Can you guys recommend me some PFETs for Q2 in this circuit 1 (figure 2) that I can use to get continuous current flow of 2400mA?

My segment current is 300mA, so I'm looking at 300mA X 8 = 2400mA for the common anode display.

Although the diagram shows P channel depletion MOSFET, looking at the functionality, it seems to be needing a P-channel enhancement. Can I get some suggestions please?

Using MAX7219 for high voltage.

  • \$\begingroup\$ I recommend you find another way. DMOSFETs are rare and expensive. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 12 '15 at 7:46
  • \$\begingroup\$ What would be another way? Can you please elaborate? Maybe a circuit diagram too if you don't mind. \$\endgroup\$ – Anup Kattel Oct 12 '15 at 7:48
  • 2
    \$\begingroup\$ The FET in figure 1 appears to be an enhancement mode device from the description of circuit operation. They are commonly drawn this way, and although that was (is) the symbol for a depletion mode device. As Ignacio notes, DMOS devices are now quite rare. For that reason, manufacturers assume we understand it is an enhancement mode device. In addition, the note describes using segment multiplexing, so the maximum current is the maximum segment current assuming you are multiplexing each segment. If not, you would need a switch for each segment anyway. \$\endgroup\$ – Peter Smith Oct 12 '15 at 8:04
  • \$\begingroup\$ Update to my previous comment: the app note is specifically expecting a switch for each segment (there is a note at the drain of the switch "Anode digit drive (1 of 8)"). \$\endgroup\$ – Peter Smith Oct 12 '15 at 8:12
  • \$\begingroup\$ "Can you guys recommend me some ...". No. We don't do that here. Read the rules (duh!). \$\endgroup\$ – Olin Lathrop Oct 12 '15 at 11:52

As far as I know discrete p-channel depletion mode MOSFETs are not commercially available at all.

However. You're in luck- those are just ordinary p-channel enhancement mode MOSFETs and are thus cheap and easily available. For example, the AO4437. You may be able to use a cheaper SOT23 packaged device, but you'd have to run the numbers in any case.

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  • \$\begingroup\$ I could only find "IRF9540N" and "MOSFET SUP53P06-20 P-CH 60V 53A" at my local electronics shop. Do you think one of these would work? If not, I will have to buy something online (which would take a while to get delivered). The later one seems to V(gs) of -4.5V, which seems okay for this. \$\endgroup\$ – Anup Kattel Oct 12 '15 at 11:05

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