ADM3251 isolated power supply


    The ADM3251 has the option of an internal isolated dc-dc converter (where it is mentioned as not so efficient) and has an option of connecting an external isolated supply.

1) How do we choose which approach is better?

2) Does one approach provide better isolation?

3) Does the choice affect the data rates?

4) Does the choice affect the operating temperature range (because the internal converter produces more heat). 

Thank you for you help and hope everyone is keeping safe,

  • +2
    •  Analog Employees 
    on Apr 22, 2020 3:24 PM 10 months ago


    1) If you already have a 3.3V isolated power supply available on side 2, then the part will use less power overall by powering Vcc = 3.3V (isolated DC-DC disabled). Otherwise, using the integrated isolated DC-DC would be preferred in order to save on PCB area, component count and design of an iso DC-DC just for the RS-232 transceiver. And at such low load on Viso, any external iso DC-DC will probably be quite low efficiency anyway.

    2) The isolation in ADM3251E is unaffected by the choice of using the integrated iso DC-DC or not. Isolation ratings will be per ADM3251E even with an external isolated DC-DC, unless the external circuit has lower isolation capability (for example, an external isolated DC-DC could be only 1kV DC withstand rating).

    3) The ADM3251E is specified for up to 460 kbps regardless of using the integrated iso DC-DC or not.

    4) The recommended temperature range is -40C to +85C regardless of mode of operation.

    An alternative option with higher temp range is LTM2882 (H-grade offers -40C to 105C); that is, if it makes sense to have 2TX & 2RX and potentially use more power for other circuits on the connector side (LTM2882 integrated iso DC-DC offers up to 200 mA). It also offers up to 1 Mbps data rate and the isolated DC-DC can be supplied on the primary side by 3.3V or 5V (choice of model). The integrated I/O level shifter may be useful if connecting to an FPGA - supports 1.8V/2.5V logic as well as 3.3V/5V.

    Best regards,

    Conal Watterson