Post Go back to editing


Category: Hardware
Product Number: LTC4418


I would like to Parallel the P-ch FETs in LTC4418 application for sharing the load/power dissipation. My concern is about removing back-to-back FET connection. Can it be removed and connected 3 P ch FETS in parallel? What is the purpose of connected back-to-back fets? To create a bidirectional switch? Can it be removed? How many total FEts can be paralleled? See attached LTspice model for clear understanding. If this is not the right way to connect them in parallel, could someone please show me how it needs to be done? 

4418 Parallel.asc

  • Hello Jay,

    Thanks for a reply. Much appreciated. Both supplies are the batteries by the way operating 20-30VDC. Please take a look at this new LTspice model that i connected back-to-back in parallel? THe problem I see, that the secondary FET's dissipating significant more power when connecting them like this way. Any advice? See the LTspice simulation and screenshots. Or if you want to parallel the FETs for dissipation do you need to have multiple LTC4418? Please advise? 


  • Try this configuration. I have adjusted the soft-start components and changed the MOSFETs to 40V MOSFETs. I would not use 30V MOSFETs if you have a 30V battery.

    You enter 'm=2' on the in the value2 line in the component attribute editor.

    CTRL-RIGHT CLICK on the part to bring up the Component attribute editor. It is a nice way of putting parts in parallel on the schematic without clutter.

    Do these changes and evaluate the model.


  • Hi Jay,

    many thanks for your input. You amazing with your help. I have only last question left. If you place more than 300uF capacitance on a load and you measure the current at a second FET drain (see photo attached) you will get a negative current there a spike of 0.3ms. I think it's because the output capacitors discharge through the FET body diode? Would that cause any problem ? Changed the FETs to 40V as well. Attaching a model of LTspice as well. 

    LTC4418 RATTLER.asc

  • If I zoom on that event, I see:

    What is happening is Q2 and Q3 are full enhanced they are on, and they should be on.

    The battery voltage V1 is lower than the output voltage, so current will flow from the output to battery, V1, The current is charging the battery. 

    In practice the battery should not go from 28V to 0V in 1ms. If the time is increased to 10ms, the load resistor, Rload, discharges the output capacitors faster than V1 is falling and there is no reverse current.

    If the battery was disconnected by a switch or a fuse there would be no current flowing in the reverse direction.


  • Thanks Jay, you are right. 

    Last question and we can close this: Would ceramic capacitors be suitable for input/output filtering rather than aluminum electrolytic ones? 

    Have a great day. 

  • Hi,

    You can use ceramic input and output capacitors. You need to be aware of the transients when 'hot-plugging'. Batteries are always 'hot-plugged'.

    Have a look at Linear Technology AN88.

    This will explain how to dampen or clamp the transients.


Reply Children
No Data