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DDC112: DDC112 power sequencing vs. digital inputs

Mark Bridges
Intellectual 270 points
Part Number: DDC112

Hello, the DDC112 data sheet specifies the following on page 24, Power Up Sequencing, "Prior to power-up, all digital and analog input pins must be LOW".   If the digital inputs are High for a short period of time (say 30 seconds) before AVdd and DVdd are applied, will this cause any damage to the part?  Also will it degrade its reliability?  Thank you.

  • 0
    TI__Expert 4590 points

    Hi Mark,

    Yeah, the immediate answer without knowing more is that yes, you could damage the device. That is just by going with the information on the abs max rating ("digital input voltage to DGND < DVDD+0.3V"). It is probably a matter of current levels besides time. I.e., what is likely to happen is that the input protection of the device (a diode between the input and the DVDD which is usually reversed bias) will turn on (become fwd bias) and try to keep a safe voltage at the inputs. At that moment, if your digital driver (generating the digital voltages going onto those pins) can put out enough current to actually drive the inputs beyond that 0.3V difference, beating the internal diodes, that same current will likely blow the protection diodes themselves. Even if the diode holds things to 0.3V the current may be already pretty big. You may want to add a series resistor with the inputs so that there would be a voltage drop across them if current is large... I can hear you asking how big that resistor but that I don't know :). Let me check but maybe 10k would work...

    Regards,

    Edu

  • 0 in reply to Eduardo Bartolome
    TI__Expert 4590 points

    Hi Martin,

    So, I'm back with some interesting info from our designer... The device is quite old so not sure what kind of ESD protection it got at the digital input (I would have to ask someone to open the database...). There are two types of protections:

    1. What we call "fail safe". These can take a lot of voltage more than the typical 0.3-0.6 diodes. We actually use these in our latest DDCs but not sure back in the days. In this "fail safe" case, if the digital driver applies, say, 5V, the current will still be small and nothing will happen (it is safe).
    2. If the protection is more like the traditional reverse diode between input and DVDD, as the DVDD is not powered up, the diode will turn on. These ESDs are actually quite powerful and can take a lot of DC current (100's of mA). Probably your digital driver will drop its output voltage before damaging that diode. Or instead, the internal LDO driving the DVDD will leak and the digital driver will actually "power" the DVDD up. I.e., you would see the digital input and the DVDD raise beyond 0.3-0.6V. How much would depend on how much the driver can put out and the DVDD takes in... The danger on either case is that the digital input trace may not be wide enough to support a lot of current for long time. Without looking at the layout we can't tell, but probably 25mA is a safe number. So, you would still need to add a resistor (likely 200 Ohms is more than enough considering that this condition is only there for short time).

    So, how do you know in which one of the two cases you are with the DDC112? Well, you have to measure the digital input voltage and the DVDD at the same time, while driven by the output driver. If you see input go up to VOH and DVDD be still near zero, then you are in case #1. If VOH doesn't raise or it does, but DVDD does too, then you are in #2 (there is current flowing into the input pin and you better put a resistor to protect both, our device and your driver, theoretically, as some drivers are pretty tough).

    Obviously, the other way is to make sure that the driver does not apply a "1" on those pins till you raise DVDD...

    Regards,
    Edu