ST8024LACTR datasheet(19/35 Pages) STMICROELECTRONICS

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ST8024L

Functional description

Doc ID 17709 Rev 5

19/35

Figure 4.

Voltage supervisor

6.2.2

With an external divider on pin PORADJ

In this case, a resistor divider is connected to the PORADJ pin (see Figure 1). Vth(ext) rise

and Vth(ext) fall are the external rising threshold voltage and the external falling threshold

voltages on pin PORADJ that switch the device on and off. By knowing these values and

using the formula:

VDD UVLO threshold (falling) = (R1+R2)/R2 x Vth(ext)fall

VDD UVLO threshold (rising) = (R1+R2)/R2 x Vth(ext)rise

it is possible to set R1 and R2 in order to get suitable values for VDD undervoltage (UVLO)

thresholds, in order to turn the device on and off (R1 + R2 = 100 kΩ typ.).

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

microcontroller off, the smartcard must also be switched off properly. The same is true for

the microcontroller startup - in such case the smartcard must be turned on after the

microcontroller. The reset pulse width tW is doubled to approximately 16 ms.

Input PORADJ is biased internally with a pull-down current source of 4 µA which is removed

when the voltage on pin PORADJ exceeds 1 V.

This ensures that after detection of the external divider by the ST8024L during power-on,

the input current on pin PORADJ does not cause inaccuracy of the divider voltage.

Note:

The Vth(ext) threshold of the ST8024L is slightly lower (by 80 mV typ.) than it was in the case

of the ST8024 device. If, for example, the microcontroller is shut down at 2.5 V, the

appropriate external resistor values must be chosen to ensure proper deactivation of the

ST8024L device.

6.2.3

Application examples

The voltage supervisor is used as power-on reset and as supply dropout detection during

a card session. Supply dropout detection is to ensure that a proper deactivation sequence is

followed before the voltage is too low. For the internal voltage supervisor to function, the

system microcontroller should operate down to 2.35 V to ensure a proper deactivation

sequence. If this is not possible, external resistor values can be chosen to overcome the

problem.

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