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HV100X Datasheet(PDF) 4 Page - Supertex, Inc |
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HV100X Datasheet(HTML) 4 Page - Supertex, Inc |
4 / 5 page 4 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 VNN through CLOAD, CRSS 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/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 ISS 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 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. 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 NN. In this case the 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 |
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