Datasheet

LTC1955
17
1955fd
For more information www.linear.com/LTC1955
DV
CC
and V
BATT
when in shutdown. For applications that
require virtually zero shutdown current, the DV
CC
pin can
be grounded. This will reduce the V
BATT
current to well
under a single microampere. Internal logic ensures that
the LTC1955 is in shutdown when DV
CC
is grounded. Note,
however, that all of the logic signals that are referenced
to DV
CC
(D
IN
, SCLK, LD, DATA, R
IN
, SYNC, ASYNC and
NC/NO) will have to be at 0V as well, to prevent ESD diodes
to DV
CC
from being forward-biased.
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
CC
and SV
BATT
/
PV
BATT
in this configuration. Recall that DV
CC
sets the
logic reference level for all the control and smart card
communication pins. The voltage on SV
BATT
/PV
BATT
can
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 flow
into CPO when the LTC1955 is in shutdown.
Charge Pump Strength
Under low V
BATT
conditions, the amount of current available
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
BATT
and the effective open-loop output resistance, R
OLCP
.
From Figure 5, the available current is given by:
I
CCA
+I
CCB
≤
2V
BATT
– V
CPO
R
OLCP
R
OLCP
is dependent on a number of factors including the
switching term, 1/(f
OSC
• C
F LY
), internal switch resistances
and the nonoverlap period of the switching circuit. How-
ever, for a given R
OLCP
, the minimum CPO voltage can be
determined from the following expression:
V
CPO
≥ 2V
BATT
– (I
CCA
+ I
CCB
)R
OLCP
The LDOs have been designed to meet all applicable smart
card standards for V
CC
with V
CPO
as low as 5.13V. Given
this information, trade-offs can be made by the user with
regard to total consumption (I
CCA
+ I
CCB
) and minimum
supply voltage.
+
–
LDO A2V
BATT
1955 F05
CPOR
OLCP
V
CCA
LDO B V
CCB
Figure 5. Equivalent Open-Loop Circuit
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 per
formance of the
LTC1955 or smart card system. Generally, a 100Ω resis
-
tor and a 20pF capacitor will accomplish this, as shown
in Figure 8.
applicaTions inForMaTion