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# LM74700-EP: Nominal Current Range, FET Selection, and Schematic Review

Part Number: LM74700-EP
Other Parts Discussed in Thread: LM74700-Q1

Could someone review this schematic? The datasheet wasn't clear on how to go through the FET selection process when there is a nominal current range. Should the higher or lower Rdson range be used for the design? Or, is it necessary to narrow the current range so that there is ample overlap with the Rdson range?

• Hi Marcus,

Generally, nominal load current is a single value. In case your load current can vary from 1A to 10A, I would recommend you to select Rds(on) based on 10A.

Below are my comments on schematic review,

• If this is an automotive application and there can be automotive pulses applicable at the input, I would recommend using SMBJ33CA TVS at the input (instead of SMAJ40CA) for 12V battery application. For more understanding on TVS selection, please refer to section '9.2.3 Selection of TVS Diodes for 12-V Battery Protection Applications' of the datasheet.
• CCAP and FET selection looks good to me.
• increase input capacitance to 100nF. This will help improve EMI performance of the IC.
• It's not for an automotive application. The nominal input voltage should be 28V. Unfortunately, the SEL_PWR (output of the OR-ing circuit) will interface with various devices (plug & play), which could draw currents ranging, as mentioned, 1 A - 10 A. Is there a reason you suggest using the higher current for the FET selection process?

• Considering the chosen FET has a Rdson of 5 - 6 mOhm @ Vgs of 4 - 10 V, can I assume:

That at 1  A the V_anode - V_cathode would be in the range of 5 - 6 mV?

That at 10 A the V_anode - V_cathode would be in the range of 50 - 60 mV?

If yes, then the  controller will be in "Regulated Conduction Mode" for most of the current range, except for maybe after ~8.5 A where the controller will enter "Full Conduction Mode"?

• Hi Marcus,

The LM74700-EP would be operating in regulation mode where the forward voltage drop of the FET (Load current x FET resistance) is Regulated at V(AK) Threshold = 20mV. To maintain a 20mV forward voltage drop, the FET resistance is increased b y LM74700-EP by reducing the gate-source voltage of the FET. This is because of linear gate control of the LM74700-Q1 which ensures zero DC reverse current.

As the Load current increases, the FET resistance is reduced by the controller and this is achieved by increasing the gate-source voltage. After a certain point, the FET resistance cannot be reduce further (Rds(on) is the lowest FET resistance) and from here on the FET forward voltage drop  keeps going > 20mV with increase in load current.

• Doesn't seem like you explained why it's best to use 10A instead of 1A.

• Hi Marcus,

Selecting Rds(on) based on the maximum current (10A in this case) leads to selection of lower Rds(on) FETs which help reduce the power dissipation in the FET. But the small draw back you have here is that the lower Rds(on) will need higher reverse current flow for the controller to detect it and turn OFF the FET to block reverse current.