Datasheet

SLUU171A - September 2003
15
TPS2350 −48-V Hot Swap/Supply Selector Evaluation Module
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 J4 (VOUT+) and J5 (VOUT−) 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 Supply Selector
The TPS2350 device monitors the input supply voltages at −48V_INA and −48V_INB. Using two external
N-channel MOSFETs, it selects the supply of larger magnitude for applying power to the back-end plane (the
load). Because of this capability, a TPS2350 circuit can be a lower-loss alternative to diodes in traditional
diode-OR telecom applications. On the EVM, the selection FETs are Q2 (for supply A) and Q3 (for supply B).
Hysteresis of the supply comparison is set internally to a nominal 400 mV. In order for the TPS2350 to switch
over to the deselected supply, its potential must become 400 mV more negative than the supply currently being
used.
The selection function can be exercised under various conditions. The voltage levels of the two supplies can
be adjusted relative to each other to generate switching events. The supplies can be turned off and back on using
their power switches, or they can even be plugged into and removed from their respective jacks J2 and J3. The
selection action can easily be viewed at the gates of the selection FETs, TP9 for the A supply, and TP8 for the
B supply. Also, by applying a steady-state load at the output terminals, current probes can be used on the bench
supply leads to monitor which supply is delivering power as input conditions are varied. And of course, the load
voltage should reflect the magnitude of the selected supply. The scope plot of Figure 6 shows an example of
what can be observed using the EVM.
NOTE:
The FET PCB patterns Q4, Q5, Q6, Q7, and certain associated components, are not used on this
EVM.
t − Time −250 µs/div
CH. 3
10 V/div
CH. 2
10 V/div
CH. 1
10 V/div
VOUT+
GATB
GATA
CH. 4
500 mA/div
I
INA
40 V
48 V
Turn off
A FET
Supply A stops
conducting when
lead pulled.
Select supply B
Figure 6. Switching From −48V_INA to −48V_INB Supply