ST8024 datasheet(16/31 Pages) STMICROELECTRONICS

16 / 31 page

Functional description

ST8024

16/31

Functional description

Throughout this document it is assumed that the reader is familiar with ISO7816

terminology.

5.1

Power supply

The supply pins for the IC are VDD and GND. VDD should be in the range of 2.7 to 6.5 V. All

signals interfacing with the system controller are referred to VDD, therefore VDD should also

supply the system controller. All card reader contacts remain inactive during power-on or

power-off.

The internal circuits are maintained in the reset state until VDD reaches Vth2 +Vhys2 and for

the duration of the internal power-on reset pulse, tW (see Figure 4). When VDD falls below

Vth2, an automatic deactivation of the contacts is performed.

A DC/DC converter is incorporated to generate the 5 or 3 V card supply voltage (VCC). The

DC/DC converter should be supplied separately by VDDP and PGND. Due to the possibility

of large transient currents, the two 100 nF capacitors of the DC/DC converter should be

located as near as possible to the IC and have an ESR less than 100 m

Ω.

The DC/DC converter functions as a voltage doubler or a voltage follower according to the

respective values of VCC and VDDP (both have thresholds with a hysteresis of 100 mV).

The DC/DC converter function changes as follows:

VCC = 5 V and VDDP > 5.8 V; voltage follower

VCC = 5 V and VDDP < 5.7 V; voltage doubler

VCC = 3 V and VDDP > 4.1 V; voltage follower

VCC = 3 V and VDDP < 4.0 V; voltage doubler.

Supply voltages VDD and VDDP may be applied to the IC in any sequence.

After powering the device, OFF remains LOW until CMDVCC is set HIGH.

During power off, OFF falls LOW when VDD is below the falling threshold voltage.

5.2

Voltage supervisor

5.2.1

Without external divider on pin PORADJ

The voltage supervisor surveys the VDD supply. A defined reset pulse of approximately 8 ms

(tW) is used internally to keep the IC inactive during power-on or power-off of the VDD supply

(see Figure 4).

As long as VDD is less than Vth2 + Vhys2, the IC remains inactive whatever the levels on the

command lines. This state also lasts for the duration of tW after VDD has reached a level

higher than Vth2 + Vhys2. When VDD falls below Vth2, a deactivation sequence of the

contacts is performed.

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