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LTM4680 closes outputs after a while

Category: Hardware
Product Number: LTM4680Y

Hi ,

We have LTM4680  uModules on our custom board. Firstly, we dont want any programming interface and arranged resistors according to what we want  at the output to make it work standalone. Our problem is pgood signal is going low within an interval like 60ms after giving power. Also, when we debug ALERT output same as pgood signal, no problem for like 100ms but later going low.

What can be the problem? We didnt changed any component in the datasheet typical application. It is as same as typical. 

further details,

vdd33 is outgoing successfully. We read it as 3.3 so no problem. Additionally, resistor connections for VOUT_CFG and VTRIM_CFG(no connection,leaved open) pins are connected as in the typical application (figure 45 in datasheet)

thank you.

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  • Hi ,

    The typical applications should work fine. May I know the conditions of your test? The input voltage and the total load current.

    The only condition for Power Good being pulled to the ground is OV and UV. Can you monitor the output voltage if it decreases below UV or above OV when power good is going low? Also, please check the input voltage if it is stable during test. 

    In the datasheet of the LTM4680, the module was tested at 0.9V Dual Channel Single output up to 60A load as shown in the efficiency data below. 

    Let me know if the PGOOD unexpectedly going low even if the input voltage and output voltage regulation are stable. 

  • Hi,

    Firstly, thank you for your time and help. We are struggling a lot because of this problem.

    Before start, I want to say we took outputs while there is no load at the output of LTM4680.Therefore, you can see voltage(Vout) is dropping very low.

    NOT: Blue probe is PGOOD always and Yellow probe is changing as Vout and Vin w.r.t figures.

    Vin-vs-PGOOD

    above figure shows Vin-vs-PGOOD. As you can see input voltage is stable when PGOOD is asserted. (Voltage scale is 5v in oscilloscope)

    Vout-vs-PGOOD

    Above figure shows Vout-vs-PGOOD.As you can see, voltage output is rippling between 0.6v and 0.85v(our output arrangement value with resistors). After a while, both PGOOD and Vout goes low and no outputs observed.(Voltage scale is 500 mv in oscilloscope)

    Vout-detailed

    Above figure is extra to show ripples at the output voltage when PGOOD is asserted.(Voltage scale is 500 mv in oscilloscope)

    BR

  • Hi MustafaDestegul

    Thanks for the waveforms. Most likely the fault causing the PGOOD going to low is under voltage (VOUT_UV_FAULT_LIMIT). Since you are using resistor (VOUT_CONFG) to set the output voltage, the under voltage limit is -7% of the output voltage 0.9V. The UV limit of your circuit is 0.9 - 0.9*0.07 = 0.837V. In your waveform, the Vout ripple voltage is ~400mV which very high and can false trigger the UV protection. 

    We can try using low ESR output capacitor to decrease the output ripple voltage and use 1% tolerance resistor for VOUT0_CFG and VOUT1_CFG to have an output voltage as close as possible to 0.9V. 

    Please check the datasheet here for design recommendations: LTM4680 Datasheet and Product Info | Analog Devices

    Datasheet Information

    Under voltage limit

    Output Capacitor Recommendation

  • hi,

    We have changed the capacitors at the output with very low ESR one and resistors with %1 tolerance. However, we have still the same problem. Also, we observed ,as you can see in the below figure, that the output voltage goes low from 0.9V to ~0.85V simultaneosly and PGOOD goes low after that drop. Do you have any guess what can cause this problem?

    NOT: yellow one Vout and green one is PGOOD.

  • Hi MustafaDestegul

    Thanks for the waveform. The waveform you posted shows that Vout drop first then PGOOD going low. 

    This time we can cross out the ripple voltage as a possible source of UV false triggering. Another factor that can affect the output voltage regulation is stability, one of the typical symptoms of unstable power supply is output voltage oscillation. Unfortunately, we cannot check the bode plot of your power supply since it is shutting down after startup.

    If we can disable the UV protection, we can check if the output voltage is indeed oscillating, but to disable or adjust the UV limit you will need a programming interface. From your original post, you mention that you don't want any programming interface. My suggestion is to use first the demo board made by Analog or copy the layout and components used in the demo board. 

    You can find the demo board information here. 

    DC2844A Evaluation Board | Analog Devices

    To tune your power supply, you can read this application note. 

    AN149 Modeling and Loop Compensation Design of Switching Mode Power Supplies (analog.com)

  • Hi reyjr,

    Thank you for your time and effort you gave us.

    We decided to take demo board and look forward with it. 

    Thanks for everything.

    Best regards.

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