LD1117S33CTR design problem

[XiZhao]

In our company's mass production product( we desgien and produce BLDC motor driver）. We stumble across  that in some PCBA (about 25% of all the mass production)  the LD1117S33CTR output have voltage oscillation.

Becasue of the 3.3V output is for MCU circuit(directly conect to analog VCC) , so the oscillation will cause MCU ADC  sample problem which  cause some noisy problem of our motor. 

circuit diagram

If we reduce or increase  capacity of C54, both method can solve the problem,as below shows.  So i think the problem is cause by our desigen do not have margin to overcome the capactior deviation. I  just want to comfrim how can we make sure the output is stable  enough when we use the LD1117 in some new product. How can we find and verify the problem in pre-desigen and not bring this probmle to mass production.

 

[Peter BENSCH]

Welcome @XiZhao, to the community!

I am very sorry that you are having such problems with the LD1117. Actually, it has been known for a long time with this LDO regulator that its capacitor at the output should have an ESR of at least 0.2-0.3 ohms up to about 18...22 ohms. Because of its small size, you are probably using an X7R, whose ESR is considerably lower, which is why the LD1117 can tend to the observed oscillation.

If you don't want to use a tantalum or electrolytic capacitor for C54, you can put a resistor of 1...10 ohms in series with it and you shouldn't notice any oscillation at all. Additionally, please make sure that C7, C8 and C54 are as close as possible to the LD1117 to minimise additional line inductance, which can also exacerbate the problem..

Hope that helps?

Good luck!
/Peter

[XiZhao]

Dear Peter：

         you are right, I use the X7R MLCC capacitor.

         And I  use 10ohms resistor in series with C54,the oscillation become bigger, as below picture shows.

       I agree with you in principle side.but the actually situation maybe not same. Do you have another idea?

[Peter BENSCH]

What about the length of the traces or the distance of the capacitors to the LD1117?

[XiZhao]

very close， as below show；

[Peter BENSCH]

Quite short indeed.

Please use a tantalum instead of the X7R without the 10ohms resistor.