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MAX9118EXK Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX9118EXK Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page SC70, 1.6V, Nanopower, Beyond-the-Rails Comparators With/Without Reference 10 ______________________________________________________________________________________ Reference (MAX9117/MAX9118) The internal reference in the MAX9117/MAX9118 has an output voltage of +1.252V with respect to VEE. Its typical temperature coefficient is 100ppm/°C over the full -40°C to +85°C temperature range. The reference is a PNP emitter-follower driven by a 120nA current source (Figure 1). The output impedance of the voltage refer- ence is typically 200k Ω, preventing the reference from driving large loads. The reference can be bypassed with a low-leakage capacitor. The reference is stable for any capacitive load. For applications requiring a lower output impedance, buffer the reference with a low-input-leak- age op amp, such as the MAX4162. Applications Information Low-Voltage, Low-Power Operation The MAX9117–MAX9120 are ideally suited for use with most battery-powered systems. Table 1 lists a variety of battery types, capacities, and approximate operating times for the MAX9117–MAX9120, assuming nominal conditions. Internal Hysteresis Many comparators oscillate in the linear region of oper- ation because of noise or undesired parasitic feed- back. This tends to occur when the voltage on one input is equal or very close to the voltage on the other input. The MAX9117–MAX9120 have internal hysteresis to counter parasitic effects and noise. The hysteresis in a comparator creates two trip points: one for the rising input voltage (VTHR) and one for the falling input voltage (VTHF) (Figure 2). The difference between the trip points is the hysteresis (VHB). When the comparator’s input voltages are equal, the hystere- sis effectively causes one comparator input to move quickly past the other, thus taking the input out of the region where oscillation occurs. Figure 2 illustrates the case in which IN- has a fixed voltage applied, and IN+ is varied. If the inputs were reversed, the figure would be the same, except with an inverted output. Additional Hysteresis (MAX9117/MAX9119) The MAX9117/MAX9119 have a 4mV internal hysteresis band (VHB). Additional hysteresis can be generated with three resistors using positive feedback (Figure 3). Unfortunately, this method also slows hysteresis re- sponse time. Use the following procedure to calculate resistor values. 1) Select R3. Leakage current at IN is under 2nA, so the current through R3 should be at least 0.2µA to mini- mize errors caused by leakage current. The current through R3 at the trip point is (VREF - VOUT) / R3. Considering the two possible output states in solving for R3 yields two formulas: R3 = VREF / IR3 or R3 = (VCC - VREF) / IR3. Use the smaller of the two result- ing resistor values. For example, when using the 120nA REF VCC VEE VBIAS Figure 1. MAX9117/MAX9118 Voltage Reference Output Equivalent Circuit Table 1. Battery Applications Using MAX9117–MAX9120 BATTERY TYPE RECHARGEABLE VFRESH (V) VEND-OF-LIFE (V) CAPACITY, AA SIZE (mA-h) MAX9117/MAX9118 OPERATING TIME (hr) Alkaline (2 Cells) No 3.0 1.8 2000 2.5 x 106 Nickel-Cadmium (2 Cells) Yes 2.4 1.8 750 937,500 1.25 x 106 1000 2.7 3.5 Yes Lithium-Ion (1 Cell) Nickel-Metal- Hydride (2 Cells) Yes 2.4 1.8 1000 1.25 x 106 MAX9119/MAX9120 OPERATING TIME (hr) 5 x 106 1.875 x 106 2.5 x 106 2.5 x 106 |
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