ADM1275:  Hot Swap Controller and Digital Power Monitor with PMBus Interface

The ADM1275 is a Hot-Swap controller that allows a circuit board to be removed from or inserted into a live backplane. It also features current and voltage readback via an integrated 12- bit analog-to-digital converter (ADC), communicated through a PMBus interface. The load current is measured using an internal current sense amplifier measuring the voltage across a sense resistor in the power path via the SENSE+ pin and the SENSE- pin. A default limit of 20mV is set but this can be adjusted if required using a resistor divider network from the internal reference voltage to the ISET pin. The ADM1275 limits the current through the sense resistor by controlling the gate voltage of an external N-channel FET in the power path, via the GATE pin. The sense voltage (and therefore the load current) is maintained below the preset maximum. The ADM1275 protects the external FET by limiting the time the FET remains on while the current is at the maximum value. This current limit time is set by the choice of capacitor connected to the TIMER pin. In addition, a foldback resistor network can be used to actively lower the current limit as the voltage across the FET is increased. This helps to maintain constant power in the FET and allows the SOA to be adhered to in an effective manner.

In case of a short-circuit event, a fast internal overcurrent detector will respond within 300ns and signal the gate to shut down. A 750mA pulldown device will ensure a fast FET response. The ADM1275 features overvoltage and undervoltage protection, programmed by using external resistor dividers on the UV and OV pins. A PWRGD signal can be used to detect when the output supply is valid, using the FLB pin to monitor the output. GPO pins can be configured as various output signals, one of which is to assert when a programmed current level is reached.

A 12-bit ADC can measure the current in the sense resistor, as well as the supply voltage on the SENSE+ pin or the output voltage. A PMBus interface allows a controller to read current and voltage data from the ADC. Measurements can be initiated by a PMBus command. Alternatively, the ADC can run continuously, and the user can read the latest conversion data whenever it is required. Up to 4 unique PMBus addresses can be selected, depending on the way the ADR pin is connected. The ADM1275-1 and ADM1275-3 can be configured with the LATCH pin to latch off or auto retry and are packaged in a 20-lead. The ADM1275-2 is available in a latch off model only (16-lead).