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TSM917 Datasheet(PDF) 11 Page - Silicon Laboratories |
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TSM917 Datasheet(HTML) 11 Page - Silicon Laboratories |
11 / 15 page TSM917 TSM917 Rev. 1.0 Page 11 move quickly past the other input, moving the input out of the region where oscillation occurs. Figure 2 illustrates the case in which an IN- input is a fixed voltage and an IN+ is varied. If the input signals were reversed, the figure would be the same with an inverted output. To save cost and external pcb area, an internal 4mV hysteresis circuit was added to the TSM917. Adding Hysteresis to the TSM917 The TSM917 exhibits an internal hysteresis band (VHB) of 4mV. Additional hysteresis can be generated with three external resistors using positive feedbackas shown in Figure 3. Unfortunately, this method also reduces the hysteresis response time. The design procedure below can be used to calculate resistor values. 1) Setting R2. As the leakage current at the IN pin is under 2nA, the current through R2 should be at least 0.2 μA to minimize offset voltage errors caused by the input leakage current. The current through R2 at the trip point is (VREF - VOUT)/R2. In solving for R2, there are two formulas – one each for the two possible output states: R2 = VREF/IR2 or R2 = (VCC - VREF)/IR2 From the results of the two formulae, the smaller of the two resulting resistor values is chosen. For example, when using the TSM917 (VREF = 1.245V) at a VCC = 3.3V and if IR2 = 0.2 μA is chosen, then the formulae above produce two resistor values: 6.23M Ω and 10.24MΩ - the 6.2MΩ standard value for R2 is selected. 2) Next, the desired hysteresis band (VHYSB) is set. In this example, VHYSB is set to 100mV. 3) Resistor R1 is calculated according to the following equation: R1 = R2 x (VHB/VCC) and substituting the values selected in 1) and 2) above yields: R1 = 6.2M Ω x (100mV/3.3V) = 187.88kΩ The 187k Ω standard value for R1 is selected. 4) The trip point for VIN rising (VTHR) is chosen such that VTHR > VREF x (R1 + R2)/R2 (where VTHF is the trip point for VIN falling). This is the threshold voltage at which the comparator switches its output from low to high as VIN rises above the trip point. In this example, VTHR is set to 3V. 5) With the VTHR from Step 4 above, resistor R3 is then computed as follows: R3 = 1/[VTHR/(VREF x R1) - (1/R1) - (1/R2)] R3 = 1/[3V/(1.245V x 187k Ω) - (1/187kΩ) - (1/6.2M Ω)] = 135.56kΩ In this example, a 137k Ω, 1% standard value resistor is selected for R3. Figure 3: Using Three Resistors Introduces Additional Hysteresis in the TSM917. Figure 2: TSM917’s Threshold Hysteresis Band |
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