HV100/HV101
Application Information
Turn On Clamp
Short Circuit Protection
Hotswap controllers using a MOSFET as the pass element all TheHV±11andHV±1±provideshortcircuitprotectionbyshut-
include a capacitor divider from VPP to VNN through C
, C
ting down if the Miller Effect associated with hotswap does not
andC .ꢁnmostcompetitivesolutionsalargeexternalLcOaApDaciRtoSSr occur. Specifically, if the output is shorted then the gate will
is addGeSd to the gate of the pass element to limit the voltage on risewithoutexhibitinga“flatresponse”. Duetothefactthatwe
thegateresultingfromthisdivider. ꢁnthoseinstances, ifagate have normalized the hotswap period for any pass element, a
capacitor is not used the internal circuitry is not available to timer can be used to detect if the gate voltage rises above a
hold off the gate, and therefore a fast rising voltage input will threshold within that time, indicating that a short exists. The
cause the pass element to turn on for a moment. This allows diagram below shows a typical turn on sequence with the load
current spikes to pass through the MOSFET.
shorted, resulting in a peak current of 4A.
The HV±11 and HV±1± include a built-in clamp to ensure
that this spurious current glitch does not occur. The built-in
clamp will work for the time constants of most mechanical
connectors. There may be applications, however, that have
rise times that are much less than ±µs (±11’s of ns). ꢁn these
instances it may be necessary to add a capacitor from the
MOSFET gate to source to clamp the gate and suppress
this current spike. ꢁn these cases the current spike generally
contains very little energy and does not cause damage even
if a capacitor is not used at the gate.
Auto-adapt Operation
The HV±11 and HV±1± auto-adapt mechanism provides an
importantfunction.ꢁtnormalizesthehotswapperiodregardless
ofpasselementorloadcapacitorforconsistenthotswapresults.
By doing this it allows the novel short circuit mechanism to
work because the mechanism requires a known time base.
The maximum current that may occur during this period can
be controlled by adding a resistor in series with the source of
the MOSFET. The lower graph shows the same circuit with
a ±11mΩ resistor inserted between source and VNN. ꢁn this
case the maximum current is 25% smaller.
The above diagram illustrates the effectiveness of the auto-
adapt mechanism. ꢁn this example three MOSFETs with dif-
ferent CꢁSS and RDSON values are used. The top waveform is
the hotswap current, while the bottom waveform is the gate
voltage. As can be seen, the hotswap period is normalized,
the initial slope of the gate voltage is approximately 2.5V/ms
regardless of the MOSFET, and the total hotswap period and
peak currents are a function of a MOSFET type dependent
For most applications and pass elements, the HV±11 and
HV±1± provides adequate limiting of the maximum current to
preventdamagewithouttheneedforanyexternalcomponents.
The 2.5s delay of the auto-retry circuit provides time for the
pass element to cool between attempts.
constant multiplied by CLOAD
.
Typically if MOSFETs of the same type are used, the hotswap
results will be extremely consistent. ꢁf different types are used
they will usually exhibit minimal variation.
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