Datasheet

SLUU155 – March 2003
17
Full-Featured –48-V Hot Swap Power Manager
4.4 Changing the Inrush Slew Rate
The TPS2392 and TPS2393 also feature slew rate limiting as current is ramped to charge the load capacitance.
The slew rate is easily programmed, once the sense resistor is determined, with a small-value capacitor
connected between the IRAMP and –VIN pins. The EVM comes equipped with three preset capacitor values,
selectable either individually or combined by closing the appropriate DIP switches of SW1. The default values
of the capacitors, and the corresponding nominal slew rates, are given in Table 7.
Table 7. –48-V Hot Swap EVM Default Slew Rates
SW1 DIP REF DES INSTALLED VALUE SLEW RATE (A/S)
1 C1 1000 pF 5000
2 C2 0.01 µF 500
3 C3 0.047 µF 106
The EVM can be used to get an illustration of the relationship between current limit, inrush slew rate, load values,
and the circuit’s fault timing requirements. With DIP switch SW1–1 only closed, the fastest of the preset slew
rates is selected, and only the hard-wired timing capacitor C6 is connected to the TPS2392 or TPS2393
controller. However, this is sufficient to allow the bulk capacitor C8 to fully charge, from 0 volts, across the full
range of input supply voltages, down to –80 V. This can be observed by connecting input power as shown in
Figure 5, displaying the VOUT– node on an oscilloscope, and enabling the device.
To observe the controller response to a load that does not charge up as expected (a shorted or otherwise
excessive load), set switches SW1–1, SW1–2, and SW1–3 to the ON position. This greatly reduces the inrush
(load charging) current slew rate at turn-on, with a corresponding increase in the amount of time needed to
successfully charge the intended load. Increase the supply level to about 60 V to 80 V, and again enable the
device. In this case, the voltage ramp time is excessively long relative to the programmed fault timer; the
controller times out and turns off the load (See Note below). This can be seen from the illumination of the red
LED. (For the TPS2393, the LED may flash briefly then turn off, indicating capacitor charging ultimately
completed on a successive retry.) If this combination represented the parameters of the target plug-in module,
then the timing capacitance of C6 and C4 (SW1–4 closed) would be more appropriate. The intended load, in
this case, the 100-µF capacitor, can again be charged up over the input voltage range.
NOTE: Due to tolerances of various EVM parameters, some units may not fault out under these
conditions. Generally, this is due to the fact that the amount of voltage ramping during the
reduced–rate turn–on period varies from device to device. Some units may be able to charge the
load almost completely during this period, when fault timing is inhibited. A more severe load fault
is needed to view the fault response. Additional capacitance, or even a resistor, can be connected
across the VOUT terminals, J3(+) and J4(–) or at C9. If the user is confident the module is operating
correctly, the load can also be shorted out to do this.
The inrush slew rate can be changed, to better match the application requirement, by replacing any capacitor
C1, C2 or C3. The PCB patterns are sized for 0805 ceramic chip capacitors. Use equation 2 to calculate the
new ramp capacitor, C
RAMP
, value in microfarads.
C
RAMP
+
11
100 R4
ǒ
di
dt
Ǔ
MAX
where:
• R4 is the selected sense resistor value, in ohms, and
• (di/dt)
MAX
is the desired maximum slew rate, in amps/second.
(2)