Datasheet

4
0 2 0 75
StraightWire
L
L . L ln . nH
D
æ ö
» ´ ´ -
ç ÷
è ø
SPIKE NOM LOAD
L
V V I
C
= +
TPS2359
SLUS792H –FEBRUARY 2008–REVISED MAY 2013 www.ti.com
Transient Protection
The need for transient protection in conjunction with hot-swap controllers should always be considered. When
the TPS2359 interrupts current flow, input inductance generates a positive voltage spike on the input and output
inductance generates a negative voltage spike on the output. The following equation estimates the magnitude of
these voltage spikes:
(10)
where V
NOM
equals the nominal supply voltage, I
LOAD
equals the load current, C equals the capacitance present
at the input or output of the TPS2359, and L equals the effective inductance seen looking into the source or the
load. The inductance due to a straight length of wire equals approximately
(11)
where L equals the length of the wire and D equals its diameter.
If sufficient capacitance to prevent transients from exceeding the absolute ratings of the TPS2359 cannot be
included the application will require the addition of transient protectors.
Output Protection Considerations for MicroTCA Power Systems
MicroTCA power systems have particular transient protection requirements because of the basic power
architecture. Traditional protection methods must be adjusted to accommodate these systems where the supplies
are OR’ed together after the inrush control and current limit circuits. However, minor changes to some standard
techniques will yield very good results.
Unlike systems which have hotswap/inrush control at the load, uTCA power modules and their hot swap circuitry
are often a significant distance (up to 1 m of trace length, two way) from the load module. Even with the best
designed backplanes this distance results in stray inductance which will store energy while current is being
delivered to the load. The inductive energy can cause large negative voltage spikes at the power module output
when the current is switched off under load. The spikes become especially severe when the channel shuts off
due to a short circuit, which drives the current well above normal levels just before shut off.
The lowest voltage allowed on the device pins is -0.3 V. If a transient makes a pin more negative than -0.3 V the
internal ESD Zener diode attached to the pin will become forward biased and current will be conducted across
the substrate to the ground pins. This current may disrupt normal operation or, if large enough, damage the
silicon. Typical protection solutions involve capacitors, TVSs ( Transient Voltage Suppressors ) and/or a Schottky
diode to absorb the energy which appears at the power module output in the form of a large negative voltage
spike.
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