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MAX3243: DOUT VOH is larger than Fig5-1 characteristics.

Kazuya Nakai59
Guru 22355 points

Part Number: MAX3243

Hello guys,

One of my customers is evaluatiing MAX3243 using own design PCB board.

When they checked DOUT VOH-IOH characteristics,
they found their MAX3243 capavility is lower than Figure 5-1 characteristics.
(it is less than 50%)

In that case, do you think what the cause is?
Is there any requirement to the capacitors? the maximin voltage? DC bias? temperature class?

Their Vcc voltage and ceramic capacitor values for charge pump are the follows?
VCC=5V,
Cbypass=0.1uF,
C1(located between C1+ and C1-)=0.1uF,
C2(located between C2+ and C2-)=0.47uF,
C3(located between V+ and GND)=0.47uF,
C4(located between V- and GND)=0.47uF

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    TI__Mastermind 28302 points

    Hi Kazuya,

    Thanks for reaching out on E2E. I am reviewing this, and I will get you a response tomorrow.

    In the meantime, you could provide a schematic and waveforms of the issue?

    -Ethan

  • 0
    TI__Mastermind 28302 points

    Hi Kazuya,

    Those capacitor values are good. So far the information you have shared is not concerning. 

    Figure 5-1 is not something we guarantee. That graph is also rated for 3.3V, not 5V. How are you measuring the current draw? What is your exact load on the DOUT pin (total resistance and capacitance)?

    The main specs I would focus on for this transceiver are found in the Electrical Characteristics section:

    What matters here is the VOH/VOL voltage output, which determines whether or not the outputs are valid. The VOH with RL=3K Ohm should be above 5V if setup correctly. The exact current performance on the bus is not something we guarantee. 

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Guru 22355 points

    Hi Ethan,

    Thank you very much for your reply.

    They said that their schematic is same as Figure 8-1
    Typical Operating Circuit and Capacitor Values in MAX3243 datasheet.

    But I will try to get their schematic and waveform.

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    Guru 22355 points

    Hi Ethan,

    Thank you very much for your supports.

    Regarding to Cbypass and C1~C4, 
    is there any requirement to the capacitors about the maximin voltage, DC bias and temperature class?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 28302 points

    Hi Kazuya,

    The voltage rating for Cbypass and C1-C4 should be above 2x VCC voltage. For 5V, we recommend >15V cap rating. Otherwise, the other specifications should not greatly impact the performance of the transceiver. 

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Guru 22355 points

    Hi Ethan,

    Thank you very much for your strong supports.

    They use a capacitors which has 35V small size capacitor (0603). Also the capacitor reducing rate by DC bias is about 20%.
    If the capacitor value is reduced by 20%, is it any problem occurred? 

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    Guru 22355 points

    Hi Ethan,

    I'm sorry I mistook to check the capacitor reduce rate.
    The reduce rate is very large due to very small capacitor size. It is -80% at 10V bias voltage.
    I think this value will have an effect to MAX3243, right?
    Is there a minimum capacitance value for each capacitor?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 28302 points

    Hi Kazuya,

    Happy to help.

    From what I know, the datasheet recommendations take into account various capacitor variations (like min capacitance). But 80% is quite large. I will need to check internally with our design team for confirmation.

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    TI__Mastermind 28302 points

    Hi Kazuya,

    In general, please use caps with 10% tolerance. But 25% might still work. Otherwise, at 80% tolerance the transceiver will have to switch much more frequently to achieve the desired output voltage. As a result, it may not be able to reach to correct output voltage. 

    You can also measure the V+ and V- pins to verify the difference between capacitor types as well.

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Guru 22355 points

    Hi Ethan,

    Thank you very much for your supports.

    Could I ask you the following additional questions?

    Q1.
    Does you mean that 10% tolerance of capacitor include the capacitance reduction due to DC bias? 

    Q2.
    If Q1 answer is yes, does it mean that you need to use a capacitor larger than the value specified on the datasheet?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 28302 points

    Hi Kazuya,

    We don't have an exact answer for you here. The value provide in the Figure 8-1 table is all we suggest. I believe it is assumed that the cap values should have a better tolerance than 80%. 

    However, I still haven't confirmed if the DOUT voltage waveform is expected or not. If possible, try to send an output waveform of DOUT1, V+, and V- so I can confirm the transceiver is operating correctly.

    Best,

    Ethan