TDA8024 Datasheet, PDF (9/29 Pages) NXP Semiconductors – IC card interface

Philips Semiconductors
IC card interface
Product specification
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:
V_{th2(ext)(rise)}
=
 ¹
+
RR-----12--_
×


V_bridge
+
V-----_{h--y--2-s--(-e---x--t-)}
V_{th2(ext)(fall)}
=
^{1 + RR-----12--}
×


V_bridge
–
-V----^h--^y--2-^s--⁽-^e---^x--^t-⁾_
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 k = _{S-----1--S--+}--1---S----2-- 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
1 + _0.98 × ^RR-----12--_
=
1
+


01----..--90---91--_
×
R-R----12--
<
1-k-
1-k- _{< 1 + }10----..--09---19--_
× ^RR-----12--
=
1
+


1.02
×
RR-----12--_
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 V_DD > ^V---k--1-
and deactivation is always possible for V_DD < ^V---k--2- .
That is V1 = 1.31 V and V2 = 1.19 V
and ^RR-----12-- < _^{3--1--.--.1-3--3--1-5--} – 1_ × 0.98 = 1.365
Suppose R1 + R2 = 100 kΩ, then
R2 = ^{1---2-0--.-0-3---6-k---5-Ω}--- = 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 t₁₂ (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:
RR-----12-- _{< }31----..--23---61---70-- _{– 1 × 0.998 = 1.491}
Therefore R2 = ^{1----0-2--0-.--4--k-9---Ω}--- = 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 t₁₂ (see Fig.9).
2004 July 12
9