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8/1/2017 11:53 PM
 

Hi,

in design with MP4470 I noticed Vout ripple of 500 mV at no load and at low load, when we increase load (above 2 A or so)  ripple goes dramatically down. We use output capacitor Panasonic 47uF 30 m ohm   OSCON 25SVPF47M directly at dc/dc output and additional 4 distributed over the PCb (which is rather large and load is distributed as well).

 

How to decrease Vout ripple at no load/low load?


 
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8/2/2017 11:20 AM
 

Hi Ales,

 

MP4470 may be working at DCM to maintain high efficiency at light load to no load condition. Higher output voltage ripple is expected at light load condition.

There are couple of ways to improve the output ripple. First, you can increase the inductance to decrease the current threshold entering DCM. Higher inductance will also help on reducing the output ripple. Second, you can add more output caps to attenuate the output voltage ripple. 

 

If you need low Vout ripple through out all load condition, then you might want to consider using a Forced PWM/CCM part. It will force the device to operate at CCM at light load condition, so the output ripple will be lower. One of the part I would recommend is MP9943A, it is a 3A part with force CCM mode.

 

Regards,

Kevin

 

 
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8/10/2017 6:20 AM
 

Hi,

 

for this particular application I can live with this ripple voltage. At Vout=13 V I have 500 m V of ripple, additional capacitor does not help reducing ripple, it only prolong ripple period. I assume I could reduce ripple by choosing inductor with more inductance, as you wrote, but it is already now physically very big ... MP9943A is not an option, I need higher output current.

 

Ripple of 500 mV goes dramatically down when Iout>1 A, which is quite high current considering that I have I out max 5 A. If you will find another approach to reduce ripple voltage in the future, let me know.

 

As mentioned, I Will not change anything for this application, but in general I'm not happy with such high ripple voltage.

 
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8/10/2017 6:01 PM
 

Hi Ales,

 

It seems like one of the reason the output ripple is high is due to the high output voltage of 13V. For the nature of buck regulators, the ripple will increase if output voltage increase. Could you please share your schematic, so we can take a look to see if there is anything else that can be improved?

 

Thanks,

Kevin

 

 

 
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8/17/2017 11:14 AM
 

Hi Ales,

 

Thank you for sending over the schematic for review. It seems like the regulator was unstable with the previous ramp compensation. I am glad to know that the issue is resolved now with output ripple reduced to 50mV after recalculating the ramp values. 

 

Regards,

Kevin

 
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