4

HV100/HV101

Application Information

Turn On Clamp

Hotswap controllers using a MOSFET as the pass element all

include a capacitor divider from V

and C

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/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.

Auto-Adapt Operation

The HV100/HV101 Auto-adapt mechanism provides an impor-

tant 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 above diagram illustrates the effectiveness of the auto-

adapt mechanism. In this example three MOSFETs with different

C

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

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.

Short Circuit Protection

The HV100/HV101 provide short circuit protection by shutting

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 normal-

ized 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.

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

maximum current is 25% smaller.

For most applications and pass elements, the HV100/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.

2A/div

NTE66 is a trademark of NTE Electronics

IRF530 is a trademark of International Rectifier Corporation

IRF120M is a trademark of International Rectifier Corporation