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LM4132DMF-2.0 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4132DMF-2.0 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 15 page  Application Information THEORY OF OPERATION The foundation of any voltage reference is the band-gap circuit. While the reference in the LM4132 is developed from the gate-source voltage of transistors in the IC, principles of the band-gap circuit are easily understood using a bipolar example. For a detailed analysis of the bipolar band-gap circuit, please refer to Application Note AN-56. SUPPLY AND ENABLE VOLTAGES To ensure proper operation, V EN and VIN must be within a specified range. An acceptable range of input voltages is V IN > VREF + 400mV (ILOAD ≤ 10mA) The enable pin uses an internal pull-up current source (I P- ULL_UP ) 2µA) that may be left floating or triggered by an external source. If the part is not enabled by an external source, it may be connected to V IN. An acceptable range of enable voltages is given by the enable transfer characteris- tics. See the Electrical Characteristics section and Enable Transfer Characteristics figure for more detail. Note, the part will not operate correctly for V EN > VIN. COMPONENT SELECTION A small ceramic (X5R or X7R) capacitor on the input must be used to ensure stable operation. The value of C IN must be sized according to the output capacitor value. The value of C IN must satisfy the relationship CIN ≥ C OUT. When no output capacitor is used, C IN must have a minimum value of 0.1µF. Noise on the power-supply input may affect the output noise. Larger input capacitor values (typically 4.7µF to 22µF) may help reduce noise on the output and significantly reduce overshoot during startup. Use of an additional optional by- pass capacitor between the input and ground may help further reduce noise on the output. With an input capacitor, the LM4132 will drive any combination of resistance and capacitance up to V REF/20mA and 10µF respectively. The LM4132 is designed to operate with or without an output capacitor and is stable with capacitive loads up to 10µF. Connecting a capacitor between the output and ground will significantly improve the load transient response when switching from a light load to a heavy load. The output capacitor should not be made arbitrarily large because it will effect the turn-on time as well as line and load transients. While a variety of capacitor chemistry types may be used, it is typically advisable to use low esr ceramic capacitors. Such capacitors provide a low impedance to high frequency sig- nals, effectively bypassing them to ground. Bypass capaci- tors should be mounted close to the part. Mounting bypass capacitors close to the part will help reduce the parasitic trace components thereby improving performance. SHORT CIRCUITED OUTPUT The LM4132 features indefinite short circuit protection. This protection limits the output current to 75mA when the output is shorted to ground. TURN ON TIME Turn on time is defined as the time taken for the output voltage to rise to 90% of the preset value. The turn on time depends on the load. The turn on time is typically 33.2µs when driving a 1µF load and 78.8µs when driving a 10µF load. Some users may experience an extended turn on time (up to 10ms) under brown out conditions and low tempera- tures (-40˚C). THERMAL HYSTERESIS Thermal hysteresis is the defined as the change in output voltage at 25oC after some deviation from 25oC. This is to say that thermal hysteresis is the difference in output voltage between two points in a given temperature profile. An illus- trative temperature profile is shown in Figure 1. This may be expressed analytically as the following: Where V HYS = Thermal hysteresis expressed in ppm V REF = Nominal preset output voltage V REF1 =VREF before temperature fluctuation V REF2 =VREF after temperature fluctuation. The LM4132 features a low thermal hysteresis of 75 ppm (typical) from -40˚C to 125˚C after 8 temperature cycles. TEMPERATURE COEFFICIENT Temperature drift is defined as the maximum deviation in output voltage over the operating temperature range. This deviation over temperature may be illustrated as shown in Figure 2. Temperature coefficient may be expressed analytically as the following: 20151338 FIGURE 1. Illustrative Temperature Profile 20151339 FIGURE 2. Illustrative V REF vs Temperature Profile www.national.com 11 |
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