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TPS62175 - Utilizing Enable pin for starting off at a set input voltage.

Other Parts Discussed in Thread: TPS62175, TPS62125, TPS54040A, TPS54336A, TPS62177, TPS62130, TPS3701

Hi,

I am powering up the EVM with a 10mA current limited supply and am seeing the TPS output go into hiccup mode if I start off with a load of 150 ohm (22mA).

The power supply is ramping up very slowly as it has to charge 1200uF caps.

I am thinking this could be the problem and would like to wait for the voltage to ramp up and then turn on, instead of at 5V when the TPS starts off on its own.

This converter does not have a soft start, so that cannot be changed, but can I use the EN pin assuming that the Vmin for turn on at this pin is guaranteed to be 0.9V?

Thanks,

Ashvin.

  • Whew, 1200uF Cout? Why do you use so much? Yes, this will definitely trip the current limit during startup.

    Also, make sure SLEEP is tied high to not be in sleep mode.

    Starting at a higher Vin may help a little, but you will still be trying to charge 1200 uF in the fixed SS time. You need to reduce the Cout to not hit current limit.

    On the EN pin question, I want to understand how you are reading the spec as this type of question is being asked more and more. Our spec means that you must drive it less than 0.3V to for sure turn it off and above 0.9V to for sure turn it on. In between is the gray area of this digital input, where it could be either high or low. How did you understand the spec? What tolerance did you assume on the turn on and turn off voltage levels?
  • Maybe I was not clear, the output of the 10mA current limited supply is having an output cap of 1200uF, so that's coming up slowly. That's acting as a holdup in our product. This is fed as the input to the TPS62175 eval board; sleep is set to high through the jumper setting (default).
    So to be clear, what I am using is a pristine eval board, except, I now have a resistor divider trying to feed in a voltage to the EN pin to make it start up after its input has reached around 10Volts, instead of the default 5V.
    I am reading the spec to say that at turn on, the voltage at the EN pin should be higher than 0.9V for it to turn on. Due to slow nature of the input to the eval board, if I let it turn on at around 5 volts, the output goes into a hiccup mode and I am trying to avoid that by turning on at a higher input voltage, so that the inrush can be managed by the big 1200uF caps.
  • Ah, thanks for clarifying.

    What is your maximum Vin and load current during operation? For example, the TPS62125 has a precise EN pin which can be used for exactly your situation.

    Back to the EN pin, if EN must be greater than 0.9V in order for the IC to turn on (i.e. it won't turn on below this level), where would you expect it to turn on at? i.e. would this be 1V or 2V? Or do I misunderstand your thinking on the 0.9V?
  • I cannot use the TPS62125 as my input voltage can go upto 24V. I have a 150 ohms resistor across the output so the load current @ 3.3V is around 22mA.

    I am putting a resistor divider across the input to give 1/10th the voltage at the EN pin, so that if the input voltage is around 10 volts, the EN pin should get enough voltage to start off, is the EN pin on this device precise enough to do this?

    If not, what other device would you suggest that can do this? The load current upto 200mA would be enough but I need to have it in the smallest possible space with minimum number of external parts.

  • So you have a slow rising input due to 1200uF on the input as well as current limit on the input of 10mA. You would like a input operating range of xx to 24 V to generate an output voltage of 3.3V with a load current of 22mA max. assuming 80% efficiency to keep your input current below 10 mA, you need minimum start up above 9 V, plus you have account for charging what ever output caps you have. You need a external SS as well.

    You could look at TPS54336A or if you can use a non synchronous solution with external diode, TPS54040A would be a better couce as it only has cycle by cycle protection and no hiccup mode.
  • Thanks for these details. Yes, TPS62175 is the smallest solution for your requirements. Actually, the TPS62177 would be a little smaller.

    I know the TPS62125 won't work for your needs, but please compare its EN threshold spec to that of the TPS62175. The TPS62125 integrates an SVS basically and thus provides a precise threshold. Most other devices, like TPS62175, just have a standard digital input which has a wide range of threshold voltage. As I mentioned in my first reply, the actual threshold voltage on the EN pin will be between 0.3V and 0.9V which is why you see it turn on at about 5V with your 1/10 divider (the threshold is around 0.5V).

    I'd very much like to get your insight into how we can improve our specification here to avoid the confusion you've had. You could also see the EN pin spec on the TPS62130 device, which has the typical EN threshold voltages. Does this help you see how that type of pin operates? Additionally, with the current spec on the TPS62175, what sort of min/typ/max spec did you consider/assume? I think you viewed the 0.9V as the voltage at which the part would not turn on below, so what was assumed as the maximum voltage at which it would turn on?

    I can think of a few ways to overcome your challenge:

    Use the TPS62125 with a zener diode on the input to clamp Vin to <17V. This is what I do for a 4-20 mA current loop source here: www.ti.com/.../slyt488.pdf

    Use the TPS62175 with an RC from Vin to the EN pin to delay the turn on until after the Vin is up to >10V or so. Because of the wide range of EN threshold voltages, this would give a wide range of delay times for startup, but I think you have quite some startup delay already. A diode back from EN to Vin is probably a good idea to avoid biasing EN above Vin.

    Use a discrete SVS IC to drive the EN pin of the TPS62175 with the SVS's precise threshold.

    Remove the load current during startup. I think the main issue you have is that your 22mA load is higher than your 10mA source. So, you must wait for your source voltage to get higher to have enough power to drive your 22mA on the output (the step-down conversion ratio is key). I think the wide UVLO level already in the IC allows you to charge the output caps just fine without turning off, but you can't deliver the DC load current. If you could disable this somehow in your system, possibly through the use of the PG pin, it would be best.

    Depending on how high your input source voltage can be guaranteed to go, you might be able to thread the needle on the EN pin voltages. i.e. if you know it will always get to at least 22V, then with a 10k/234k EN divider, you would be sure to turn on by 22V and wouldn't turn on below 7.3V.
  • Hi Chris,

    My main confusion was whether the EN pin on this device was an analog pin or a digital pin. I did not see this spelt out clearly. I could have guessed by looking at the numbers.

    I assumed that if the specification table said that  for a high threshold on the EN min. is 0.9V to start off, then, that is a guaranteed spec. which I could work off.

    I expected the chip to turn on only if I gave a voltage above 0.9V at EN pin. It is not clear from the data sheet that the EN pin has a wide band of threshold voltage.

    I have checked with the load connected and a resistor divider at the EN pin and it seems to work, but again, is it guaranteed that I will switch off only above 0.9 and switch off below 0.3?

    Thanks,

    Ashvin.

  • It is a digital input like most EN pins that I've seen. If it were an analog input with a precise threshold, we would have spec'd it as in the TPS62125.

    This means that there is a wide range of where it is specified to turn on and off. We specify what is needed to drive a digital input. The worst case logic high and low levels.

    Did any of the options I listed sound possible in your system?
  • Hi Chris,
    Could you point me to any SVS IC to use for this?
    Among the solutions that you provided, I think I need to work on what part of the load I could turn off during startup. Seems possible.
    I agree that using an RC from the Vin to the EN is possible but I think it would give a variable on time. Also would add more components.
    My input voltage can go down as low as 15V, so that could mean I am not guaranteed to turn on if I use a resistor divider at the EN pin.

    I was thinking of using something like a TPS54162 which would have a precise analog EN pin, , but need more components for external compensation and frequency setting.

    What is the closest that can come to the TPS62177 which would need least external components?
    Thanks,
    Ashivn.
  • Hi Ashivn,

    TPS3701 should work for you. This offers a window coparator, but the over and under detection have separate outputs.

    Very Respectfully,
    Ryan