SLUU150 – February 2003
14
Evaluation Modules for Simple –48-V Hot Swap Controllers
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 between approximately 60 V and 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
(1)
. This can be seen from the illumination of the red FAULTED
LED. (When evaluating the TPS2391, 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 C9 and C4 (SW1–4
closed) would be more appropriate. The intended load, in this case, the 100-
µ
F capacitor, can
again be charged up over the input voltage range.
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
•
(di/dt)
MAX
is the desired maximum slew rate, in amperes/second.
4.5
Fault Timing with the TPS2390/TPS2391
Whenever the hot swap controller is limiting current to the load, an on-chip timer is monitoring
this operation against an established time limit. The timeout period is generated by the
constant-current charging of a capacitor at the FLTTIME pin. If current regulation ceases prior to
expiration of the timer, the capacitor is discharged, and normal steady-state operation of the load
either starts or resumes. However, if the timer expires, then the pass FET is turned off, disabling
power to the load, and the FAULT output is asserted.
On the –48-V hot swap EVM, several capacitor patterns are provided for increasing or otherwise
modifying the timeout period. Capacitor C9 is hard-wired to the device FLTTIME pin, and
provides a minimum fault timer for the default load. C4 and C5 can be switched into the circuit
via DIP switches SW1–4 and SW1–5, respectively. The EVM ships from the factory with a
0.1-
µ
F capacitor installed at C4; C5 is not populated for easier subsequent user modification as
required.
(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, J3(+) and J4(–) or at C7. If the user is confident the module
is operating correctly, the load can also be shorted out to do this.
(2)
