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[3V TO 5.5V] current and VL=0V state

Category: Hardware
Product Number: LTC2870

Dear ADI team

I have two questions about LTC2870.

1.In VL Logic Supply and Logic Pins item.
[RS232 and RS485 driver outputs are undriven and the
RS485 termination resistors are disabled when VL or VCC
is grounded or VCC is disconnected.]

When Vcc > 3V,VL=0V,
is it correct to use 125kΩ pull-down for Y and Z pins?
Also, is it correct to use 125kΩ pull-down for A and B pins?

2.Please tell me how to calculate the current supply capacity of [3V TO 5.5V] that is safe even if the output is short-circuited in RS485.
・Vcc is 3.3mA max.
・SW is 250mA when  Y and Z is short

Is this correct?

Best regards,

  • Hi SYVALION,

    If you can list all logic pins setting, I would have a better understand what mode the part operates at. Please let me know if you referred to a different operating condition.

    1)Vcc = 3.3V and VL = 0V is not in the recommend operation conditions. VL should be between 1.7V to Vcc. For the “125kΩ pull-down” on Y & Z and A & B, are you referring to when the driver is disabled and is at half-duplex mode.

     

    2) Neither “Vcc is 3.3mA max.” nor “SW is 250mA with Y and Z are shorted” is a normal operating condition. If you referred to typical quiescent current is 3.3mA, the part is unloaded, not connect to the bus lines. Please see TPCs on the datasheet for estimate supply current with different operating conditions. To estimate the supply current when the driver outputs are shorted, please see the below TPC for the typical short-circuit current vs. Y or Z voltage.

    Best,

    Jane

  • Hi Jane

    Question (1) was asked because I was concerned about the state of the pins from Vcc power-on to VL power-on.
    Normal operation is planned to be Vcc=3.3V, VL=1.8V.

    Q(2)

    I got it.

    Best,

    SYVALION,

  • Did you understand about (1)?

    The conditions are
    VL and other digital IO are all 0V
    and only Vcc is being supplied
    to them.

    When the power is turned on, I'm wondering whether ABYZ is high impedance or pulled down between the time Vcc is supplied and the time Vl is supplied.

    Best,

    SYVALION,

  • Hi SYVALION,

    I understand the conditions in your question 1. Since the conditions are not under recommended operating conditions, the outputs are undefined. We can only guarantee output states under recommended operating conditions. VL is critical and is the logic high supply for all I/Os, so Vcc=3.3V and VL=0V should be considered unpowered state for LTC2870. 

    Best,

    Jane