Datasheet
SLUU155 – March 2003
16
Full-Featured –48-V Hot Swap Power Manager
4.2 Load Capacitors
Capacitor patterns C8 and C9 are available on the EVM for installation of components to represent the module
input bulk capacitance; i.e., the load capacitance seen by the hot swap interface circuit. As supplied from the
factory, the EVM contains a 100-µF aluminum electrolytic installed at C8. Further customization to approximate
the user’s application can be done using either C8 or C9. When installing capacitors in these mounting locations,
care should be taken to observe the polarity marking on the PCB silkscreen, and to use appropriately rated
capacitors for voltage withstanding. Generally, telecom applications should use 100-V minimum rated
capacitors.
Banana jacks J3 and J4 are also connected across the output terminals, in parallel with C8 and C9. These jacks
can be used to connect additional loads to the EVM board.
4.3 Changing the Current Limit Threshold
During power-up of a plug-in card, the TPS2392 and TPS2393 limit the peak inrush current drawn by the
discharged bulk capacitance. The LCA senses load current as the drop across an external sense resistor.
Current is regulated by slewing the gate of the pass FET to maintain the voltage drop at an internally set level,
nominally 40 mV. Therefore, the peak current level can be established by selecting the appropriate sense
resistor value. On the –48-V Hot Swap EVM, this resistor is R4. The default value of R4 is 20 mΩ. To modify
the current limit threshold, a new sense resistor value can be determined from Equation 1.
R4 v
VMAX
IMAX
where:
• VMAX is the sense voltage limit, and
• IMAX is the desired current limit threshold.
Using the device minimum value of 33 mV for VMAX along with the required minimum load current ensures that
minimum amount of current can always be supplied to the load. For example, a particular line card is expected
to draw a maximum of 1.2 A, when the power bus is at its operating minimum level of –33 V, once the card is
powered up and operating normally. For this load characteristic, a sense resistor value less than 33 mV/1.2 A,
or 27 mΩ, would be selected. A 25-mΩ resistor is generally the closest standard value readily available; smaller
values also work, but with a corresponding increase in the maximum current limit.
(1)