Datasheet

SLUU155 – March 2003
19
Full-Featured –48-V Hot Swap Power Manager
Table 8. Nominal UVLO and OVLO settings.
PARAMETER DESCRIPTION VALUE (V)
V
UV_L
UVLO threshold, supply low (V
IN
< V
UV
) 32.8
V
UV_H
UVLO threshold, supply high (V
IN
> V
UV
) 30.8
V
OV_L
OVLO threshold, supply low (V
IN
< V
OV
) 72.6
V
OV_H
OVLO threshold, supply high (V
IN
> V
OV
) 70.5
The thresholds are easily modified by changing the resistor values. When the desired trip voltages and the UV
hysteresis have been established for the protected load, new values are determined as follows. Generally, the
process is simplest by first selecting the top leg of the divider (R1) needed to obtain the desired hysteresis. This
value is calculated from Equation 4.
R1 +
V
HYS_UV
10 mA
where V
HYS_UV
is the amount of undervoltage hysteresis.
Once a value for R1 is selected, it is used to calculate R2 and R3 using Equations 5 and 6.
R2 +
1.4 R1
ǒ
V
UV_L
* 1.4
Ǔ
ȧ
ȱ
Ȳ
1 *
V
UV_L
ǒ
V
OV_L
) 10
*5
R1
Ǔ
ȧ
ȳ
ȴ
R3 +
1.4 R1
ǒ
V
UV_L
* 1.4
Ǔ
ȧ
ȱ
Ȳ
V
UV_L
ǒ
V
OV_L
) 10
*5
R1
Ǔ
ȧ
ȳ
ȴ
where:
• V
UV_L
is the UVLO threshold when the input supply is low; i.e., less than V
UV
, and
• V
OV_L
is the OVLO threshold when the input supply is low; i.e., less than V
OV
4.7 TPS2392/93 Powergood Output (PG)
The –48-V hot swap EVM contains two possible circuits for interfacing to the device PG output pin. The outputs
of these circuits are available at the PCB headers J6 and J7 as the EN1
and EN2 signals, respectively. An
example application of these signals is to drive the enable input of downstream converters. As such, both
outputs are referenced to the VOUT– node of the EVM, as this would be the low side of input power to the brick.
The EN1
output is generated via the resistive translation of the device PG output. The EVM schematic of
Figure 2 shows the circuit details. This active-low output is capable of sinking a minimum of 10 µA at a maximum
output voltage of 0.8 V(TTL–compatible V
OL
), across the complete range of the 48-V input supply and ambient
operating temperature. (The default UVLO and OVLO thresholds are assumed.)
The EN2 output demonstrates an isolated signal implementation using an opto device. The EVM schematic
shows the circuit details. This active-low output is capable of sinking a minimum of 150 µA across the complete
range of input supply voltage and ambient operating temperature.
The use of these enable signals requires a pull-up source at the VS input, J5. See Tables 2 and 3 for the
requirements of the VS supply.
(4)
(5)
(6)