LTC1955EUH Linear Technology, LTC1955EUH Datasheet - Page 17

• Manufacturer Part Number: LTC1955EUH
• Description: IC INTERFACE DL SMART CARD 32QFN
• Manufacturer: Linear Technology

Specifications of LTC1955EUH

• Controller Type: Smart Card Interface
• Interface: 4-Wire Serial
• Voltage - Supply: 1.7 V ~ 5.5 V
• Current - Supply: 10µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS INFORMATION
DV
require virtually zero shutdown current, the DV
be grounded. This will reduce the V
under a single microampere. Internal logic ensures that
the LTC1955 is in shutdown when DV
however, that all of the logic signals that are referenced
to DV
NC/NO) will have to be at 0V as well, to prevent ESD diodes
to DV
Operation at Higher Supplies
If a 5.5V to 6V supply voltage is available, it is possible
to achieve some power savings by bypassing the charge
pump. The higher supply can be connected directly to the
CPO pin. As long as the voltage on CPO is higher than that
at which it ordinarily regulates (5.35V or 3.7V depending
on voltage selections), the charge pump’s oscillator will
not run. This confi guration can give considerable power
savings since the charge pump is not being used.
A voltage source is still needed on both DV
PV
logic reference level for all the control and smart card
communication pins. The voltage on SV
be any convenient level that meets the parameters in the
Electrical Characteristics table.
The 5.5V to 6V supply can be left permanently connected
to CPO, but there will be approximately 5μA of current fl ow
into CPO when the LTC1955 is in shutdown.
Charge Pump Strength
Under low V
to the smart cards is limited by the charge pump.
Figure 5 shows how the LTC1955 can be modeled as a
Thevenin equivalent circuit to determine the amount of
current available given the effective input voltage, 2V
and the effective open-loop output resistance, R
From Figure 5, the available current is given by:
BATT
CC
I
CCA
CC
CC
and V
in this confi guration. Recall that DV
+
from being forward-biased.
(D
I
CCB
IN
BATT
BATT
, SCLK, LD, DATA, R
≤
conditions, the amount of current available
when in shutdown. For applications that
2
V
BATT
R
OLCP
–
V
CPO
IN
CC
, SYNC, ASYNC and
BATT
is grounded. Note,
BATT
current to well
CC
and SV
/PV
CC
CC
BATT
sets the
OLCP
pin can
BATT
BATT
.
can
/
R
switching term, 1/(f
and the nonoverlap period of the switching circuit. How-
ever, for a given R
determined from the following expression:
The LDOs have been designed to meet all applicable smart
card standards for V
this information, trade-offs can be made by the user with
regard to total consumption (I
supply voltage.
Changing the Smart Card Supply Voltage
Although the LTC1955 control system will allow the smart
card voltage to be changed from one value to the next
without an interim power-down, this is not recommended.
When changing from a higher voltage to a lower voltage
there will generally not be a problem; however, changing
from a lower voltage to a higher voltage will result in both
an undervoltage condition and an overcurrent condition
on that channel. The likely result is that the channel will
automatically deactivate. Applicable smart card standards
specify that the smart card supply be powered to zero
before applying a new voltage.
Compliance Testing
Inductance due to long leads on type approval equipment
can cause ringing and overshoot that leads to testing
problems. Small amounts of capacitance and damp-
ing resistors can be included in the application without
compromising the normal electrical performance of the
LTC1955 or smart card system. Generally, a 100Ω resis-
tor and a 20pF capacitor will accomplish this, as shown
in Figure 8.
OLCP
V
CPO
+
–
is dependent on a number of factors including the
R
2V
≥ 2V
OLCP
BATT
Figure 5. Equivalent Open-Loop Circuit
CPO
BATT
LDO A
OLCP
– (I
OSC
CC
CCA
, the minimum CPO voltage can be
• C
with V
+ I
FLY
V
CCB
CCA
), internal switch resistances
CPO
CCA
)R
1955 F05
OLCP
as low as 5.13V. Given
+ I
LDO B
CCB
LTC1955
) and minimum
V
CCB
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