Datasheet
SLUS610 − JULY 2004
17
www.ti.com
DETAILED DESCRIPTION
UDG−20118
4 s
50 A
0.4 A
+
+
0.5V
4V
SQ
QR
FAULT
LOGIC
4
6
FAULT
FLTTIME
RETRY
S
R
RESET
OL
OC
ON
EN
Figure 24. Fault Timer Block Operation
The fault logic within the timer block automatically manages capacitor charge and discharge rates (RESET
signal), and the operational status of other device-internal circuits (ON signal). The FAULT
output remains
asserted continuously during retry mode; it is only released if the fault condition clears.
The TPS2393A also features a fast-acting overload comparator which clamps large current transients from
catastrophic faults occurring once the pass FET is fully enhanced, such as short circuits. This function provides
a back-up protection to the LCA by producing a hard gate discharge action when the LCA is saturated and the
pass FET is fully enhanced. If sense voltage excursions above 100 mV are detected, this comparator rapidly
pulls down the GATE output, overriding the response of the LCA, and bypassing the fault timer, to terminate
the short-circuit condition. Only a 4-µs deglitch filter is applied to the OVERLOAD signal to help reduce the
occurrence of nuisance trips. However, overload faults are not immediately latched in the device. Instead, once
the spike has been brought down below the 100-mV threshold, the pull-down is released, returning control to
the LCA. The FET is turned on again in either current ramp or current limit mode. Now, with load current once
again under closed-loop control, fault timing is initiated. This permits the persistence of the fault to be assessed
prior to fully interrupting the load.
Other noise events within the system can also produce large current spikes. For example, the sudden
switchover in a diode-OR circuit to a supply of greater voltage potential may generate transients. Also, the
temporary dropout and sudden reapplication of input power can cause a surge of current to plug-in cards.
Generally, these are brief transients, and not associated with a load fault. However, the sudden inrush of current
to charge the module bulk capacitance to the new supply level appears as a load fault to the hot swap controller.
The TPS2393A transient response addresses this issue by providing rapid circuit-breaker protection against
true load faults, along with minimal interruption of power flow during other supply noise events.
In order for downstream loads (bricks, etc.) to operate through such power bus disturbances, it is important to
properly size the filtering capacitance to supply the needed energy during the OFF-time of the pass FET.
Sufficient capacitance should be provided to supply the converters, at full anticipated load, for the 50 to 200 µs
period during which the FET gate is below its ON threshold. The length of the actual OFF-time is dependent
on several factors, including the FET input capacitance, FET threshold voltage, and the size of the ramp
capacitor.