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TS12011ITD1022T Scheda tecnica(PDF) 9 Page - Touchstone Semiconductor Inc |
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TS12011ITD1022T Scheda tecnica(HTML) 9 Page - Touchstone Semiconductor Inc |
9 / 11 page TS12011/TS12012 TS12011_12DS r1p0 Page 9 RTFDS Adding Hysteresis to the TS12012 Open-Drain Option The TS12012 has open-drain output and requires an external pull-up resistor to VDD as shown in Figure 3. Additional hysteresis can be generated using positive feedback; however, the formulae differ slightly from those of the push-pull option TS12011. The procedure to calculate the resistor values for the TS12012 is as follows: 1) As in the previous section, resistor R2 is chosen according to the formulae: R2 = VREFOUT/150nA or R2 = (VDD- VREFOUT)/150nA - R4 where the smaller of the two resulting resistor values is the best starting value. 2) As before, the desired hysteresis band (VHYSB) is set to 100mV. 3) Next, resistor R1 is then computed according to the following equation: R1 = (R2 + R4) x (VHYSB/VDD) 4) The trip point for VCOMPIN+ rising (VTHR) is chosen (again, remember that VTHF is the trip point for VCOMPIN+ falling). This is the threshold voltage at which the comparator switches its output from low to high as VCOMPIN+ rises above the trip point. 5) With the VTHR from Step 4 above, resistor R3 is computed as follows: R3 = 1/[VTHR/(VREFOUT x R1) - (1/R1) - (1/R2)] 6) As before, the last step is to verify the trip voltages and hysteresis band with the standard resistor values used in the circuit: For VCOMPIN+ rising: VTHR = VREFOUT x R1 x (1/R1+1/R2+1/R3) For VCOMPIN+ falling: VTHF = VREFOUT x R1 x(1/R1+1/R3+1/(R2+R4)) -(R1/(R2+R4)) x VDD and Hysteresis Band is given by VTHR – VTHF Pilot Light Flame Detector with Low-Battery Lockout Circuit The TS12011 can be used to create a pilot flame detector with low-battery lockout circuit as shown in Figure 4. The circuit is able to detect when the thermocouple does not detect the pilot flame and when the battery in the circuit drops to 1.39V. This circuit makes use of the op-amp, comparator, and 0.58V reference in the TS12011. In this example, a type R thermocouple is used. It generates a voltage range from 9mV to 17mV that corresponds to a temperature range of 900ºC to 1500ºC, which is typical of a methane pilot flame. If the pilot flame is removed, the temperature drops; hence, the output voltage generated by the thermocouple is drops to a minimum voltage of 0.1mV that is applied to the non- inverting input of the op-amp. This switches the output voltage of the op-amp to a LOW state and in turn, switches Q1 off. If, however, the battery voltage drops from 1.5V to 1.39V, the comparator output will switch from an output HIGH to a LOW. This will turn off Q2 and the output of the op-amp will turn Q1 off. The complete circuit consumes approximately 95µA of supply current at VDD = 1.5V. PC Board Layout and Power-Supply Bypassing While power-supply bypass capacitors are not typically required, it is good engineering practice to use 0.1uF bypass capacitors close to the device’s power supply pins when the power supply impedance is high, the power supply leads are long, or there is excessive noise on the power supply traces. To reduce stray capacitance, it is also good engineering practice to make signal trace lengths as short as Figure 3. Using Four Resistors Introduces Additional Hysteresis in the TS12012 |
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