LM7480-Q1: Priority power mux for two 12V/6A inputs

Hi Abin,

The details of the design procedure for Power Mux Configuration are mentioned in section 'Design # 6: Reverse Battery Protection With Priority Power MUXing' of the Six System Architectures With Robust Reverse Battery Protection Using an Ideal Diode Controller app note. I am copying the related text below for your reference. Please let me know if any point is not clear to you, I will elaborate it. 

The OV 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 6-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 keeping Q2 ON, as its Vs supply is powered from the auxiliary path. 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 3 µs (typical).

Hi Abin,

Please see my response inline below,

1) Did you mean Design # 6: Reverse Battery Protection With Priority Power MUXing?

Yes, I meant to refer to 'Design # 6: Reverse Battery Protection With Priority Power MUXing'

2)Would you be able to state the status of Q1 & Q3 too?

Say if there is a voltage difference  between two supplies and primary is active and at a higher voltage, the document says specifically of Q4 being off only. Isn't there a possibility of Q4's body diode conducting? What is the state of Q3 (I assume Q3 would be ON to keep the U2 ON)? Wouldn't there be a threat of reverse current flow ?

When the output voltage is greater than input voltage (Secondary),the gate linear regulation scheme will pull DGATE low and Q3 will be turned OFF. 

3) Say if I already have a reverse voltage protection implemented at the input of VPrimary, What is the use of LM74800 on VPRIMARY path? Since the VPRIMARY has the priority, whenever Vprimary is connected, the upper LM74800 has to turn on rt? It is as simple as a shorted wire. If we can use a cheaper Ideal diode controller say LM74700 (this seems not stocked anywhere though), wouldn't that save cost (replacing primary LM74800 with LM74700 in design #6, topology being the same)? 

With LM74800-Q1 used on the primary, you have the additional benefit of

1. Over voltage protection
2. Primary path ON/OFF control 

If the above features are not required, then yes you can use LM74700 instead of LM74800. 

Hi Abin,

1. The voltage rating of Q5 transistor should be > [VAUX(MAX) + Charge pump voltage,15V]
2. The value of R5 resistance is depends on how much current you want to inject into the HGATE,

• • Without R5 = 0 ohms, the current injected would be around 2.7 mA
  • If you want to reduce the current injection into HGATE to slow down the turn on of the HFET, you can increase the value of R5. For example, if you want to limit the max current injection into the HGATE to 500 uA, then R5 = [(VAUX(max)+15V)/500 uA]