Philips Semiconductors
Product specification
IC card interface
8.2.2
W
ITH AN EXTERNAL DIVIDER ON PIN
PORADJ (
NOT
FOR THE
TDA8024AT)
Transposed, this becomes
R1
0.99
R1 1
1
+
0.98
×
------
=
1
+
----------
×
-------
<
--
-
-
- -
1.01
R2 k
R2
1
1.01
R1
R1
-
--
<
1
+
----------
×
------
=
1
+
1.02
×
------
-
-
-
0.99
k
R2
R2
TDA8024
If an external resistor bridge is connected to pin PORADJ
(R1 and R2 in Fig.1), then the following occurs:
•
The internal threshold voltage V
th2
is overridden by the
external voltage and by the hysteresis, therefore:
V
hys(ext)
R1
V
th2(ext)(rise)
=
1
+
------
×
V
bridge
+
-------------------
-
-
R2
2
V
hys(ext)
R1
V
th2(ext)(fall)
=
1
+
------
×
V
bridge
–
-------------------
-
-
R2
2
where V
bridge
= 1.25 V typ. and V
hys(ext)
= 60 mV typ.
•
The reset pulse width t
W
is doubled to approximately
16 ms.
Input PORADJ is biased internally with a pull-down current
source of 4
µA
which is removed when the voltage on
pin PORADJ exceeds 1 V. This ensures that after
detection of the external bridge by the IC during power-on,
the input current on pin PORADJ does not cause
inaccuracy of the bridge voltage.
The minimum threshold voltage should be higher than 2 V.
The maximum threshold voltage may be up to V
DD
.
8.2.3
A
PPLICATION EXAMPLES
If V1 = V
th(ext)(rise)(max)
and V2 = V
th(ext)(fall)(min)
activation will always be possible if V
PORADJ
> V1
and deactivation will always be done for V
PORADJ
< V2.
V1
Activation is always possible for V
DD
>
------
-
k
V2
and deactivation is always possible for V
DD
<
------ .
-
k
That is V1 = 1.31 V and V2 = 1.19 V
R1
3.135
and ------
<
--------------
–
1
×
0.98
=
1.365
-
-
R2
1.31
Suppose R1 + R2 = 100 kΩ, then
100 kΩ
R2
=
------------------ = 42.3 kΩ and R1 = 57.7 kΩ.
-
2.365
Deactivation will be effective at
V2
×
(1 + 1.02
×
1.365) = 2.847 V in any case.
If the microcontroller continues to function down to 2.80 V,
the slew rate on V
DD
should be less than 2 V/ms to ensure
that clock CLK is correctly delivered to the card until
time t
12
The voltage supervisor is used as Power-on reset and as
supply dropout detection during a card session.
Supply dropout detection is to ensure that a proper
deactivation sequence is followed before the voltage is too
low.
For the internal voltage supervisor to function, the system
microcontroller should operate down to 2.35 V to ensure a
proper deactivation sequence. If this is not possible,
external resistor values can be chosen to overcome the
problem.
8.2.3.2
Microcontroller requiring a 3.3 V
±
10% supply
For a microcontroller supplied by a 3.3 V with a
±1%
regulator and with resistors R1, R2 having a
±0.1%
tolerance, the minimum supply voltage is 3.267 V.
The same calculations as in Section 8.2.3.1 conclude:
R1
3.267
------
<
--------------
–
1
×
0.998
=
1.491
-
-
R2
1.310
100 kΩ
Therefore R2
=
------------------ = 40.14 kΩ and R1 = 59.86 kΩ.
-
2.49
Deactivation will be effective at
V2
×
(1 + 1.002
×
1.491) = 2.967 V in any case.
If the microcontroller continues to function down to 2.97 V,
the slew rate on V
DD
should be less than 0.20 V/ms to
ensure that clock CLK is correctly delivered to the card
until time t
12
8.2.3.1
Microcontroller requiring a 3.3 V
±
20% supply
For a microcontroller supplied by 3.3 V with a
±5%
regulator and with resistors R1, R2 having a
±1%
tolerance, the minimum supply voltage is 3.135 V.
S1
V
PORADJ
= k
×
V
DD
, where k
=
-------------------- with S1 and S2
-
S1
+
S2
the actual values of nominal resistors R1 and R2.
This can be shown as
0.99
×
R1 < S1 < 1.01
×
R1 and
0.99
×
R2 < S2 < 1.01
×
R2
2004 July 12
9
PHILIPS [ NXP SEMICONDUCTORS ]
