This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Power dessipation for RS422 and RS232 transreciver

Other Parts Discussed in Thread: AM26LV32E, AM26LV31E, MAX3232E

Hello,

I am planning to do Thermal analysis for our project.

So please explain me how to find power dissipation(PD) for AM26LV32EIPWR ,AM26LV31EIPWR and MAX3232EIDR

Regards,

Prasanna G 

  • Hi Prasanna ,

    I will move this post into appropriate forum for support .
  • Prasanna,

    The generic answer is PD = VCC * ICC - PL;  where PL is the power to the load.

    Tomorrow, I will give a more detailed answer.

     

  • Hello Ronald,

    Waiting for your suggestion.

    Regards,
    Prasanna G
  • AM26LV32E

    Total Pd is sum of these terms. Assumption is that outputs are only capacitively loaded.
    Static power = Vcc * Icc
    CH1 power = (Cpd + Cout) * Vcc^2 * F
    CH2 power = (Cpd + Cout) * Vcc^2 * F
    CH3 power = (Cpd + Cout) * Vcc^2 * F
    CH4 power = (Cpd + Cout) * Vcc^2 * F
    where Cpd is 150pF per data sheet, F = frequency= number of bit changes / 2 per second)
    For random data, F = bits/s * 30%
  • AM26LV31E

    Total Pd is sum of these terms. Assumption is that outputs drive transmission lines.
    Static power = Vcc * Icc
    CH1 power = (VCC - VOL + VOH) * IOUT * (1 + F / 75 MHz)
    CH2 power = (VCC - VOL + VOH) * IOUT * (1 + F / 75 MHz)
    CH3 power = (VCC - VOL + VOH) * IOUT * (1 + F / 75 MHz)
    CH4 power = (VCC - VOL + VOH) * IOUT * (1 + F / 75 MHz)
    where VOL and VOH are in data sheet,
    IOUT = (VOH - VOL) / [line impedance]
    F = frequency = number of bit changes / 2 per second)
    For random data, F = bits/s * 30%
    Power increase slightly with higher frequency, empirical data gives the (1 + F/MHz) scaling factor.
  • MAX3232E

    The power dispassion is simply VCC * ICC when the device in shutdown or the output is unloaded (RS232 cable disconnected).
    When the RS232 cable is connected and transmission is active, it is more complicated.
    The receivers won't add much to the power consumption so I’ll focus on the drivers.

    For each driver the DC output current is: I = DOUT / RL = 1.08mA (RL is load of remote RS232 receiver, normally 5k ohms. DOUT typical is +/-5.4V)

    For each driver the switching (AC) output current is: I=F*C*10.8V^2/5.4V this is simplifies to I=F*C*21.6V. F is the driver frequency which is not the same as bits per second. With a continuous alternating bit pattern (worst case), F=bps/2. For random data F=bps*30%. C is capacitive load on the driver line. 10.8V is the peak to peak voltage and 5.4V is the charge pump voltage. Also only one driver toggles at a time.

    Add both AC & DC currents (for all drivers) calculated above to create a total current required of the charge pump. Now ICC can be calculated.
    ICC = 0.3mA + 2.3 * I[total]. The 0.3mA value is the typical static ICC, the 2.3 is the measured charge pump efficiency factor. If supply power is needed, multiply ICC by VCC. This will provide a good estimation of typical active current or power consumption. Temperature rise is P * TJA.

    For example. 38400 baud, continuous alternating bit pattern, 2500pF 5k load, VCC=3.3V.
    DC current is 2*5.4V/5k = 2.16mA
    AC current is 1*38400/2*2500pF*21.6 = 1.04mA
    Total ICC is 0.3mA + 2.3*(2.16mA+1.04mA) = 7.7mA
    Power is 3.3V*7.7mA= 25.4mW
  • Hello Ronald,

    Am very thank full to your information. I understood the PD for RS232.
    But for AM26LV31E what does mean F ,Can you please give an example as it was given in RS232 reply.

    Regards,
    Prasanna G
  • Prasanna,

    Quote from AM26LV31E text:
    F = frequency = number of bit changes / 2 per second)
    For random data, F = bits/s * 30%
    end quote:

    If no data is being transmitted then F =0
    If data is being sent on a channel then F = bps * 30%. If data was 10 Mbps then F = 3 MHz, meaning 6 million bit changes per second. 4 million times per second the bit won't change because the next bit is the same as the last.