TPS3808: pullup resistor setting

Tess Chen

Part Number: TPS3808

Hi Team,

We use the TPS3808G33DBV to do voltage monitoring for TMS570 MCU.

I am confusing about the R5 pull resistor value selection. I am referring to TPS3808G33 datasheet, R5 should keep above 10Kohm.

And below the the requirement for nPORRST from TMS570 MCU datasheet.

because R5 must be higher than 10k, so the high level output Vreset_n=3.3V-100uA*10k=2.3V . It is too low for electrical level.

I want to change R5 to 4.7K, so Vreset_n=3.3V-100uA*4.7k=2.83V , it's tolerance become better.

But in the reference of TPS3808, i am confused about no smaller than 10k.

Could you please give some guide for the R5 value setting? Thanks.

over 4 years ago

0 Ben C over 4 years ago

TI__Guru 56345 points

Hi Tess,

The TPS3808G33 has an open drain input to pulldown on the /RESET signal.

I am not sure about the TMS570 MCU nPORRST pin. Does the pin have an internal pullup resistor? It seems that the TMS570 already has a pulldown circuit if the MCU senses a low voltage on I/O supply or core voltage.

Anyway, based on the information you have given, the proper VOL should be no lower than 0.3V (logic value). Therefore, the calculation should be:

(3.3V - 0.3V) / 100uA = 30Kohms. Any value higher than 30Kohms will work for the application.

The high level will be 3.3V because in a non-fault condition, the nPORRST is not sinking 100uA; only when there is a fault condition.

Ben

0 user_CAP over 4 years ago in reply to Ben C

Intellectual 290 points

In real circuit, R5=10K.

When TPS3808G33 outputs hi level, the measured voltage is 2.42V which is approximity to 3.3V-10k*100uA=2.3V.

When TPS3808G33 outputs low level(reset), the measued voltage is 0.026 which is violate your calculation.

I want to know , can i change the value of R5 (from 10k to 4.7k), then i can improve the output hi level of TPS3808G33.

I worry about the datasheet of TPS3808G33. No lower than 10k.

6.5V must higher than 10k?

3.3V must higher than 3.3/6.5*10k=5.07k?

0 Ben C over 4 years ago in reply to user_CAP

TI__Guru 56345 points

Hi,

My calculation is only for the TPS3808G33 and 0.026V does not violate the VOL spec. The high-level that was measured is not from the TPS3808G33 but from voltage drop across R5 to the nPORRST.

I am confused with the high level from the TMS570. Is the 100uA current sink always present on the nPORRST?

My opinion is using a 4.7Kohm instead of 10Kohm should be ok. The recommended current through the /RESET pin is shown below:

Therefore, if the we take worst case scenario of VRESET = 6.5V, the current going into /RESET pin is 6.5V/4.7Kohms = 1.38mA which is below the 5mA max spec.

Ben