Latching on power up

Document created by analog-archivist Employee on Feb 23, 2016
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We have a module built with using AD7665 and power circuits made exactly as it
specified in datasheet on page 13 with DVDD connected to AVDD through 47 ohm
resistors and two capacitors – 10nf and 10uF.

OVDD is coming from 3V3 switching regulator and delayed by about 100ms from
5V(AVDD) settles. It was working fine until it was powered from DC/DC converter
with 5V rise time about 10ms. But when it been connected to 5V through MOSFET
switch, which would apply 5V relatively sharp(50us rise time), we found that
AD7665 device is latching and beginning to consume excessive power and won’t
operate. Voltage on DVDD was dropping to 2.5V, which means that due resistor in
series it consumes 50mA, while under normal operation consumption on DVDD was
very small and it was matching AVDD even with resistor in series.

We also found that if 10uF tantalum capacitor will be removed, it wont happen.
For now we have provisional fix where 10uF capacitor is removed and 10nF
ceramic capacitor is replaced by 0.47uF ceramic capacitor of the same
size(0603). I can see small lose of performance on high frequencies above 2K,
where SINAD drops from 87dB to 86dB, but it still within specification.

Any explanation why it can happen?

 

There is a requirement that OVDD does not exceed DVDD by more than 0.3V. Kindly
refer to the Poewr Supply section on page 15 of the www.analog.com/AD7665
datasheet.

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