Simple PWM interface can program regulators for Vout < Vsense
I recently published a Design Idea (DI) showing some very simple circuits for PWM programming of standard regulator chips, both linear and switching, “Revisited: Three discretes suffice to interface PWM to switching regulators.”
Figure 1 shows one of the topologies “Revisited” visited, where:
R1 = recommended value from U1 datasheet
DF = PWM duty factor = 0 to 1
R2 = R1/(Vomax/Vsense – 1)
Vout = Vsense(R1/(R2/DF) + 1) = DF(Vout_max – Vsense) + Vsense
DF = (Vout/Vsense – 1)(R2/R1) = (Vout – Vsense)/(Vout_max – Vsense)
DF = (Vout – 0.8)/9.2 for parts shown
Figure 1 Five discrete parts comprise a circuit for linear regulator programming with PWM.
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An inherent limitation of the Figure 1 circuit is its inability to program Vout < Vsense. Its minimum
Vout = Vsense @ DF = 0. For most applications this doesn’t amount to much, if any, of a problem. But sometimes it would be useful, or at least convenient, for Vout to be zero (or thereabout) when DF = zero. Figure 2 shows an easy modification that can make that happen, where:
R1 and R2 chosen as in Figure 1
(R4 + R5/2) = (5v – Vsense)/(Vsense/R1) – R2
R5 ~ R4/5
Vout = R1 DF(Vsense(1/R2 + 1/R1)) = DF Vout_max
DF = Vo/(R1(Vsense(1/R2 + 1/R1)) = Vout/Vout_max
DF = Vout/10 for part values shown
Figure 2 In order to make Vout programmable down to zero volts, add R4 and (optionally) R5 trimmer.
A cool feature of the Figure 1 topology is that, unlike some other schemes for digital power supply control, only the precision of R1, R2, and the regulator’s own internal voltage reference determines regulation accuracy. Precision is wholly independent of external voltage references. It remains equal to the precision of R1, R2, and Vsense (e.g., ±1%) for all output voltages.
Unfortunately, as the ancient maxim says, something’s (usually) lost when something’s gained. In gaining Vout < Vsense capability, the Figure 2 circuit loses that feature, and for outputs less than full scale, Vout precision becomes somewhat dependent on the +5v rail. This is where the R5 trimmer comes in handy.
The design equation (R4 + R5/2) = (5v – Vsense)/(Vsense/R1) – R2, makes the values chosen for the R4, R5 pair dependent on the accuracy of the 5v rail. They can only be as correct as it is. This makes output voltages somewhat suspect, especially when they approach zero. Including R5 and adjusting it for Vout = 0 @ DF = 0 makes low Vout settings accurately programmable. If that isn’t a critical factor, then R5 can be omitted, just make R4 = (5v – Vsense)/(Vsense/R1) – R2.
The simplicity of the arithmetic for computing DF from the desired Vout is also a desirable feature of Figure 2.
In closing: This DI revises an earlier submission: “Three discretes suffice to interface PWM to switching regulators.” My thanks go to comment makers oldrev, Ashutosh Sapre, and Val Filimonov for their helpful advice and constructive criticism. And special thanks go to editor Shaukat for her creation of an environment friendly for the DI teamwork that made it possible.
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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