HV100/HV101
Application Information
Turn On Clamp
Hotswap controllers using a MOSFET as the pass element all
include a capacitor divider from V
PP
to V
NN
through C
LOAD
, C
RSS
and C
GS
. In most competitive solutions a large external capacitor
is added to the gate of the pass element to limit the voltage on
the gate resulting from this divider. In those instances, if a gate
capacitor is not used the internal circuitry is not available to
hold off the gate, and therefore a fast rising voltage input will
cause the pass element to turn on for a moment. This allows
current spikes to pass through the MOSFET.
The HV100 and HV101 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 1µs (100’s of ns). In 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. In 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.
Short Circuit Protection
The HV100 and HV101 provide short circuit protection by shut-
ting down if the Miller Effect associated with hotswap does not
occur. Specifically, if the output is shorted then the gate will
rise without exhibiting a “flat response”. Due to the fact that we
have normalized the hotswap period for any pass element, a
timer can be used to detect if the gate voltage rises above a
threshold within that time, indicating that a short exists. The
diagram below shows a typical turn on sequence with the load
shorted, resulting in a peak current of 4A.
Auto-adapt Operation
The HV100 and HV101 auto-adapt mechanism provides an
important function. It normalizes the hotswap period regardless
of pass element or load capacitor for consistent hotswap results.
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 100mΩ resistor inserted between source and V
NN
. In this
case the maximum current is 25% smaller.
The above diagram illustrates the effectiveness of the auto-
adapt mechanism. In this example three MOSFETs with dif-
ferent C
ISS
and R
DSON
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
constant multiplied by C
LOAD
.
Typically if MOSFETs of the same type are used, the hotswap
results will be extremely consistent. If different types are used
they will usually exhibit minimal variation.
5
For most applications and pass elements, the HV100 and
HV101 provides adequate limiting of the maximum current to
prevent damage without the need for any external components.
The 2.5s delay of the auto-retry circuit provides time for the
pass element to cool between attempts.
SUPERTEX [ Supertex, Inc ]
