[C5535] Internal Pull-down/up value

Kaka-san

I am pasting Mark's response to the escalation on this thread internally , to Hitoshi-san. Hope this helps close it with your customer.

Regards

Mukul

Hi Hitoshi-san,

There was a similar question answered with an explanation of the internal pullup/downs on this E2E thread:
e2e.ti.com/.../81850

"Internal pull-up/pull-down resistors are implemented with weak transistors. As the voltage present on the I/O pin varies the relative gate voltage to this weak transistor changes which will cause the effective pull-up/pull-down resistance to change. Therefore, internal resistors do not have a linear response like external resistors. That is part of the reason you see internal pull-up/pull-down resistors specified as having a wide range of resistance. The wide range of resistance also provides margin for process variation."

"The ranges shown in the nominal column represent the typical variation of current with stated conditions at the values shown and unstated conditions varying over their full recommended ranges. Conditions may include voltage, temperature, and process."

...

TI places a range in the typical column for this device family because we cannot guarantee the min and max values of these internal pulls.

Since the strength of the internal pulls changes with voltage (biasing of transistors), we recommend relying on internal pulls only when the pin is floating (not connected to any capacitive load or possible external leakage path). Under these conditions, the weak pull does not have to overcome any external load, and the voltage on the pin will be very close to DVDD. This minimizes current through the CMOS input which can partially turn on both PMOS and NMOS transistors when presented with an uncontrolled input.

Choosing external resistors values to override the internal pulls (contention) can be calculated with the strongest internal pull values from the typical range. Internal pulls are strongest when DVDD = 3.3V, but when an external resistor pulls against the internal pull, the voltage present at the pin will be somewhere in between GND and DVDD (a resistor divider), so the strength of the internal pull will be weaker than the worst case - this is to our advantage when trying to size external resistors.

Calculate the worst case resistance of the internal pull at worst case DVDD voltage with Ohm's law: R = V/I

Treat the external resistor and internal "resistor" as a resistor divider network and ensure the VIH or VIL is satisfied. Margin will be added since the resistor divider network lowers the voltage to the pin, and reduces the strength of the internal pull.

Lets work through an example:

DVDD = 3.3V
Worst case, strongest internal pull down I_IHPD = 158uA (at 3.3V about 20.9k, R = 3.3V / 158uA)
We need to choose an external resistor that always meets VIH while the internal pull-down is enabled
For DVDD = 3.3V, VIH = 0.7*DVDD = 2.31V (close to 2.5V for which we have internal pull values)
Solve for X - the external pull up value...
VIH < 3.3V * 20.9k / (20.9k + X)
2.31 < 3.3V * 20.9k / (20.9k + X)
X < 8.96k

However, 2.31 is close to 2.5V and the datasheet provides internal pull range for 2.5V: 83uA (at 2.5V about 30.1k)
2.31x < 3.3V * 30.1k / (30.1k + X)
X < 12.90k

So a 10k resistor is probably adequate, but a resistor stronger/ less resistance than 8.96K has more margin...

Consider also the worst case with 5% tolerance for the external resistors (subtract 5% from the calculated resistor value, 5% stronger)

Regards,
Mark