LTC1955 Linear Technology, LTC1955 Datasheet - Page 17

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

APPLICATIO S I FOR ATIO
both DV
that require virtually zero shutdown current, the DV
can be grounded. This will reduce the V
under a single microampere. Internal logic ensures that
the LTC1955 is in shutdown when DV
Note, however, that all of the logic signals that are refer-
enced to DV
and 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 configuration 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 5 A of current flow
into CPO when the LTC1955 is in shutdown.
Charge Pump Strength
Under low V
able to the smart cards is limited by the charge pump.
Figure 6 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 6, the available current is given by:
www.DataSheet4U.com
BATT
I
CCA
CC
in this configuration. Recall that DV
I
CCB
and V
CC
BATT
CC
(D
from being forward biased.
BATT
2
IN
conditions, the amount of current avail-
V
, SCLK, LD, DATA, R
BATT
U
R
when in shutdown. For applications
OLCP
–
U
V
CPO
W
BATT
IN
CC
BATT
, SYNC, ASYNC
CC
current to well
is grounded.
and SV
/PV
CC
U
BATT
OLCP
sets the
CC
BATT
BATT
.
can
pin
/
R
switching term, 1/(f
tances and the nonoverlap period of the switching circuit.
However, for a given R
be 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 damping
resistors can be included in the application without com-
promising the normal electrical performance of the
LTC1955 or smart card system. Generally a 100 resistor
and a 20pF capacitor will accomplish this as shown in
Figure 9.
OLCP
V
CPO
is dependent on a number of factors including the
+
–
R
2V
2
OLCP
Figure 6. Equivalent Open-Loop Circuit
V
BATT
BATT
CPO
– (
LDO A
CC
OSC
I
OLCP
CCA
with V
• C
, the minimum CPO voltage can
I
CCB
FLY
CPO
V
CCA
CCA
), internal switch resis-
)
R
as low as 5.13V. Given
OLCP
+ I
1955 F06
CCB
LDO B
LTC1955
) and minimum
sn1955 1955fs
V
17
CCB