This circuit provides on/off switching, soft starting, current monitoring, current tripping, and protection against overcurrent for a 30 Vdc power supply at normal load currents up to 2 A. The switch is turned on by an `on` command pulse; it is turned off by an `off` command pulse. An overcurrent trip can also be set on the bus side by a 6-digit binary signal, which is converted to an analog voltage and compared with the amplified voltage developed across a load-current-sensing resistor. Resistor/capacitor combinations (0.027 Âµ, 2 kfi) at the inputs of the current-sensing amplifiers act as low-pass filters: this introduces a few hundred /is of delay in the response to overcurrent, thereby providing some immunity to noise.
The 0.022 Âµ capacitors connected to the drain terminals of the PFETs provide a Miller effect, which reduces the rate of change of the drain voltage and therefore the rate of rise of current at turn-on. The soft-turn-on time depends upon the load impedance and is typically 100 to 200 ms.