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Original question:

[FAQ] ISOM8110: How do I design ISOM8110 to have the best timing performance for switching applications?

ISOM8115: designing for max operating frequency at 50% duty cycle

Amit Halfi
Prodigy 100 points

Part Number: ISOM8115
Other Parts Discussed in Thread: ISOM8113

Tool/software:

Hi,

I'm currently designing a digital input using the ISOM8115, and I would like to validate my understanding of the datasheet and the design process, along with requesting some information not on the datasheet.

My requirement is to achieve maximum operating frequency, assuming duty cycle = 50%.

To my understanding, the limiting factor is the tON or tOFF (the higher between them), which must be as small as possible.

According to section 6.8 switching characteristics, it seems tOFF is considerably larger, so, using the part number ISOM8115, I should operate around Iforward = 1.6mA, and not 16mA.

Would reducing Iforward further, to ~0.8mA for example, improve tOFF?

Figure 6-3 is difficult to read around the low mA range. A zoomed in version around 0-3mA would be very useful. Please share it if it is available.

According to figure 6-22, tOFF can be further decreased by lowering RL.

The tON = tOFF point (RL=~2.5KOhm) occurs at the crossing of the black and red lines in figure 6-22, which is the ideal operating point for 50% duty cycle. Is my understanding correct?

Then, I would have liked to also incorporate the effect of ambient temperature. Figure 6-26 refers to this information, but it is for RL=4.7KOhm.

Is similar information, but for RL=~2.5kOhm instead of RL=4.7kOhm, available? If so please share it.

Additionally, in figure 6-26 temperatures -20 and down, tON rises considerably. Marked yellow.

How would this effect appear when RL=~2.5kOhm? would it shift higher in temperature? Have a steeper slope?

My operating temperature is -20 to +85 Celsius.

I'm 

Thank you

  • 0
    TI__Mastermind 34555 points

    Hello Amit, 

    Please allow me until Friday to look at what details we have available in the datasheet. 

    Best,
    Andrew

  • 0
    TI__Mastermind 34555 points

    Hello Amit,

    Great questions and thank you for your patience. Unfortunately, the data trends you see in the datasheet are all that is available. 

    Amit Halfi said:

    I should operate around Iforward = 1.6mA, and not 16mA.

    Would reducing Iforward further, to ~0.8mA for example, improve tOFF?

    Reducing the forward current should help to improve the storage time (Ts) of the isolator which is a component of TOFF. However, please keep in mind that the forward current should not go below 0.7mA which meant that 0.8mA is the minimum. 

    Amit Halfi said:

    According to figure 6-22, tOFF can be further decreased by lowering RL.

    The tON = tOFF point (RL=~2.5KOhm) occurs at the crossing of the black and red lines in figure 6-22, which is the ideal operating point for 50% duty cycle. Is my understanding correct?

    Yes, your understanding is correct. According to the typical application curve 2.5k is the ideal pint for a 50% duty cycle since TON and TOFF are the same. 

    Amit Halfi said:

    Is similar information, but for RL=~2.5kOhm instead of RL=4.7kOhm, available? If so please share it.

    Additionally, in figure 6-26 temperatures -20 and down, tON rises considerably. Marked yellow.

    How would this effect appear when RL=~2.5kOhm? would it shift higher in temperature? Have a steeper slope?

    My operating temperature is -20 to +85 Celsius.

    I suspect that this shift in TON is due to CTR at cold temperatures dropping which causes the device to move out of saturation mode which then worsens the TON spec. The smaller resistance 2.5kohms should output strongly driven and help keep the device in saturation mode 

    You could also try using the high-CTR variants (ISOM8113) which would ideally allow for the least amount of IF to drive the device to saturation mode. This would also allow for a smaller RL and could further improve switching times. 

    Best,
    Andrew