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LM3670: The output voltage of this DC-DC converter is higher than expected

Wilfred Baluja
Prodigy 30 points
Part Number: LM3670

According the the datasheet the maximum output voltage over the full specified operating temperature range should be 1.2 V + 4% = 1.248 V but the voltage measured at room temperature is very close to 1.33 V.
This part is used for around 3 years in a product with Safety Integrity Level (SIL) certification and as result this voltage is periodically measured by the MCU but only recently we found multiple cases where the diagnostic flag was set due to this voltage being way out of tolerance. The threshold value to set the diagnostic flag in our firmware is 1.28 V.
Since this design is also Intrinsically Safe we are required to limit the capacitance in this board, so Cin is only 1 uF, L1 = 10 uF, Cout is 1 uF plus 5 x 0.01 uF. The capacitance is lower than specified in your datasheet but typical load current is only 1 mA, and rarely reaches 2 mA. Additionally, he don't have any evidence showing that we had this problem before, even when the same capacitance has been used for the last 3 years.
We have not observed any misbehavior in the components powered by this converter, we are just concerned that the Vout is way out of tolerance and not sure it could lead to premature failures.
If you don't believe we are not exposed to any risks, we could just increase the diagnostic threshold but first we need to hear from you a reasonable explanation about why Vout is not in accordance to the specification in the datasheet.

  • 0
    TI__Expert 7585 points

    Hi User, 

    Can you please send the schematic?

    If I understand correctly, you have used the devices for three years and you are noticing this problem only now.

    Such a low COUT can be the reason because the device is not able to respect the datasheet specifications (that should be verified with the reference schematic and good layout), however is very strange that you are noticing this problem only now.

    Have you done any BOM changes recently (for example inductor/capacitor part number)?

    Can you try to place a LM3670 device, for which you read a VOUT of 1.33V, on an 'old' PCB (build before you started having this problem) to see if the problem still persist?

    Do you have any info of the batch number of the new samples?

    Thanks,

    Stefano

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).