Here introduces how to calculate pull-up resistance / pull-down resistance when they are not specified for an IC that has built-in pull-up resistance / pull-down resistance.
Step1：Confirmation of block diagram
Before calculating pull-up resistance/pull-down resistance, it is important to confirm the block diagram.
This is because that pull-up/pull-down may not be executed by resistance, but the pull-up/pull-down may be executed by a constant current element.
If pull-up / pull-down is executed by constant current element, the current does not vary depending on the applied voltage and the current becomes constant unlike resistance. Let's refer to the terminal current shown on the data sheet.
Step2：Calculation of resistance value
If the block diagram shows that pull-up / pull-down is executed by resistance, check the terminal current on the data sheet. Here explains it referencing the voltage regulator XC6228 that has CE terminal equipped with built-in pull-down resistance.
As XC6228 is equipped with pull-down resistance, check the terminal current at CE "H" where the current flows into pull-down resistance.
XC6228 CE "H" terminal current
| PARAMETER | SYMBOL | CONDITIONS | MIN. | TYP. | MAX. | UNITS |
|---|---|---|---|---|---|---|
| CE "H" Current | ICEH | VCE=VIN=5.5V | 3.0 | 5.5 | 9.0 | μA |
Here, check the measurement conditions (CONDITIONS) specifying CE "H" Current.
" VCE=VIN=5.5V " described for CONDITIONS means that "H" Current when CE = 5.5 V is applied is specified. The voltage to which CE is applied to the pull-down resistance. Thus, if 5.5 V is applied to pull-down resistance, CE "H" Current flows.
If you understand this far, you can calculate the pull-down resistance value using Ohm's law.
The following formula is calculated by Ohm's law "R = V / I".
CE applied voltage = CE “H” Current x “pull-down resistance”
Using the above formula, the pull-down resistor can be calculated as follows.
Pull-down resistance MIN = 5.5V / CE “H” Current MAX = 611kΩ
Pull-down resistance TYP = 5.5V / CE “H” Current TYP = 1000kΩ
Pull-down resistance MAX = 5.5V / CE “H” Current MIN = 1833kΩ
Here calculated the resistance value referencing the pull-down resistance, however, you can also calculate the pull-up resistance in the same way.