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TPS735: TPS73533 LDO Fixed 3v3 passes 6.5v

Guru 54027 points
Part Number: TPS735
Other Parts Discussed in Thread: SN65HVD230, INA240

Hello,

Accidently placed two resistor 90 degrees on +5v buck regulator, outputting +6.5v to LDO input. That passed +6.5v onto 3v3 bus immediately shorting VDD on TM4C1294KCPDTT3 MCU. After fixing buck regulator resistors it passed +5v to input of LDO, passing +3v3 to later and newly placed TM4C1294KCPDTI3.

How can +6.5v input to fixed +3v3 LDO output +6.5vdc after the MCU was removed? Oddly with shorted VDD the LDO output level was +3v3, immediately dropped to +2.09v via DMM check. Perhaps something in LDO was pulling the +5v buck down to +6.5v but how LDO output still remain +6.5v? 

Absolute maximum input TPS735 is noted as 7.0v [SBVS087M –JUNE 2008–REVISED JUNE 2018]

  • Hi GI,

    This is a little confusing. At first you said:

    "After fixing buck regulator resistors it passed +5v to input of LDO, passing +3v3 to later and newly placed TM4C1294KCPDTI3."

    Then you said:

    "How can +6.5v input to fixed +3v3 LDO output +6.5vdc after the MCU was removed?"

    Are you saying that the LDO was outputting 6.5V when the MCU was removed, but was outputting 3.3V when the MCU was replaced?

    If the output voltage was the same as the input voltage, this to me would imply one of two things. Either the device was damaged and was just passing the input to the output, or there was a short external to the IC that was causing this. 

    Regards,

    Nick

  • Hi Nick,

    Are you saying that the LDO was outputting 6.5V when the MCU was removed, but was outputting 3.3V when the MCU was replaced?

    Right, since the +5v buck down was actually +5.02v after rotating two resistors 90° in feed back loop, buck input was +11.3v. Later replaced the MCU after discovering the LDO output was +6.5v with MCU VDD (62Ω) short removed. The LDO was unloaded and VDD 7.9kΩ on 3v3 test point. 

    Either the device was damaged and was just passing the input to the output, or there was a short external to the IC that was causing this. 

    The LDO seems to be ok though the output was +6.5v same as the input. The shorted VDD of MCU pulled the LDO +6.5v output down 2.09v. Point being the LDO was not regulating the output to 3v3 with 6.5v input. Amazed LDO still works not getting hot to the touch and regulating +3v302 with new MCU. 

  • Hi GI,

    I don't know why the input would get passed through to the output if the LDO was not damaged, unless there was a physical short. 

    Was the LDO getting hot when the output was 2.09V? I wonder if the LDO was in current limit when the damaged MCU was still connected.

    I'm not sure what you mean by "The LDO was unloaded and VDD 7.9kΩ on 3v3 test point." Can you explain this?

    Regards,

    Nick

  • Hi Nick,

    "The LDO was unloaded and VDD 7.9kΩ on 3v3 test point." Can you explain this?

    Before installing the new MCU the VDD bus was unloaded. After more review there was still a CAN driver SN65HVD230 on LDO output separated by 0Ω resistor, absolute max VCC +6v. It's (CAN) not getting hot but was in forced standby mode on POR.

    I don't know why the input would get passed through to the output if the LDO was not damaged, unless there was a physical short. 

    BTW forgot to mention there are two 3v3 LDO's supplied by the +5v bus. Neither LDO was getting hot, hence 2.0v on the output as VDD (3v3) was very near shorted 62Ω with bad MCU still installed. Oddly there is CAN driver on same LDO as MCU and 3x INA240's (ABSlute +6v) on other 3v3 LDO output. I didn't leave power on long after seeing +6.5v on the first LDO output, after 62Ω short was removed. The first LDO 3v3 bus impedance was roughly 7.9KΩ with a new MCU and other 3v3 bus peripherals.

    I wonder if the LDO was in current limit when the damaged MCU was still connected.

    It must have been limited since I saw the DMM show 3v3 then suddenly drop all VCC voltages, even on 2 other buck regulators. Next time I will use 6vdc supply to check the +5v buck is working correctly. Logical assumption the LDO pass through (+6.5v) must have been DMM counting PWM pulses from +5v buck being mostly unregulated, how bazar.