ST72411R datasheet(47/71 Pages) STMICROELECTRONICS

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ST72411R

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SMARTCARD SUPPLY SUPERVISOR (Cont’d)

5.5.4 Functional Description

The core of the SSS is the internal reference volt-

age generator that is used for the output voltage

level regulation and for the Current Overload de-

tection.

Output regulation is achieved from the MCU VDD

with a follower transistor used as output stage, as-

sociated to a feedback regulation.

Software control through the Status/Control regis-

ter allows software to:

Turn-on / turn-off the SSS

Enable the overload detector

Enable interrupt in case of overload

Smartcard Power Supply

When disabled, the whole SSS is stopped in order

to achieve minimum consumption. The SC_PWR

line is tied to ground, and the I/O lines supplied by

SC_PWR, are also tied to ground.

When the SSS module is enabled, the SC_PWR

line provides a regulated voltage to the smartcard,

and the I/O lines have a logic “1” level identical to

the smartcard supply ensuring a safe interface.

Current Overload protection

When a current overload occurs on the SC_PWR

supply output, SC_PWR level drop is detected by

the Current Overload detector when enabled. As a

consequence, the SSS is turned-off with the

SC_PWR line tied to ground and the SSSEN bit is

cleared. On top of that, the OVLD flag is set into

the Status/Control Register and an interrupt re-

quest can be initiated.

Note: The Current Overload detector must be en-

abled by setting OVLDEN bit only after setting the

SSSEN bit.

Figure 37. Current Overload Detection

Switching SC_PWR from 3V to 5V output and

vice versa

The usual (and safe) procedure is to test the card

at 3V before selecting 5V output.

The application should avoid making a direct tran-

sition from 5V to 3V as a delay is required before

the 3V level is reached (the delay depends on the

external capacitor and card type).

For a controlled transition from 5V to 3V it is rec-

ommended to clear the SSSEN bit to tie the

SC_PWR to ground before enabling 3V output.

See Figure 52.

Figure 38. Recommended transitions when switching the voltage regulator (SSSR bit).

ISC_PWR

0mA

SHORT CIRCUIT

VOVLD

OVLDF FLAG

VSC_PWR

SSSR =0

SSSEN=1

SSSR=1

SSSEN=1

SSSR=1

SSSEN= 0

SSSR=0

SSSEN=1

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