CDC3RL02: Unused LDO and Output

Brian Angiel

Part Number: CDC3RL02

Hi team,

I have a question regarding CDC3RL02. Here is my basic setup:

VBATT = 3.3V
MCLK = 24MHZ 1.8V CMOS
LDO = unused/floating

With the above, my output looks like Image 1 (below). However, when I drop VBATT to 1.8V, the output looks like Image 2 (below)--it cleaned up quite a bit. So I have two questions:

Can I operate at VBATT = 1.8V - are there any potential long-term problems with that approach? This isn't within spec according to the datasheet, but the results look better.
Is my problem potentially related to the floating LDO? Does the LDO need to be connected to a within-spec capacitor even if it's unused?

Bonus third question - is there anything else that might be causing this problem? Thanks in advance for your help!

1. Output when VBATT = 3.3V (bouncing back and forth between 3.3V and 1.8V)

2. Output when VBATT = 1.8V

Regards,

Brian Angiel

over 5 years ago

0 Alan O over 5 years ago

TI__Mastermind 32575 points

The LDO output pin cannot be left floating (unstable condition) and must be connected to a within spec cap (1uF to 10uF, 0.1 mOhm Min. ESR) for proper operation. Note the output drivers are internally supplied by the LDO.

Alan

0 Brian Angiel over 5 years ago in reply to Alan O

TI__Expert 3345 points

Thanks, Alan. I saw that note in the d/s (Section 9.1.3.), and I figured this might be my issue. I appreciate the feedback.

Brian

0 Brian Angiel over 5 years ago in reply to Alan O

TI__Expert 3345 points

Follow-up question:
A quick test confirms that the inclusion of the cap on the LDO pin drastically improves the performance of my device. However, I've already had a number of PCB's built up, and these don't have room for a cap. As such, for the short term, I plan to run these devices at VBATT = 1.8V. I understand this is outside the recommended specification of the device (but inside the abs max ratings)--can you help me understand the worst case scenario in doing this? Could this stress or otherwise damage the device over time?

Thanks,
Brian