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LM612AIN Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM612AIN Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 10 page Application Information VOLTAGE REFERENCE Reference Biasing The voltage reference is of a shunt regulator topology that models as a simple zener diode With current IR flowing in the ‘‘forward’’ direction there is the familiar diode transfer function IR flowing in the reverse direction forces the refer- ence voltage to be developed from cathode to anode TLH11058 – 8 FIGURE 1 124V Reference is Developed between Cathode and Anode Current Source IR is External The reference equivalent circuit reveals how VR is held at the constant 12V by feedback for a wide range of reverse current TLH11058 – 9 FIGURE 2 Reference Equivalent Circuit To generate the required reverse current typically a resistor is connected from a supply voltage higher than the refer- ence voltage to the Reference Output pin Varying that volt- age and so varying IR has small effect with the equivalent series resistance of less than an ohm at the higher currents Alternatively an active current source such as the LM134 series may generate IR TLH11058 – 10 FIGURE 3 12V Reference Capacitors in parallel with the reference are allowed See the Reference AC Stability Range typical curve for capaci- tance valuesfrom 20 mA to 3 mA the reference is stable for any value of capacitance With the reference’s wide sta- bility range with resistive and capacitive loads a wide range of RC filter values will perform noise filtering when neces- sary Reference Hysteresis The reference voltage depends slightly on the thermal his- tory of the die Competitive micro-power products varyal- ways check the datasheet for any given device Do not as- sume that no specification means no hysteresis COMPARATORS Either comparator or the reference may be biased in any way with no effect on the other sections of the LM612 ex- cept when a substrate diode conducts (see Electrical Char- acteristics Note 3) For example one or both inputs of one comparator may be outside the input voltage range limits the reference may be unpowered and the other comparator will still operate correctly The inverting input of an unused comparator should be tied to Vb and the non-inverting tied to Va Hysteresis Any comparator may oscillate or produce a noisy output if the applied differential input voltage is near the compara- tor’s offset voltage This usually happens when the input signal is moving very slowly across the comparator’s switch- ing threshold This problem can be prevented by the addi- tion of hysteresis or positive feedback as shown in Figure 4 TLH11058 – 11 FIGURE 4 RS and RF Add Hysteresis to Comparator The amount of hysteresis added in Figure 4 is VH e Va c RS (RF a RS) Va c RS RF for RF ll RS A good rule of thumb is to add hysteresis of at least the maximum specified offset voltage More than about 50 mV 7 |
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