The LTC4354 negative voltage diode-OR controller is designed to operate with inputs of up to 100V. As shown in Figure 1, the maximum voltage between –48V, VA and –48V, VB is limited to 100V, and the voltage applied to either input is similarly limited to 100V relative to –48COM. A careful study of the LTC4354 data sheet reveals that the drain pins, DA and DB, which are the only pins exposed to high voltage, are limited to 80V. Nevertheless, with a 2k series limiting resistor, these pins can handle up to 100V.
If it were possible to further increase the series resistance, even higher voltages could be tolerated by simply changing the 2k resistor. Unfortunately the drain pin input bias current sets a practical limit of 2k, so as to avoid interfering with the operation of the ideal diode function itself.
In systems where the inputs are subjected to spikes in excess of 100V, MOSFET breakdown clamps the maximum voltage, although admittedly bereft of characterization and guarantee.
If spikes in excess of 100V are an issue, the high voltage capability of the drain pins is easily extended beyond 100V by simply adding a Zener clamp, as shown in Figure 2. Input spikes above 75V are clamped by the Zener, with current limiting provided by the 2k resistor.
Zeners in the 250mW to 500mW range are capable of absorbing the peak current generated by a 150V, 10µs spike. Higher voltage and longer duration spikes may be accommodated by larger devices.
For sustained conditions, a simple Zener clamp is made untenable by the dissipation in both the resistor and Zener. The circuit shown in Figure 3 uses small, high voltage MOSFETs for limiting and can handle up to 400V. 11V gate bias is conveniently obtained from the shunt-regulated VCC pin without the need for any extra components, making this useful configuration a very simple modification of the basic circuit.
Under normal conditions, the –48V inputs are at or near the VSS potential, and the small MOSFETs M3 and M4 are driven fully on as their gates are biased to ~11V with respect to VSS by the VCC pin. If one input rises with respect to VSS, the small MOSFET remains on and the associated drain pin tracks the input. If the input continues to rise to the point where it is ≥10V with respect to VSS, the small MOSFET turns into a source follower, safely limiting the drain pin to about 10V with respect to VSS. MOSFETs M1 and M2 can be expected to avalanche and clamp any positive-going spikes exceeding 300V, to less than 400V.
While the circuit in Figure 3 was designed for a –48V system, changing RIN to a 100k, 1W unit allows the circuit to operate with inputs of –200V to –300V DC. Higher voltage standoff is possible with appropriate selection of MOSFETs.