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AN3275 Datasheet(PDF) 11 Page - STMicroelectronics |
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AN3275 Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 32 page AN3275 Emergency deactivation/fault detection Doc ID 17962 Rev 1 11/32 3.2 PORADJ VDD undervoltage with external divider In this case, a resistor bridge is applied to the PORADJ pin (see Figure 7). 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: VPORADJ =(R2/R1 + R2) x VDD it is possible to set R1 and R2 such that the device powers on and off at the values of VDD desired by the user (R1 + R2 = 100 kΩ typ). In particular, R1 and R2 have to be set so that, when VDD is getting low, before turning the microcontroller off, the smartcard has to be switched off properly as well. The same is true for the microcontroller startup in that the smartcard has to be turned on after the microcontroller. Figure 8 and Figure 9 on page 13 show the VTH(ext) rise and VTH(ext) fall on the PORADJ pin (1.196 V and 1.155 V, respectively). The VTH(ext)fall threshold of the ST8024L is slightly lower (80 mV typ.) than the ST8024 device. If for example, the microcontroller is shut down at 2.5 V, appropriate resistor values must be chosen to ensure proper deactivation of the ST8024L device. Table 2 shows an example of the resistor values between the ST8024 and ST8024L devices if the microcontroller is shut down at 2.5 V. Table 2. Resistor values for VTH(ext)fall trip point ST8024 ST8024L R1 50 k Ω 55.5 k Ω R2 50 k Ω 44.5 k Ω VTH(ext)fall 1.25 V 1.14 V Table 3. VPORADJ trip point VDD VPORADJ ST8024 ST8024L 5.0 2.500 2.275 4.5 2.250 2.048 4.0 2.000 1.820 3.5 1.750 1.593 3.0 1.500 1.365 2.5 1.250 1.138 2.0 1.000 0.910 |
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