Three discretes suffice to interface PWM to switching regulators

It’s exceptional what number of switching regulator chips use the identical primary two-resistor community for output voltage programming. Determine 1 illustrates this characteristic in a typical (buck sort) regulator. See R1 and R2 the place:

Vout = Vsense(R1/R2 + 1) = 0.8v(11.5 + 1) = 10v

Determine 1 Typical regulator output voltage programming with a primary two-resistor community.

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Quantitatively, the Vsense suggestions node voltage varies from sort to sort and advisable values for R1 can differ too, however the topology doesn’t. Most conform faithfully to Determine 1. This defacto uniformity is helpful in case your utility wants digital management of Vout by way of PWM. 

Determine 2 reveals the simplistic three-component answer it makes potential the place:

Vout = Vsense(R1/(R2 + R3/DF) + 1) = 0.8v to 10v as DF = 0 to 1

All that’s required so as to add PWM management to Determine 1 is to separate R2 into two equal halves, join filter cap Cf to the center of the pair, and add PWM swap Q1 in collection with its floor finish.

Determine 2 Easy circuit for regulator programming with PWM the place Vout ranges from 0.8 V to 10 V because the responsibility issue (DF) goes from 0 to 1.

The Cf capacitance required for 1-lsb PWM ripple attenuation is 2^(N-2)Tpwm/R2, the place N is variety of PWM bits and Tpwm is the PWM interval. Since Cf won’t ever see greater than maybe a volt, its voltage score isn’t a lot of a difficulty.

A cool characteristic of this easy topology is that, not like many different schemes for digital energy provide management, solely the regulator’s inside voltage reference issues to regulation accuracy. Precision is due to this fact impartial of exterior voltage sources, e.g. logic rails. It is a good factor as a result of, for instance, the tempco of the TPS54332’s reference is simply 15 ppm/^oC.

Determine 3 graphs Vout versus the PWM DF for the Determine 2 circuit the place the X-axis is DF, the Y-axis is Vout and,

Vout = Vsense(R1/(R2 + R3/DF) + 1)
Vout(min) = Vsense
Vout(max) = Vsense(R1/(R2 + R3) + 1)
R1/(R2 + R3) = Vout(max)/Vsense – 1

Determine 3 Graph exhibiting Vout versus the Determine 2 PWM DF.

Determine 4 plots the inverse operate with DF vs Vout the place,

DF = R3/(R1/(Vout/Vsense – 1) – R2)

The nonlinearity of DF versus Vout does incur the price of a little bit of software program complexity (two subtractions and three divisions) to do the conversion. However because it buys substantial circuitry simplification, it appears an inexpensive (perhaps zero) value. Or, if the mandatory reminiscence is on the market, a lookup desk is one other (easy!) risk.

 Determine 4 DF versus Vout; the non-linearity necessitates a little bit of software program complexity to carry out the conversion.

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.

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The put up Three discretes suffice to interface PWM to switching regulators appeared first on EDN.

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