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LM1117: Output voltage ripple

Gene Qiu
Intellectual 1720 points
Part Number: LM1117

Hi team,

Here come across some LM1117 chips output voltage ripple too much case need your help to guide whether what is risk/potenial reason with below schematic or layout ?  if moved the not working fine chip to other w/o LED board, the output is normal without such high ripple.

           C87-C88:    100nF±10%-50V-0603-X7R

           C164:  10nF±10%-50V-0603-X7R

           C156:10uF±10%-16V-1206-X7R

           R197:1/10W-1KΩ±1%

           LED1 :80 mW

  • 0
    TI__Genius 13085 points

    Hi Gene,

    I can think of two reasons why this could be happening.

    • The input voltage is drooping due to the load and putting the LDO into dropout 
      • Measure Vin and Vout using the scope to ensure the input voltage is stable.
    • The circuit you described does not have a high enough output capacitance 
      • Increase Cout to 10uF minimum  and ensure it meets the ESR requirements as well. 
      • Ceramic capacitors should not be used due to their very low ESR unless you add a series resistor to increase the effective ESR

  • 0 in reply to Kyle Van Renterghem
    TI__Intellectual 1720 points

    Hi Kyle,

    Thank you for your help. I went to onsite debug, and check the two points you suggest, 

    1. according to the specification conditions to verify the actual load capacity, conditions: 5V input and output, parallel 10uF capacitor, load current 500mA, output voltage can be stable at about 3.3V, normal waveform (stable 3.3V, very small ripple 20mV and no other abnormalities);
    2. The failed components put on other fine PCBs, and the display screen is continuously dark (observe output 3.3V abnormal ripple around 200mV)
    3. Use external 5V directly as input, the failed LM1117 can observe small ripple and display screen recover

    Do you have any other comments about the abnormal high 200mV ripple of LDO LM1117 in above scenario?

    Best Regards

    Gene

  • 0 in reply to Gene Qiu
    TI__Genius 13085 points

    Hey Gene,

    There is always part to part variation but it is good to know that the failure follows the part when put on working PCBs. It is interesting that the failing part recovers when an external 5V input is used. This makes me concerned that the input voltage may be too low Can you capture a scope shot showing Vin?