine C
FLT(MIN)
, and the result is approximately 0.039-µF.
Setting the Undervoltage and Overvoltage Thresholds
The UV and OV 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 GAT pin is held low, disconnecting power from the
load, and the PG output is deasserted. When input voltage is within the UV/OV window, the GAT pin 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.
Note (1): Additional details omitted for clarity.
Figure 10. Programming the Undervoltage and Overvoltage Thresholds
The UV and OV thresholds can be individually programmed with a three-resistor divider connected to the
TPS2394 as shown in the typical application diagram, and again in Figure 10a. 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. First, select the top leg of the divider (R1 in the diagram) to obtain the
threshold hysteresis. This value is calculated using Equation 9.
(9)
Where:
V
HYS_UV
is the undervoltage hysteresis value
For example, assume the typical application design targets have been set to undervoltage turn-on at 33 V (input
supply rising), turn-off at 31 V (input voltage falling), and overvoltage shutdown at 72 V. Then Equation 9 yields
R1 = 200 kΩ for the 2-V hysteresis. Once the value of R1 is selected, it is used to calculate resistors R2 and R3.
(10)
Copyright © 2010, Texas Instruments Incorporated 13
Product Folder Link(s): TPS2394