PWM power DAC incorporates an LM317

Instead of the conventional approach of backing up a DAC with an amplifier to boost output, this design idea charts a less traveled by path to power. It integrates an LM317 positive regulator with a simple 8-bit PWM DAC topology to obtain a robust 11-V, 1.5-A capability. It thus preserves simplicity while exploiting the built-in fault protection features (thermal and overload) of that time proven Bob Pease masterpiece. Its output is proportional to the guaranteed 2% precision of the LM317 internal voltage reference, making it securely independent of vagaries of both the 5-V logic supply rail and incoming raw DC supply.

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Figure 1 diagrams how it works.

Figure 1 LM317 regulator melds with HC4053 CMOS switch to make a 16-W PWM power DAC.

CMOS SPDT switches U1b and U1c accept a 10-kHz PWM signal to generate a 0 V to 9.75 V “ADJ” control signal for the U2 regulator via feedback networks R1, 2, and R3. The incoming PWM signal is AC coupled so that U1 can “float” on U2’s output. U1c provides an inverse of the PWM signal, implementing active ripple cancellation as described in “Cancel PWM DAC ripple with analog subtraction.” Note that R1||R2 = R3 to optimize ripple subtraction and DAC accuracy.

This feedback arrangement does, however, make the output voltage a nonlinear function of PWM duty factor (DF) as given by:

Vout = 1.25 / (1 – DF(1 – R1/(R1 + R2))
= 1.25 / (1 – 0.885*DF)

This is graphed in Figure 2.

Figure 2 The Vout (1.25 V to 11 V) versus PWM DF (0 to 1) where Vout = 1.25 / (1 – 0.885*DF).

Figure 3 plots the inverse of Figure 2, yielding the PWM DF required for any given Vout.

Figure 3 The inverse of Figure 2 where PWM DF = (1 – 1.25/Vout)/0.885.

The corresponding 8-bit PWM setting works out to: Dbyte = 255 (1 – 1.25 / Vout) / 0.885

Vfullscale = 1.25 / (R1/(R1 + R2)), so design choices other than 11 V are available. 11 V is the maximum consistent with HC4053’s ratings, but up to 20 V is feasible if the metal gate CD4053B is substituted for U1. Don’t forget, however, the requirement that R3 = R1||R2.

The supply rail V+ can be anything from a minimum of Vfullscale+3V to accommodate U2’s minimum headroom dropout requirement, up to the LM317’s absmax 40-V limit. DAC accuracy will be unaffected due to this chip’s excellent PSRR, although of course efficiency may suffer.

U2 should be heatsunk as dictated by heat dissipation caused by required output currents multiplied by the V- to Vout differential. Up to double-digit watts is possible at high currents and low Vout.

Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.

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