AN1488.0 INTERSIL [Intersil Corporation], AN1488.0 Datasheet

AN1488.0

Manufacturer Part Number

AN1488.0

Description

Fixed DC/DC Regulator Output Margining Using Digitally Controlled Potentiometer

Manufacturer

INTERSIL [Intersil Corporation]

Datasheet

1.AN1488.0.pdf (2 pages)

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VIN

Many applications using a DC/DC regulator are implemented

with a resistor divider in the feedback path to set up Vout.

However, for some high end applications, the voltage needs

to be adjusted in order to optimize the application’s

performance. A Digitally Controlled Potentiometer (DCP)

can be used in both high end and low end applications to

provide this flexibility.

The typical Buck DC/DC converter scheme includes a PWM

driver, switches, inductor, filter, and feedback divider (Figure 1).

The feedback voltage, FB, from the resistor divider R1 and

R2, is a negative input for the error amplifier of the voltage

control loop. In addition to the point of load regulation, the

feedback divider also sets up the desired output voltage,

Vout. In other words, Vout is programmable through an

external resistor divider based on Equation 1.

where Vref is a reference voltage and a positive input for the

error amplifier.

The minimum output voltage can be set as low as Vref,

according to Equation 1. The maximum output is also

determined by the resistor divider ratio, as well as input

voltage and the maximum duty cycle. The reference voltage,

Vref, can be obtained from a PWM controller datasheet.

Thus, making the feedback divider adjustable makes the

DC/DC regulator also adjustable, as shown in Figure 2.

The adjustable output voltage of the DC/DC regulator can be

expressed by Equation 2.

where R1 and R2 are fixed resistors, Rw is a wiper resistance,

Rtotal is the total resistance of the DCP, code is a decimal

code of wiper position and n is a total number of wiper taps.

Vout

FIGURE 1. TYPICAL BUCK DC/DC CONVERTER

Vref

ERROR

CONTROLLER

•

⎛

⎝

PWM

⎛

⎝

---------------------------------------------------------------------------------------- -

------- -

VREF

1 )

Rtotal code

•

Application Note

Fixed DC/DC Regulator Output Margining Using

⁄

(

n 1

–

)

⎞

⎠

VOUT

(EQ. 1)

(EQ. 2)

1-888-INTERSIL or 1-888-468-3774

CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.

Digitally Controlled Potentiometer

VIN

FIGURE 2. ADJUSTABLE POINT OF LOAD DC/DC REGULATOR

The advantages of having the DCP in serial with the R2 resistor

(rather than in parallel) are easier calculation and bandwidth

limitation of the DCP. The DCP terminals have a parasitic

capacitance of 10pF to 25pF, which is shown in Figure 2 as

Crw. Connecting any of the DCP terminals to the FB node will

cause phase shift and may decrease DC/DC regulator

performance, such as transient response and stability.

The right DCP should be chosen to meet the application

requirements based on knowing the Vout adjusting range

and desired resolution. For example, we would like to adjust

the Vout output voltage of a DC/DC regulator, built on the

ISL85001 PWM controller, in a range from 0.8V to 3.3V.

The DCP total resistance, Rtotal, R1 and R2 resistor values

can be derived from Equation 2. For simplicity, it can be

overwritten as Equation 3, where the wiper resistance is

counted together with the R2 serial resistor. The suggested

value of the R2 resistor should be less than or equal to

0.1*Rtotal in order to have a bigger adjusting range.

The maximum Vout will be at code = 0

and the minimum Vout will be at max code = n -1

According to the ISL85001 datasheet, the internal Vref = 0.6V

and the recommended value for the R1 resistor is from 1kΩ to

10kΩ. Since the number of Rtotal values of the DCPs are

limited, let’s select the Rtotal = 10kΩ first and then calculate

R1 and R2.

Vout

Vout max

Vout min

≤

1kΩ

(

August 17, 2009

0.1Rtotal

Vref

All other trademarks mentioned are the property of their respective owners.

ERROR

CONTROLLER

)

Intersil (and design) is a registered trademark of Intersil Americas Inc.

PWM

•

Vref

---------------------------------------------------------------------------------------------------------- -

•

0.1Rtotal

Vout max

------------------------------------------------ -

VREF

•

0.1Rtotal

---------------------------------------- -

(

1.1Rtotal

0.1Rtotal

Vref

1.1Rtotal

0.1Rtotal

) Vref

–

Rtotal code

•

1kΩ

•

⁄

3.3V 0.6V

------------------------------ -

(

n 1

⁄

(

–

0.6V

n 1

–

AN1488.0

–

VOUT

)

CRW

(EQ. 3)

(EQ. 4)

(EQ. 5)

(EQ. 6)

4.5kΩ

DCP

Related parts for AN1488.0

AN1488.0 Summary of contents

Page 1

... Rtotal = 10kΩ first and then calculate (EQ ⎠ ⁄ – R1 and R2 0.1Rtotal ≤ R2 1kΩ CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 AN1488.0 L1 VOUT D1 C1 PWM CONTROLLER FB ERROR VREF • ⁄ 0.1Rtotal + Rtotal code n 1 – ...

Page 2

... Vout resolution per step changes with the tap position, from 50mV per tap at the low end to 2mV per tap at the high end of the DCP settings in our example. 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 FIGURE 3. THE ISL85001 VOUT vs THE ISL22317W TAP POSITION 100 120 TAP POSITION (DECIMAL) AN1488.0 August 17, 2009 ...

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