• TI Thinks Resolved

TLV3201: Relaxation Oscillator Output Stuck at 0.5VDD

Prodigy 20 points

Replies: 4

Views: 39

Part Number: TLV3201

Hello,

I am using the TLV3201 in a relaxation oscillator circuit to measure changing capacitance in a system by counting the number of edges outputted. The circuit receives +3.3V (+3.3V_CM) in this way: +5V_USB -> LDO -> +3.3V_USB -> load switch -> +3.3V_CM. The 3.3V_CM is controlled via an enable/disable signal. When I was testing this on the bench getting 5V from my laptop USB, I was noticing some edge count dropouts, maybe about a few times a day to once an hour. Using a scope, I was able to observe noise on the USB 5V rail, which then translates to noise on the 3.3V rail. I saw that if this noise occurred on the input as the output edge was transitioning, the oscillations would stop and the IC would output 0.5VDD. The only way to get out of this state is to disable then re-enable the 3.3V going to the chip. I can not add filters to the input as we are trying to measure small changes in capacitance and don't want to increase the baseline capacitance. Adding some additional filtering and powering this from our system's USB 5V has stopped this from occurring, and we are able to monitor this signal for weeks without a single dropout. Although the problem has went away while on the system, I am planning to implement a way to automatically recover if and when this occurs, maybe through the use of a watchdog that toggles the enable/disable when it notices no oscillations when there should be.

Revisiting this project, I really want to understand this problem better. My main question is why does the output get stuck at 0.5VDD when this happens? Is this related to the input changing faster than the propagation delay of the TLV3201? Below are scope shots taken a while ago on the issue. I also looked at different rail voltages in the scope shots in order to determine where the noise is coming from.

Channel 1: Yellow, TLV3201 Output | Channel 2: Blue, TLV3201 Input | Channel 3: Purple, +3.3V_CM

Channel 1: Yellow, TLV3201 Output | Channel 2: Blue, TLV3201 Input | Channel 3: Purple, +3.3V_CM | Channel 4: Green, +3.3V_USB

Channel 1: Yellow, TLV3201 Output | Channel 2: Blue, TLV3201 Input | Channel 3: Purple, +3.3V_USB | Channel 4: Green, +5V_USB

Channel 1: Yellow, TLV3201 Output | Channel 2: Blue, TLV3201 Input | Channel 3: Purple, +3.3V_USB | Channel 4: Green, +5V_USB

Thank you,

Thi Le