73S8024RN-ILSLASHF datasheet(8/27 Pages) MAXIM

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73S8024RN Data Sheet

DS_8024RN_020

Rev. 2

3 Power Supply and Voltage Supervision

The 73S8024RN smart card interface IC incorporates a LDO voltage regulator. The voltage output is

controlled by the digital input 5V/#V. This regulator is able to provide either 3V or 5V card voltage from

the power supply applied on the VPC pin.

Digital circuitry is powered by the power supply applied on the VDD pin. VDD also defines the voltage

range to interface with the system controller.

Three voltage supervisors constantly check the presence of the voltages VDD, VPC and VCC. A card

deactivation sequence is forced upon fault of any of these voltage supervisors. The two voltage

supervisors for VPC and VCC are linked so that a fault is generated to activate a deactivation sequence

when the voltage VPC becomes lower than VCC. It allows the 73S8024RN to operate at lower VPC voltage

when using 3V cards only. The voltage regulator can provide a current of at least 90mA on VCC that

comply easily with EMV 4.0 and NDS specifications. The VPC voltage supervisor threshold values are

defined from applicable standards (EMV and NDS). A third voltage supervisor monitors the VDD voltage.

It is used to initialize the ISO 7816-3 sequencer at power-on, and to deactivate the card at power-off or

upon fault. The voltage threshold of the VDD voltage supervisor is internally set by default to 2.3V

nominal. However, it may be desirable, in some applications, to modify this threshold value. The pin

VDDF_ADJ (pin 18 in the SO package, pin 17 in the 32QFN package, not supported in the 20QFN

package) is used to connect an external resistor REXT to ground to raise the VDD fault voltage to another

value VDDF. The resistor value is defined as follows:

REXT= 56kΩ /(VDDF - 2.33)

An alternative method (more accurate) of adjusting the VDD fault voltage is to use a resistive network of

R3 from the pin to supply and R1 from the pin to ground (see applications diagram). In order to set the

new threshold voltage, the equivalent resistance must be determined. This resistance value will be

designated Kx. Kx is defined as R1/(R1+R3). Kx is calculated as:

Kx = (2.789 / VTH) - 0.6125 where VTH is the desired new threshold voltage.

To determine the values of R1 and R3, use the following formulas.

R3 = 24000 / Kx

R1 = R3*(Kx / (1 – Kx))

Taking the example above, where a VDD fault threshold voltage of 2.7V is desired, solving for Kx gives:



Kx = (2.789 / 2.7) - 0.6125 = 0.42046.

Solving for R3 gives:



R3 = 24000 / 0.42046 = 57080.

Solving for R1 gives:



R1 = 57080 *(0.42046 / (1 – 0.42046)) = 41412.

Using standard 1 % resistor values gives R3 = 57.6KΩ and R1 = 42.4KΩ.

These values give an equivalent resistance of Kx = 0.4228, a 0.6% error.

If the 2.3V default threshold is used, this pin must be left unconnected. The 20QFN package has the VDD

fault threshold fixed at this default value.

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