Datasheet

SLUU171A - September 2003
16
TPS2350 −48-V Hot Swap/Supply Selector Evaluation Module
In the Figure 6 plot, the TPS2350 is switchng from the A supply to the B supply when the initially higher-potential
A supply lead is rapidly pulled from the input jack. At the time the lead is disconnected, current from that source
is interrupted. The bulk capacitor holds up the sense node voltage momentarily (seen here on the VOUT+ trace);
however, the 1-A load eventually causes it to decay below the magnitude of the B supply, which was 40 V. At
this time, the A-channel FET gate is rapidly pulled low, and the B-channel is turned on.
4.4 Changing the Current Limit Threshold
During power-up of a plug-in card, the TPS2350 limits 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 42 mV.
Therefore, the peak current level can be established by selecting the appropriate sense resistor value. On the
EVM, this resistor is R5. The default value of R5 is 20 mΩ. To modify the current limit threshold, a new sense
resistor value can be determined from Equation 1.
R5 +
V
MAX
I
MAX
where:
− VMAX is the sense voltage limit, and
− IMAX is the desired current limit threshold.
Using the device minimum value of 34 mV for VMAX along with the required minimum load current will ensure
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 2.5 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
34 mV/2.5 A, or 13 mΩ, would be selected. A 10-mΩ resistor is generally the closest standard value readily
available; smaller values also work, but with a corresponding increase in the maximum current limit.
4.5 Changing the Inrush Slew Rate
The TPS2350 also features 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 RAMP and SOURCE pins. The EVM comes equipped with three preset capacitor values,
selectable either individually or in combination 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. Supply Selector 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
(1)