Many microcontrollers feature a pulse-width-modulated (PWM) output that can be low-pass filtered to produce a variable dc voltage. Without additional circuitry, however, this technique is limited to controlling very low-power loads.
The circuit here illustrates a scheme that lets this dc voltage control a high-power load, such as a motor, actuator, or heating element (see the figure). Furthermore, the load voltage may be higher or lower than the microcontroller's supply voltage. On top of that, it may be adjusted over any suitable range with a resolution equal to the PWM signal.
The PWM signal, VPWM, together with resistor network R1-R3 and filter capacitor C1, generates the control voltage (VC), which is buffered and level shifted by IC1, R6, and R7. The load voltage (VL) appears at the output of the adjustable, positive voltage regulator (IC2) and can be set to any value from around 1.25 V up to a volt or so below the level of the high-power supply voltage (VS).
Although a bipolar or MOSFET transistor could be used as the pass device, the regulator has the advantage of intrinsic protection mechanisms, such as short-circuit current limiting and thermal overload shutdown. Furthermore, regulators such as the LM317 or LM1084IT-ADJ are relatively inexpensive and can deliver considerable load power with appropriate heatsinking.
These devices feature an internal bandgap reference that sets the output voltage to 1.25 V (typical) above the potential at the adjust (ADJ) pin. The closed loop around IC1, IC2, R6, and R7 produces the following relationship:
VL = VC[1 + (R6/R7)] (1)
where VC is the control voltage appearing at the op amp's non-inverting input terminal. This equation can be rearranged to give R6 in terms of VL, VC, and R7:
R6 = R7[(VL/VC) - 1] (2)
Now, provided VPWM can swing rail-torail and cover a 0 to 100% duty-cycle range, the following equations may be used to determine R1, R2, and R3:
R2 = R1[(VC(MAX) VC(MIN))(VD - VC(MAX))] (3)
R3 = R1[(VC(MAX)/VC(MIN)) - 1] (4)
The following two examples illustrate the design process.
EXAMPLE 1 VL = 3.0 to 12.0 V; VD = 3.3 V. First, we allow some margin on the limits of VL and let the range be 2.8 to 12.2 V. Also, we can simplify the circuit by assuming that VC(MAX) = VD = 3.3 V. From Equation 2, we find that:
Inserting the values of VC(MIN), VC(MAX), and VD into Equations 3 and 4, we find that R2 = ∞ (i.e., R2 is omitted), and R3 = 3.359 R1. Suitable preferred values are: R1 = 270kΩ, R3 = 910kΩ. The supply voltage VS should be set high enough to account for IC2's dropout voltage, typically 1.5 to 2.0 V for the LM317 and 1.0 V for the LM1084IT-ADJ (assuming 1-A load current, T = 25°C). In a test circuit built using the resistor values quoted above, VL ranged from 2.80 to 12.26 V.
EXAMPLE 2 VL = 2.5 to 4.5 V; VD = 5.0 V. Again, we allow some margin on VL and let the range be 2.3 to 4.7 V. Since VL(MAX) is less than VD, the potential divider action provided by R6 and R7 isn't required, so R7 may be omitted and the value of R6 is chosen to suit stability capacitor C3; say R6 = 100Ωk . Thus, VC(MAX) = VL(MAX) = 4.7 V, and VC(MIN) = VL(MIN) = 2.3 V. Inserting these values into Equations 3 and 4 yields:
R3 = 1.043 R1≈ R1, and R2 = 8 R1
Suitable preferred values are R1 and R3 = 150Ωk , R2 = 1.2 M Ω. A test circuit built using these values produced a VL range of 2.350 to 4.709 V.
Filter capacitor C1 determines the ripple on VC. If the PWM frequency isn't very low, a value of 100 nF to 1µF should be suitable. Op-amp IC1 should be chosen to accommodate the full range of VC at its input, and its output voltage (Vo) must satisfy Vo = VL - 1.25 V for all values of VL. If IC1's positive output swing is somewhat limited, optional resistor R4 may be included. This will set IC1's output voltage to Vo = (VL - 1.25 V) - (125 µA + IADJ)R4 , where IADJ is the regulator's adjust pin current, typically around 50µA.
However, R4 should be added with caution, especially if VL(MIN) is fairly low. For cases where VL(MAX) > VD, it will usually be necessary to power IC1 from the VS rail. However, for applications such as the one outlined in Example 2, it may be possible to use an op amp with rail-to-rail output powered from the VD rail.
Capacitor C3 is necessary to ensure stability of the op-amp/regulator loop. If R6 and R7 are in the hundreds of kilohms range, a value of 100 µF to 1 nF should be suitable. Smaller values of R6 and R7 may require a larger value of C3. Capacitors C2 and C4 must be chosen to suit the requirements of the regulator type used for IC2. Note that the precision of VL depends directly on VD, which should be well regulated.
I just tried to run this circuit in LTSpice. Looks like there is a problem with R5. It should be 220 ohm, or? Otherwise it does not function properly, at least in the simulation.
Rufinus -December 10, 2008
When you already have a PWM output, why would you use a linear regulator to control a high power load? It would be more efficient and simpler to use the PWM signal to control a switching Mosfet. Of course the output would need to be filtered.
Ken Lundgren -December 07, 2006
@ Tobin Gimber
Well spotted, Tobin. A typo has, indeed, crept into the final draft of the text. You are correct in pointing out that the range for C3 is 100pF (as shown in the figure) to 1nF, not 100uF to 1nF.
@ Dr. Shyam
Readers should be aware that protection diodes are not essential, but depend on the type of regulator and the application. The LM317 regulator generally requires a protection diode connected between the adjust pin and output pin only when the adjust pin is bypassed by a large capacitance (not the case in this application). For regulators from the LM1084-1086 range, the internal resistance between the adjustment and output terminals limits the discharge current. No diode is needed to divert the current around the regulator even with a capacitor on the adjustment terminal. For cases where a large value of output capacitance (C4) is required, and where there is a possibility that the input terminal could be short-circuited to ground, a diode connected between input and output will divert the capacitor’s discharge current safely around the regulator.
Thanks to all for your comments.
Anthony H. Smith Design Brief Author.
Anthony H. Smith -September 08, 2006
Idea is good one. However, protection devices required for programmable regulator should not be omitted. Like output excess voltage discharge diodes are missing. These Ics like LM317 need that else may get damaged easily. Not good for inductive loads.
Shyam
www.sensorstechnology.com/
Dr. Shyam -August 13, 2006
Is there a typo in the final paragraph? I'm guessing that the range for C3 is 100pF to 1nF, not 100uF to 1nF. I'm just comparing the schematic with the text.
Tobin Gimber -August 07, 2006 (Article Rating: )
Good idea.
Aubrey Kagan -August 04, 2006 (Article Rating: )
This application would work nicely with another recent design idea, published in EDN. Focusing on the controller side it generates tabled sequences intended to be programmed and externally filtered to provide an inexpensive signal generator. EDN 7/20/06, Design Ideas, 'Microprocessor generates programmable clock sequences' http://www.edn.com/article/CA6351286.html?spacedesc=designideas&industryid=44217
William Grill -August 03, 2006 (Article Rating: )
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