Datasheet

SLUU171A - September 2003

TPS2350 −48-V Hot Swap/Supply Selector Evaluation Module

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 TPS2350 controller. However,

this is sufficient to allow the bulk capacitor C8 to fully charge, from 0 V, across the full range of input supply

voltages, down to −80 V. This can be observed by connecting input power as shown in Figure 4, 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 72 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.

(1)

This can be seen from the illumination of the red LED. (The LED

may only flash briefly then turn off, indicating load 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 on the first attempt, for supply values across the input voltage range.

NOTE: (1)

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 will vary 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, J4(+) and J5(−) or at C9. If the

user is confident the module is operating correctly, the load can also be shorted out to do

this demonstration.

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.

RAMP

11.3

100 R5

dińdt

MAX

where:

− R5 is the selected sense resistor value, in ohms, and

− (di/dt)

MAX

is the desired maximum slew rate, in amps/second.

(2)