Controller eases multi-input PSU mediation and hot-swapping

Linear Tech is easing the pain of high-rel power supply design with a controller that mediates two power sources and a single load.

LTC4236 - Controller eases multi-input PSU mediation and hot-swapping

LTC4236 includes a dual ‘ideal diode’ wired-OR, a hot-swap controller and a load current monitor output for external ADCs.

For supply ORing, the chip uses two external n-channel mosfets – cutting dissipation compared with Schottky diodes. An internal charge pump provides gate drives above the input voltage for the n-channel devices.

To keep the ideal diodes well-behaved and to smooth the transition from one supply to the other (see plot), rather than saturate the OR-ing mosfets, Linear has opted to regulate the voltage drop across them (to 15mV or Iload x RSDon if higher).

LTC4236 switching smoothlyEither input can range from 2.9 to 18V, and output current is dependant on the mosfets – the data sheet includes a 7A 12V application example.

Should the live input supply get shorted to ground, a fast (0.5μs) circuit cuts gate drive to prevent heavy reverse flow through the conducting mosfet channel.

For situations where a priority must be enforced amongst the input suppies and the mosfet source-drain body diodes would cause uncontrollable flow – when OR-ing a main supply with an auxiliary supply of higher voltage, for example, the chip allows an OR-ing mosfet to be replaced with a pair of back-to-back mosfets (see diagram).

LTC4236 priority controlThe external downstream hot-swap mosfet (top diagram) allows safe insertion and removal from live backplanes by limiting in-rush current and providing output over-current protection. An active current limit amplifier servos the third mosfet (in ≤1μs) to deliver 25mV or less across the current sense resistor – with an adjustable timer to limit mosfet dissipation during current limit.

Hot-swap contact bounce is accommodated by a filtering delay – adjustable with an external capacitor.

For an external ADC monitoring output current, the sense resistor voltage is  amplified by 100x and level-shifted from the positive rail to ground to allow

Under-voltage lock-out (2.2V) forces all external mosfets off, and the output mosfet can be turned off by external hardware via a pin to shut the output down.

This is a simplified story. Comprehensive control logic makes the chip flexible to use, with more than one control input provided and several status outputs – that can drive LEDs directly.

LTC4236 is available in two options: LTC4236-1 remains off after an over-current fault, while the -2 version automatically re-tries with a 0.15% duty cycle.

Operation is 0 to 70°C or -40°C to 85°C, and the package is 4 x 5mm 28pin QFN.

Application are expected servers, network routers and solid-state drives.

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