Datasheet

2004 July 12 9
Philips Semiconductors Product specification
IC card interface TDA8024
8.2.2 WITH AN EXTERNAL DIVIDER ON PIN PORADJ (NOT
FOR THE TDA8024AT)
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:
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 APPLICATION EXAMPLES
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.
8.2.3.1 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.
V
PORADJ
=k× V
DD
, where with S1 and 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
Transposed, this becomes
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.
Activation is always possible for
and deactivation is always possible for .
That is V1 = 1.31 V and V2 = 1.19 V
and
Suppose R1 + R2 = 100 kΩ, then
= 42.3 kΩ and R1 = 57.7 kΩ.
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
(see Fig.9).
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:
Therefore = 40.14 kΩ and R1 = 59.86 kΩ.
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
(see Fig.9).
V
th2(ext)(rise)
1
R1
R2
-------
+


V
bridge
V
hys(ext)
2
--------------------
+


×=
V
th2(ext)(fall)
1
R1
R2
-------
+


V
bridge
V
hys(ext)
2
--------------------
–


×=
k
S1
S1 S2+
---------------------
=
1 0.98
R1
R2
-------
×


+ 1
0.99
1.01
-----------


R1
R2
--------
1
k
---
<×+=
1
k
---
1
1.01
0.99
-----------


R1
R2
-------
×+< 1 1.02
R1
R2
-------
×


+=
V
DD
V1
k
-------
>
V
DD
V2
k
-------
<
R1
R2
-------
3.135
1.31
---------------
1–


0.98× 1.365=<
R2
100 kΩ
2.365
-------------------
=
R1
R2
-------
3.267
1.310
---------------
1–


0.998× 1.491=<
R2
100 kΩ
2.49
-------------------
=