AN2269 datasheet(9/25 Pages) STMICROELECTRONICS

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AN2269 - Application Note

Emergency Deactivation/Fault Detection

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3.2

PORADJ VDD Undervoltage with External Divider

In this case, a resistor bridge is applied to the PORADJ pin (see

Figure 6). VTH(ext) rise and

VTH(ext) fall are the External Rising Threshold Voltage and the External Falling Threshold

Voltage on VDD, respectively. They are the voltages on pin PORADJ that switch the device

ON and OFF. By knowing these values and using the formula:

Where:

it is possible to set R1 and R2 in order to get suitable values for VDD in order to turn the

device ON and OFF (R1+R2 = 100kΩ typ).

In particular, R1 and R2 have to be set so that, when VDD is getting low, before turning the

microcontroller off, the smart card has to be switched off properly as well. The same goes

for microcontroller start-up. The smart card has to be turned on after the microcontroller.

Figure 7 and Figure 8 on page 11 show the VTH(ext) rise and VTH(ext) fall on the PORADJ pin

(0.21V and 1.15V, respectively).

As long as VDD gets the proper start-up value (so that VTH(ext) rise = 1.196V), OFF goes low

for tw + tdebounce (tw ≈ 16ms, in this case). During this time, the device can not be turned on

by the CMDVcc. To turn the device on, CMDVcc must go low at least by the end of about

16ms (while OFF is high).

Figure 6.

External Resistor Bridge Applied to PORADJ

PORADJ

2 R1

⁄

() V

VDD

GND

To PORADJ

AI11885

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