Create a List. Summary Current Sources and Voltage References provides fixed, well-regulated levels of current or voltage within a circuit. Read on the Scribd mobile app Download the free Scribd mobile app to read anytime, anywhere. Start your free 30 days. Page 1 of 1. Close Dialog Are you sure?
Also remove everything in this list from your library. Are you sure you want to delete this list? Remove them from Saved? No Yes. Explore now. Figure 2: Typical Zener diode current versus voltage curve.
Click graphic to zoom Figure 3 is a basic schematic of a shunt voltage reference. Figure 3: Circuit schematic of a shunt reference. Click graphic to zoom The shunt reference in Figure 3 regulates the reference voltage by acting as a variable current sink, effectively creating a variable resistor divider with R S to provide a constant voltage at the output node. The external resistor, R S , is required for regulation, but there is no maximum input-voltage rating for a shunt reference.
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The resistor sets the total current sourced by the input supply as a function of the supply voltage. The total supply current is the sum of the load current and operating current of the shunt reference.
The input supply, V DD , always sees the maximum load current determined by the input-voltage level and external resistor. The shunt reference also sinks more or less current as the current requirements of the load change. The shunt is effectively acting as a current-controlled current sink, where the load current determines the current that the part sinks from the input supply. In this respect, the shunt reference is always on, and cannot be put into a sleep or shutdown mode. An increase in input voltage results in an increase in current through the external resistor, and more current is shunted across the reference device.
Two loading-condition extremes can help calculate the resistor value. The first loading condition is when the input voltage is at its minimum and the load current is at its maximum.
When the input voltage is at its maximum and the load current is at its minimum, the current through the shunt reference will be the largest for the application. For example, an industrial application requires a 2. The system input-voltage supply ranges from 12 V to 48 V and the load device draws between 0. The LM 2.http://co.organiccrap.com/12625.php
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You can calculate the external resistor using Equations 1, In this example, R S can be any value between 3. A higher resistor draws less current from the input supply. Table 1: Total supply current and shunt current for multiple values of Rs. Click graphic to zoom Shunt references are flexible because they can create a floating reference voltage not referenced to ground as well as negative references.
To use a shunt to create a floating reference voltage, simply use the shunt as described. However, connect it to a lower potential in the circuit instead of connecting the second terminal to ground Figure 4 , where V2 marks the lower potential. To use a shunt as a negative reference, the procedures and calculations remain the same, except R S is connected from the negative supply rail to the ground terminal of the reference.
The output is between the resistor and the ground terminal, and what was the input terminal is now grounded Figure 5. A typical application for a negative voltage reference is a bipolar data converter, such as a digital-to-analog converter DAC , which requires positive and negative references to generate the full range of possible output voltages.
Figure 6 shows an example using two shunt references in a bipolar reference configuration.
Figure 4: Floating shunt reference. Click graphic to zoom Negative reference. Figure 5: Negative shunt reference. Click graphic to zoom Bipolar reference. Figure 6: Positive and negative voltage reference rails. Click graphic to zoom Series. The second type of voltage reference device is a series voltage reference.
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Figure 7 shows a series voltage reference device that does not require an external resistor. It only consumes as much current as required by the load, plus a small quiescent current. Therefore, the input supply sees a decreased load and a decrease in power consumption when the load is not using as much current. However, the series device does have a maximum-rated input voltage, and is not easily configured for floating or negative references.
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Figure 7: Series voltage reference device schematic. The dropout voltage is the minimum voltage difference between V IN and V OUT for that device under a given load, and is stated in the device datasheet. A series voltage reference also allows for additional functionality with extra pins on the device for shutdown or sleep to limit power drawn from the input supply.
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