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TDA8024T Datasheet(PDF) 9 Page - NXP Semiconductors |
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TDA8024T Datasheet(HTML) 9 Page - NXP Semiconductors |
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9 / 29 page  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 Vth2 is overridden by the external voltage and by the hysteresis, therefore: where Vbridge = 1.25 V typ. and Vhys(ext) = 60 mV typ. • The reset pulse width tW 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 VDD. 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. VPORADJ =k × VDD, 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 = Vth(ext)(rise)(max) and V2 = Vth(ext)(fall)(min) activation will always be possible if VPORADJ >V1 and deactivation will always be done for VPORADJ < 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 k Ω, then = 42.3 k Ω 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 VDD 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: Therefore = 40.14 k Ω 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 VDD 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). 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 k Ω 2.365 ------------------- = R1 R2 ------- 3.267 1.310 --------------- 1 – 0.998 × 1.491 = < R2 100 k Ω 2.49 ------------------- = |
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