7 Design 5: LM749x0-Q1 as Priority Power MUX Controller

Another architecture of power-supply redundancy is based on a system with one primary source like a vehicle battery and an auxiliary supply source like a DC/DC converter or a large storage capacitor. When both the supplies are present, then the priority should be given to the primary source and the system should be powered from the primary source path independent of the supply voltage levels. In the absence of the primary supply or when the primary supply drops to a specified undervoltage level, then the downstream should switch over to the auxiliary power path. The transition should be fast enough ensuring that the output voltage does not drop below the undervoltage lock out level of the downstream circuit.

Figure 7-1 shows LM749x0-Q1 application circuit for Priority Power MUX design.

Figure 7-1 Typical Application Circuit for Priority Power MUXing Designs

The OV pin connection of the AUX channel is from the primary supply path. The OV falling threshold of U2 is set at the undervoltage level of primary supply. During normal operation, primary path is ON. OV of U2 is high and this keeps GATE of Q₄ OFF, disconnecting the auxiliary supply path to the load. When the primary supply is dropped or when it reaches a set undervoltage level, then Q₂ turns OFF, disconnecting the primary path. At the same time, OV threshold of U2 goes low and HGATE starts rising after a delay of 8-µs (typical). HGATE has gate drive strength, I_(HGATE) of 53 µA. Q₅ enables higher GATE drive levels up to 600-µA for fast turn ON of Q₄. U1 on the primary path remains active as its Vs supply is powered from the auxiliary path through the body diode of FET Q2. When the primary supply is connected back, then the system output immediately gets connected to the primary path. OV of U2 goes high and Q₄ turns OFF within 4-µs (typical).