Electrically quiet isolated DC-DC convertor

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316a

316a

Member
Joined 2002
#1
I have a portable boombox built with a Raspberry Pi and IQaudio DAC . This runs from a 12V SLA with the Rpi powered via a Murata NDY1205C isolated DC-DC convertor . I am experiencing issues with 5V noise , which manifests at random as quiet whistling and crackles . No issues running from a separate 5V power bar or external PSU . The NDY device is connected as per the datasheet with low-Z 100uF bypass capacitors on input and output and cables are as short as possible . Minimum current for the device is 150mA , I measure 165mA drawn by the Rpi/DAC combination . Maybe I'm expecting too much from the NDY in terms of noise , but has anyone else used this device or similar DC-DC convertors and obtained decent noise performance without adding additional LC filters ?

Most frustrating , my scope is on the blink . First time in years I've needed to poke around in equipment 🙁

cheers

316a
 
#3
The whole point to to isolate the Rpi/DAC from the 12V power amp supply to avoid noise as the Rpi CPU core derives power prior to the DAC and the end result is even more noisy . If the DAC had direct 5V power input to the board like the Hifiberry Dac pro , and the layout made as tight as possible , I would probably get away with a linear reg . The convertor is also there to conserve battery power , it's reputed to be 71% efficient so a small gain in battery life

For the time being I have placed a 2.5mH 0.3R DCR choke and 470uF cap LC filter in circuit , much more acceptable . I suspect changing the chipamp from TDA7297 to TDA7377 , that has much less gain , may make the situation improve even more but I'd rather get this 5V isolated power situation nailed than apply band-aids

cheers

316a
 
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