| Tiny Programmable Oscillator Spans Three Decades
from 5Khz to 20Mhz
By merging a digital potentiometer with an oscillator chip, we can make a very small programmable oscillator (1 uMAX and 1 SOT23 package). Not only does this circuit take up very little board space but it can be controlled with only three signals from a microprocessor. The MAX5160 is a 200K digital pot with 32 taps. To move up to the next tap, set UP/DOWN high and pulse INCREMENT low. To move down, set UP/DOWN low before pulsing INCREMENT. The LTC1799 is a programmable oscillator from Linear Technology. The frequency of oscillation is proportional to the value of the set resistor between V+ and the SET pin. Internal to the chip is a divider which is controlled by the DIV pin. The following table illustrates the relationship between the divide ratio and the input on DIV:
The equation for the oscillator frequency is:
In the schematic shown below, R1 was added to satisfy the minimum Rset requirement for the LTC1799. The circuit can be further simplified by fixing the DIV input and operating over a narrower output range.
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Where N is the divider value. A plot of
this equation for each value of N and each of the tap locations on
the digital potentiometer can be seen in the figure below.
