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Using Digital Potentiometers in Adjustable Step-Down DC-DC Converter Design.

Posted: 28 Jun 2003     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2003JUN/A/2003JUN28_AMD_POW_AN4.PDF

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Introduction

This application note describes how to use a digital potentiometer to generate an adjustable voltage supply

using a MAX1776 Step-Down DC-DC Converter. A reference design using the DS3903 is shown that

features the ability to produce a regulated supply between 1.25V and 5.50V, and the potentiometer allows

the supply to be calibrated to within 1.8% of the desire voltage level. Depending on the current limit settings

used with the MAX1776, the design is capable of supplying up to 600mA of current.

The DS3903 is an ideal part for this type of application because the voltage allowed on the potentiometer

terminals is 0V to 5.5V independent of the DS3903's supply voltage. This allows the DS3903 to be designed

into the feedback loop of the MAX1776 without ensuring that the DS3903's supply level is always greater

that the output voltage being generated. Additionally, the DS3903 provides 30PPM/0C stability when used

as a voltage divider over temperature, which will minimize the effects of temperature on the performance of

the circuit. For further reading on the use of digital potentiometers in power supply designs, AN226 discuses

using digital potentiometers with step-up DC-DC converters.

Benefits of Using a Digital Potentiometers

The primary benefit for using a digital potentiometer for creating an adjustable power supply is that they

allow the automation of the calibration process. This is particularly true for 2-wire and 3-wire digital

potentiometers because their digital interface is easily computer controlled. When interfaced to a

test/measurement system, this allows the voltage supply to be calibrated without introducing a labor-

intensive process into the production of the voltage supply circuitry. Other benefits of using digital

potentiometers for this type of application include: nonvolatile (NV) position settings; small form factors;

several potentiometers per package; low ratiometric temperature coefficient; and last but not least, they are

relatively inexpensive.

Using the DS3903 with the MAX1776

To generate an adjustable supply, the digital potentiometer is used to set the ratio of the output voltage VOUT,

to the MAX1776's feedback voltage, VFB. This is done by connecting the output voltage to the high-side

terminal of the potentiometer (H), the low-side terminal (L) to ground, and the wiper output (W) to the

feedback pin FB, see Figure 1.

Application Note 225

Using Digital Potentiometers in

Adjustable Step-Down DC-DC Converter

Designs

www.maxim-ic.com

AN225

2 of 7

Figure 1. Using a Digital Potentiometer in the Feedback Loop with a MAX1776

U2

MAX1776

FB

GND

ILIM

LX

OUT

SHDN

ILIM2

IN

VIN

VOUT

C1

22uF

L1

47uH

D1

REE

RH

RL

Digital

Potentiometer

H

W

L

VFB

The output voltage will be set by adjusting the position of the potentiometer, which will cause the MAX1776

to raise or lower the output voltage until VFB is eventually maintained within the specified range of VFB for

the MAX1776. The output voltage is shown as function of resistance in Equation 1, where VOUT is the output

voltage, RL is the resistance from the wiper to ground, and REE is the end-to-end resistance of the digital

potentiometer.

EE

L

FB

R

R

VoutV = Equation 1

( )1-D= PRREE Equation 2

( )PRRL D= Equation 3

P

PV

V FB

OUT

)1( -

= Equation 4

( )

P

V

P

V

VOUT

75.158112825.1

=

-

= Equation 5

Equation 2 and Equation 3 show REE and RL as a functions of R and P, where R is the incremental

resistance increase per position, P is the current position setting, and the constant P is the total number of

positions for a given digital potentiometer. In the case of the DS3903, its potentiometers are 128 positions

each.

Equation 4 is the result of substituting Equations 2 and 3 into Equation 1, then solving for VOUT and

simplifying the expression. Equation 5, is Equation 4, assuming VFB for the MAX1776 is constant at its

nominal value, 1.25V, and the DS3903 with 128 positions is used.

The circuit shown in Figure 2 is a simple circuit that allows the DS3903 to operate from the MAX1776's

input voltage (VIN = 4.5V to 24.0V), and provide the feedback required to regulate the output. Key elements

that make it possible for the DS3903 to operate in this circuit are:

AN225

3 of 7

1. The DS3903's wide VCC supply range (2.7V to 5.5V) allows it to operate using a Zener

diode to regulate its VCC supply. The DS3903's supply changes from approximately 2.7V

to 4.3V as VIN is varied from 4.5V to 24V.

2. The potentiometer signals of the DS3903 are allowed to operate from 0V to 5.5V as long

as the DS3903 is powered, independent of the DS3903's VCC supply level. Most digital

potentiometers require the potentiometer terminals to remain between 0V and VCC. This

would require the output voltage of the MAX1776 to be below the digital potentiometer's

supply voltage. Because the DS3903 does not have this requirement, it provides added

flexibility in this type of application.

3. The SDA and SCL pullups (R1 & R2) are operated off the same supply as the DS3903

(VDS3903), which also ensures that the part will work over a wide range of input voltages.

If the DS3903 is disconnected from the 2-wire bus, the pullups should remain attached to

SDA and SCL to prevent them from floating low.

4. The DS3903 is NV, so once it has been used to calibrate the power supply it will

remember its settings until changed at a future time, even if power is removed from the

circuit.

Figure 2. Using the DS3903 with the MAX1776

L1

47uH

VIN

D2

MA2YD15

VOUT

C4

22uF

R3

2.4k

D1

1N4682

U2

MAX1776

FB

GND

ILIM

LX

OUT

SHDN

ILIM2

IN

U1

DS3903

SDA

SCL

A0

WP

N.C.

L0

W1

L1

L2

GND

VCC

N.C.

N.C.

N.C.

N.C.

H0

W0

H1

W2

H2

C2

10uF

C1

0.1uF

VDS3903

Production

Tester

OR

2-wire master

C3

10uF

R1

10k

R2

10k

The passive components shown used with the MAX1776 should be chosen based on the desired current limit

inputs ILIM and ILIM2 in accordance with the Recommended Components selection guide shown in Table 3

of the MAX1776 data sheet. The values shown in this application note correspond with circuit 3 shown in

this table. The circuit is able to provide 150mA DC, with a peak inductor current of 300mA. As mentioned

in the introduction, other circuit configurations are available to supply up to 600mA of current.

Voltage Supply Accuracy, Precision, and Temperature Performance

Three parameters that are often used to determine a voltage supply's performance in an application are its

accuracy, precision, and performance over temperature. This section shows some of the analysis required to

determine how the digital potentiometer's integral and differential non-linearity and its temperature

performance effect the voltage supply's ability to set its output voltage, calibrate its output voltage, and

maintain its output voltage over temperature. These techniques can be applied to analyze the effect of

placing any digital potentiometer into a similar DC-DC converter design.

AN225

4 of 7

Supply Accuracy

Supply accuracy is being defined as error that can be expected in setting the supply to a particular voltage by

calculating the digital potentiometer's position using Equation 5. Equation 5 assumes the MAX1776 has a

typical feedback voltage (VFB = 1.25V) and ideal Integral Non-Linearity (INL) characteristics for the

DS3903's potentiometer. In reality, the MAX1776's feedback voltage is a 13% value, and the data sheet

value for the INL of the DS3903 is 11 LSB. Because the output voltage is determined by the multiplying

VFB by the 1/ratio of the potentiometer, any error associated with INL is multiplied by the ratio of VOUT/VFB

as well. Figure 3 shows the output accuracy as a function of the desired output voltage using both worst case

and typical (based on the typical operating characteristic in the DS3903's data sheet) INL models for the

potentiometer. This curve was calculated using Equation 4 with the worst case VFB (either 1.212V or 1.288V

for low and high bounds respectively) and RL adjusted by 11 LSB or the typical INL value of 10.090LSB.

The output voltage's accuracy in most cases will not be important because the potentiometer's precision will

be what effects the supply's accuracy after calibration.

Figure 3. Output Voltage Accuracy as a Function of Desired Output Voltage

OUTPUT VOLTAGE ACCURACY vs OUTPUT VOLTAGE

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000 5.500

OUTPUT VOLTAGE (V)

OUTPUTVOLTAGEACCURACY(%ERROR)

Worst Case DS3903

INL Model Bounds

Typical DS3903

INL Model Bounds

Supply Precision

Supply precision determines the resolution that can be used to calibrate the supply to the desired output

voltage. To analyze supply precision, VFB is assumed to be constant for a given part, and the quantizing error

due to the discrete steps of the digital potentiometer including differential non-linearity (DNL) is evaluated.

It must be assumed that either the worst case or typical DNL will always affect the discrete step about the

desired voltage. Figure 4 shows the effects of DNL on supply precision.

To calculate a supplies precision, Equation 1 is used, and RL will be adjusted to account for the DNL such

that RL 1 DNL causes an increased quantization interval. The calibration error is = the quantization interval.

AN225

5 of 7

Figure 5 shows the example circuit's (Figure 2) worst case calibration error as a function of the output

voltage.

Figure 4. Analyzing Supply Precision Considering Quantization Error and DNL

Figure 5. Calibration Error Due to Supply Precision as a Function of the

Desired Output Voltage

CALIBRATION % ERROR vs OUPUT VOLTAGE

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

1.000 2.000 3.000 4.000 5.000 6.000

OUTPUT VOLTAGE (V)

CALIBRATIONERROR(%)

Ideal Calibration Error Bounds

(Quantization Only)

Typical Calibration Error Bounds

(Quantization + Typical DNL)

Worst Case Calibration Error Bounds

(Quantization + Worst Case DNL)

Temperature Performance

One of the advantages of using digital potentiometers in this type of a feedback loop is that digital

potentiometers provide very good performance over temperature when they are used as voltage dividers.

This is a direct result of the fact that the materials that make up the resistive elements within a digital

Quantizing error with no DNL

Desired voltage

Worst case

quantizing error

with no DNL

No DNL,

quantization

intervals even

Digital Pot

Position

VOUT

Worst case quantizing

error increased by

DNL error

Quantizing error with DNL

Desired voltage

DNL error,

shortened interval

DNL error,

increased interval

Digital Pot

Position

Digital Pot

Position

VOUT

AN225

6 of 7

potentiometer are matched within a single die. Looking at the VOUTPUT ACCURACY vs. TEMPERATURE

typical performance curve in the MAX1776's data sheet, it shifts approximately -1.1% to +0.7% over the

-400C to +850C temperature range. Most all of Dallas Semiconductor's digital potentiometers provide 130

PPM/0C temperature stability when used as a voltage divider. If the worst case scenarios are analyzed, the

DS3903 will only add another 0.2% (30 PPM/0C x (250C - (-400C)) = 1950PPM = 0.195%) error to the

temperature performance of the circuit. Typically the DS3903's voltage divider temperature coefficient is

approximately 15 PPM/0C, so this effect would only typically cause a 0.1% change in the output voltage.

When compared to external resistors that will each have their own temperature coefficients, the digital

potentiometers are generally superior.

Techniques to Improve the Circuit's Precision

With the design shown in Figure 2, only 99 of the 128 positions will produce output voltages under 5.5V.

A simple improvement that dramatically improves the circuit's precision is to add resistors between the

potentiometers low terminal and ground, and the high terminal and output voltage, see Figure 6. By

calculating intelligent values for R1 and R2, it becomes possible to use all the positions of the potentiometer

to adjust the output voltage over a smaller range (e.g. all 128 positions adjust between 3.0V and 3.6V).

Because there will be more positions of adjustment working over a smaller range, the precision of the circuit

is greatly enhanced. One thing that must be carefully accounted for is that REE has a wide tolerance.

Generally digital potentiometers specify REE as 120%, and additionally REE will shift over temperature.

Another easy way to improve the performance of the circuit is to choose a digital potentiometer that has

more positions such as the DS1845. The upside of choosing the DS1845 is that it offers 256 positions instead

of 128. The downside of choosing the DS1845 is that it requires that the potentiometers be operated within

the supply level of the DS1845. Thus, the circuit may have to be operated from a regulated 5V supply, and

require that the output voltage is less than 5V. This may be practical in some instances, and it is an easy way

to adapt the circuit to specific needs.

Figure 6. Improving the Circuit's Precision by Adding External Biasing

Resistors

U2

MAX1776

FB

GND

ILIM

LX

OUT

SHDN

ILIM2

IN

VIN

VOUT

C1

22uF

L1

47uH

D1

REE

RH

RL

Digital

Potentiometer

H

W

L

VFB

R1

R2

Conclusion

This application note provides an example design using a digital potentiometer in the feedback loop of a

step-down DC-DC converter, and a discussion of how to analyze the accuracy, precision and temperature

performance of the circuit. Using digital potentiometers in power supply circuitry is beneficial in

applications because it can reduce the circuit's size, calibration time, in many instances the overall

component cost.

AN225

7 of 7

Dallas Semiconductor/Maxim Contact Information

Company Addresses:

Maxim Integrated Products, Inc

120 San Gabriel Drive

Sunnyvale, CA 94086

Tel: 408-737-7600

Fax: 408-737-7194

Dallas Semiconductor

4401 S. Beltwood Parkway

Dallas, TX 75244

Tel: 972-371-4448

Fax: 972-371-4799

Product Literature / Samples Requests:

(800) 998-8800

Sales and Customer Service:

World Wide Website:

www.maxim-ic.com

Product Information:

http://www.maxim-ic.com/MaximProducts/products.htm

Ordering Information:

http://www.maxim-ic.com/BuyMaxim/Sales.htm

FTP Site:

ftp://ftp.dalsemi.com

Email Support:

MixedSignal.Apps@dalsemi.com

(408) 737-7600





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