
Keywords: digital potentiometer, variable voltage, power supply application note, app notes, dig pot, digital pots, linear regulator, xicor
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Controlling a Variable Voltage Power Supply Using the DS1809 (Pushbutton Control)
© May 21, 2002, Maxim Integrated Products, Inc.


Abstract: Semiconductor topic: Many linear and switchedmode power supplies and voltage regulators use a resistor network as feedback into the regulator to adjust the voltage output. This is typically done either by varying the values of the individual resistors or using a mechanical potentiometer. This article discusses controlling a voltage regulator using a digital potentiometer with pushbutton controls, the DS1809, which give a "human interface" to the control of the voltage regulator. This can be used a reference for other digitally controlled power supplies using other digital potentiometers which can be controlled by a microprocessor. This lends itself to applications where system voltage levels are adjusted to optimum levels (during the manufacturing process) to provide the optimum margin of error between the supply voltage and the operating voltage of the devices in the circuit.
Variable Voltage Supply Design
In this example application, the DS1809 is used to control the voltage output of an LP2951
voltage regulator.
The intent is to create a variable power supply that has precise voltage control and defined voltage limits.
This circuit can be powered by many sources including wall socket adapters and batteries, which provide
portability for the supply circuit.
The circuit in
Figure 1 shows how the output of the LP2951 is controlled by the DS1809. Any lowoutput
noise voltage regulator will do. The DS1809 is a pushbutton controlled, 64 position
digital potentiometer. It is
available in 10kΩ, 50kΩ, and 100kΩ sizes. The voltage divider circuit composed of R1, R2, and the DS1809
adjusts the voltage seen on the Feedback pin (pin 7) of the LP2951. By changing the potential here, the output
voltage of the LP2951 is adjusted.
Figure 1. Variable Power Supply Circuit, 5.0V To 5.5V Range
C1 is a decoupling
capacitor used to provide voltage stability for the input. C2 is a bypass capacitor used to
reduce AC noise on the output voltage signal. C3 is used to provide voltage stability when a load is present on
the output of the regulator. C4 is a decoupling capacitor for the DS1809. D1 is used to protect the LP2951
from damage when C2 is fully charged and the input is disconnected. D2 and C4 are needed to allow the
DS1809 to autostore the wiper position when the circuit powers down. The Rh terminal of the DS1809 is
connected to the wiper terminal Rw to minimize noise.
R1, R2, and the size of the digital potentiometer are used to determine the maximum and minimum voltage
range of output voltage signal. Equation 1 shows the formula that is used to select the sizes of R1, R2, and the
DS1809.
Equation 1. V_{OUT} Formula.
V
_{OUT} is the voltage seen on the output pin of the LP2951, and V
_{REF} is the internal reference
comparator voltage
seem the FeedBack pin (pin 7), and is typically 1.235V. The current through R2 and the DS1809 is the same
current seen through R1. This current is calculated by dividing the reference voltage by the combined
resistance of R2 and the DS1809. The voltage seen on the output pin is this current times the sum of R1, R2,
and the resistance seen through the DS1809.
The first step is to determine the voltage range the circuit will supply. We can design a supply for a range of
5.0V to 5.5V. This was done by substituting 5.0V and 5.5V for V
_{OUT} and 1.235V for V
_{REF} into Equation 1. The
result will look like the formulas in
Equation 2.
Equation 2. V_{OUT} Calculation for 5.0V to 5.5V range.
From the data sheet of the DS1809, we use the maximum wiper resistance value and R2 to determine R1 for
the maximum voltage value. We use the maximum resistance of the DS1809 and R2 to calculate R1 for the
minimum value of the voltage supply. We solve the two equations for R1, then substitute and solve for R2.
Using the above equations, we get 66.75kΩ for R2 and 233.90kΩ for R1. Using a 68kΩ 1/8 watt resistor and
a 220kΩ 1/8 watt resistor, we get the circuit in Figure 1. The .5V range divided by the 64 positions provided
by the DS1809 gives us 7.8mV/step from 5.0V to 5.5V.
To use a broader range, from 4.5V to 5.5V, simply use the same equations substituting in the desired voltage
ranges.
Equation 3. V_{OUT} Calculation for 4.5V to 5.5V range.
Using these equations, we find using a 10kΩ DS1809 that R1 is 101.4kΩ and R2 is 28.3kΩ. We get a
15.6mV/step accuracy using this 1V range.
Equation 4. V_{OUT} Calculation for 3.0V to 5.5V range.
Using the above equation, we choose the 50kΩ pot for a wider voltage range. We determine that R1 is
119.5kΩ and R2 is 33.6kΩ. The 2.5V range allows for 39mV/step accuracy.
Current Limitations
The maximum current that can be drawn from the LP2951 is 100mA. For stable functionality, the maximum
current that can flow through the resistive elements of the DS1809 is 4mA. In this circuit, the current drawn
through the DS1809 wiper is 32µA and the current into the VCC pin of the DS1809 115µA in standby, and
350µA when actively adjusting the wiper positions. The LP2951 Feedback pin (pin 2) is internally connected
to an
amplifier that compares the voltage from the R1/(R2+R1) voltage divider to the internal reference
voltage of 1.235V. Typical current drawn from the Feedback pin is 20nA.
Summary
This supply design is portable and easy to build. It allows precise control over a variety of voltage ranges.
More precise control (7.8mV/step) is achieved when using a small (0.5V) voltage range. Less precise control
(39.0mV/step) is possible when using a larger (2.5V) voltage range. The pushbutton interface of the DS1809
lets single step adjustments to be made to the output voltage. If desired, the wiper can be shifted continuously
by pressing and holding either pushbutton. After an initial 0.5 second delay, the wiper will automatically step
in the desired direction once every 100 milliseconds until the endposition is reached.
Other digital potentiometers can be used, such as the DS1804, which uses 3wire communication, or a
DS1803, which uses 2wire communication to adjust the wiper position. Thus, it is possible to use a
microprocessor to control the output voltage of this circuit.
The DS1809 will not increment or decrement the wiper position past the digital potentiometer’s endpositions,
and will not autostep unless either pushbutton is depressed for more than 0.5 seconds. This means
that the voltage will never exceed the maximum and minimum values designed into the circuit. Pressing the
SW1 pushbutton decrements the output voltage. Similarly, SW2 increments the output voltage by the same
amount per press. Therefore, even if SW1 or SW2 pushbuttons are inadvertently pressed, the output voltage
changes very little and is never allowed to exceed the set range limits.
Use of a DS1809 digital potentiometer allowed convenient manual control of a voltage regulator. The circuit
was built on a breadboard with a 9V battery supply and alternatively, a 20V power supply. Because the
standby current of the LP2951 is only 60µA and the low level of standby current drawn by the circuit as a
whole, a 9V battery can be used for a power
source. This makes the circuit small and portable. The fixed
voltage range of this supply prevents accidental overvoltage of the ICs.
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APP 183: May 21, 2002
APPLICATION NOTE 183,
AN183,
AN 183,
APP183,
Appnote183,
Appnote 183
