LTC1955 Linear Technology, LTC1955 Datasheet - Page 16

• Manufacturer Part Number: LTC1955
• Description: Dual Smart Card Interface
• Manufacturer: Linear Technology

LTC1955
APPLICATIO S I FOR ATIO
Using S.A.M. Cards
For applications using one or more installed S.A.M. cards,
the PRES A/PRES B pins for those sockets must be
grounded before operation of the card can occur (assum-
ing NC/NO is grounded). The PRES A/PRES B pull-up
current is designed for very low consumption, but ultralow
current can be achieved in shutdown by using a
microcontroller output to pull down on the PRES A/PRES
B pins only when communication is necessary. The fault
detection circuitry will not allow a card socket to be
operated unless a card is detected.
Asynchronous Channel A Card Detection
Since the shift register is transparent when LD is held low,
D
indicates the status of the card detection channel for
channel A. Thus, it is not necessary to perform an entire
read/write operation to determine the card detection sta-
tus of channel A. With LD low, D
generate a real time card detection interrupt. This could be
useful for one S.A.M. card, one smart card applications.
Inter Card Communication
Communication is possible directly from one card socket
to the other when both cards are selected at the same time.
This can be achieved by the following sequence of actions.
1) Start with both cards off and deselected
2) Activate the supply of the slave card
3) Select the slave card only
4) Initiate a reset on the slave card
5) Deselect the slave card
6) Activate the supply of the master card
7) Select the master card only
8) Initiate a reset on the master card
9) Select both cards
16
OUT
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is the same as D15. Recall from Table 1 that D15
U
U
W
OUT
can be used to
U
Using the UNDERV Pin
The UNDERV pin can be used to add protection against a
supply undervoltage fault. By using two external program-
ming resistors, the undervoltage detection can be set to an
arbitrary level (Figure 8). To ensure that the smart cards
are properly shut down, there must be sufficient energy
available in the input bypass capacitor to run one or both
smart cards until the deactivation cycle begins. It can take
approximately 30 s from the detection of a fault until the
deactivation sequence begins. It is desirable to maintain
the V
Consider the following (worst-case) example:
1) The UNDERV pin is programmed to trip below 3.1V.
2) It is possible to have both cards activated at 5V and
drawing 60mA.
Since the output voltage is programmed to 5V, the charge
pump will be acting as a voltage doubler. With two cards
drawing 60mA each, the input current will be 2 • (60mA +
60mA) or about 240mA. Allowing the V
droop from 3.1V to 2.7V during the 30 s timeout period,
the input capacitance would need to be at least
240mA / [(3.1V – 2.7V) / 30 s] or 18 F.
Thermal Management
To minimize power dissipation, the LTC1955 will actively
decide whether to step up or down depending on the
required output voltages and available input voltage. How-
ever, for optimum efficiency, the LTC1955 should be
powered from a 3.3V supply.
If the input voltage is above 3.6V, and both cards are
drawing maximum current, there can be substantial power
dissipation in the LTC1955. If the junction temperature
increases above approximately 150 C, the thermal shut-
down circuitry will automatically deactivate both chan-
nels. To reduce the maximum junction temperature, a
good thermal connection to the PC board is recom-
mended.
Zero Shutdown Current
Although the LTC1955 is designed to have very low
shutdown current, it can still draw over a microampere on
BATT
supply at 2.7V or greater during this period.
BATT
supply to
sn1955 1955fs