Datasheet

DS_8023C_019 73S8023C Data Sheet
Rev. 1.5 9
5 Voltage Supervision
Two voltage supervisors constantly check the presence of the voltages V
DD
and V
CC
. A card deactivation
sequence is triggered upon a fault detected by these voltage supervisors.
The digital circuitry is powered by the power supply applied on the VDD pin. V
DD
also defines the voltage
range for the interface with the system controller. The V
DD
Voltage supervisor is also used to initialize the
ISO-7816-3 sequencer at power-on, and also to deactivate the card at power-off or upon a fault. The
voltage threshold of the V
DD
voltage supervisor is internally set by default to 2.3 V nominal. However, it
may be desirable, in some applications, to modify this threshold value. The pin VDDF_ADJ (pin 17) is
used to connect an external resistor R
EXT
to ground to raise the V
DD
fault voltage to another value, V
DDF
.
The resistor value is defined as follows:
R
EXT
= 180 kΩ /(V
DDF
An alternative (more accurate) method of adjusting the V
- 2.33)
DD
Figure 11: 73S8023C – Typical Application
Schematic
fault voltage is to use a resistive network of
R3 from the pin to supply and R1 from the pin to ground (see
). 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 = (2.649 / V
TH
) - 0.6042 where V
TH
is the desired new threshold voltage.
To determine the values of R1 and R3, use the following formulas:
R3 = 72000 / Kx R1 = R3*(Kx / (1 – Kx))
Taking the example above, where a V
DD
fault threshold voltage of 2.7 V is desired, solving for Kx gives:
 Kx = (2.649 / 2.7) - 0.6042 = 0.377
Solving for R3 gives:  R3 = 72000 / 0.377 = 191 kΩ.
Solving for R1 gives:  R1 = 191000 *(0.377 / (1 – 0.377)) = 115.6 kΩ.
Using standard 1 % resistor values gives R3 = 191 kΩ and R1 = 115 kΩ. These values give an
equivalent resistance of Kx = 0.376, a 0.3% error.
Using 1% external resistors and a parallel resistance of 72 k ohms will result in a +/- 6% tolerance in the
value of VDD Fault. The sources of variation due to integrated circuit process variations and mismatches
include the internal reference voltage (less than +/- 1%), the internal comparator hysteresis and offset
(less than +/- 1.7% for part-to-part, processing and environment), the internal resistor value mismatch and
value variations (less than 1.8%), and the external resistor values (1%).
If the 2.3 V default threshold is acceptable, this pin must be left unconnected.