There are many ways to implement multiple power systems with Schottky diodes. For example, high-availability electronic systems such as μTCA networks and storage servers use Schottky diode OR circuits in their redundant power systems. The diode OR circuit is also used in systems that use a backup power source, such as an AC AC adapter and a backup battery feed. The problem is that the Schottky diode consumes power due to the forward voltage drop, and the heat generated must be dissipated by a dedicated copper foil area on the PCB or by a heat sink that is bolted to the diode. The way all takes up a lot of space.
One of Linear Technology's product families uses external N-channel MOSFETs as pass-through components to minimize power consumption, minimizing voltage drop from the supply to the load when these MOSFETs are turned on. The product line includes the LTC4225, LTC4227 and LTC4228. When the input supply voltage drops below the output common-mode supply voltage, the appropriate MOSFET is turned off, matching the function and performance to the ideal diode.
As shown in Figure 1, by adding a current sense resistor and configuring two back-to-back MOSFETs with separate gate control, the LTC4225 improves the performance of the ideal diode with inrush current limiting and overcurrent protection. This allows the board to be safely inserted or removed from the live backplane without damaging the connector. The LTC4227 can be used to add a current sense resistor and a hot swap MOSFET after the ideal diode MOSFET connected in parallel to save one MOSFET. By configuring a sense resistor between the ideal diode and the hot-swap MOSFET, the LTC4228 is an improvement over the LTC4225, and the LTC4228 recovers faster from the input voltage undervoltage to keep the output voltage constant.
Figure 1: Different configurations of the LTC4225, LTC4227, and LTC4228 with sense resistors and external N-channel MOSFETs
* ADDITIONAL DETAILS OMITTED FOR CLARITY:* For clarity, some details have been omitted
The LTC4225-1, LTC4227-1, and LTC4228-1 feature a lock-off circuit breaker, while the LTC4225-2, LTC4227-2, and LTC4228-2 provide automatic retry after failure. Both versions of the LTC4225, LTC4227, and LTC4228 are available in 24-pin, 20-pin, and 28-pin 4mm x 5mm QFN and SSOP packages.
Ideal diode control
The LTC4225 and LTC4228 use an internal gate drive amplifier to monitor the voltage between the IN and OUT pins (IN and SENSE+ for the LTC4227), which acts as an ideal diode that drives the DGATE pin. When this amplifier detects a large forward voltage drop (Figure 2), it quickly pulls the DGATE pin high to turn on the MOSFET for ideal diode control.
Figure 2: Pulling the ideal diode controller CPO and DGATE pins when the IN power is turned on
An external capacitor connected between the CPO and the IN pin provides the charge required for the ideal diode MOSFET to turn on quickly. The internal charge pump charges the capacitor when the device is powered up. The DGATE pin provides current from the CPO pin and sinks current into the IN and GND pins. The gate drive amplifier controls the DGATE pin to follow the forward voltage drop across the sense resistor and the two external N-channel MOSFETs up to 25mV.
If the load current causes a voltage drop of more than 25mV, the gate voltage rises to enhance the MOSFET used to achieve the ideal diode control. When the MOSFET is turned on, if the input supply is shorted, a large reverse current will begin to flow from the load to the input. When a fault occurs, the gate drive amplifier detects the fault condition and pulls DGATE low to turn off the ideal diode MOSFET.
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