A current Design Thought, “Three discretes suffice to interface PWM to switching regulators,” demonstrated a method to make use of PWM to regulate the output of a typical switching voltage regulator. There have been some discussions within the feedback part about circuit conduct, which influenced design modification. Right here’s a low-cost design that advanced in mild of these discussions. A logic gate IC, an op-amp, and some resistors and capacitors buffer a PWM and provides a sign to the regulator’s suggestions pin, Determine 1.
Determine 1 A microprocessor produces a 12-bit, 20 MHz, PWM sign that controls a switching voltage regulator utilizing an inverter IC, an op-amp, resistors, and capacitors to buffer the sign for the regulator’s suggestions pin.
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For numerous causes, it’s troublesome, if not inconceivable, to regulate a regulator’s output voltage past a sure degree of accuracy. This design proceeds with a PWM having 12 bits of decision in thoughts, working at a frequency of roughly 4900 Hz.
It’s straightforward today to search out microprocessors (µPs) that may produce a PWM clocked at 20 MHz. Nonetheless, the provision currents operating by that IC’s energy provide bonding wires could cause voltage drops. Which means that the PWM indicators don’t fairly swing to the rails. Worse but, if the currents differ considerably with the µP’s duties, the voltage drops can change, and it may not be attainable to calibrate the errors out. A easy answer is to buffer the µP with logic gates (sometimes inverters), which draw negligible present besides throughout switching transients. The gates may be powered from a clear, correct voltage supply, the identical as or shut in worth to that which powers the µP.
The inverter in Determine 1 is a 74LVC2G14GW,125 IC whose paralleled outputs drive an op-amp-based low-pass filter whose passive elements are of sufficiently excessive impedances to negligibly load these outputs. When powered from 3 V or extra, this twin inverter has an output resistance of lower than 15 Ω from -40°C to +85°C. (If that you must function the µP at 1.8 V, parallel the 6 inverters of a 74LVC14AD,118 for a lower than 19 Ω outcome.)
The TLV333IDBVR op-amp has a most enter offset voltage of 15 µV (the utmost offset is specified for a 5-V provide; an unknown enhance may be anticipated if the provision voltage is decrease).
Its typical (most aren’t specified) enter currents are 150 pA from -40°C to +85°C, contributing an offset by R1, R2, and R3 of 115 µV. At 1.8 V, ½ LSb for a 12-bit sign is 220 µV (370 µV with a 3.0-V provide.) The filter settles to a lot lower than that 12-bit ½ LSb voltage in 10mS and has a peak (not peak-peak) ripple of lower than 50µV.
R4 and R5 must be chosen in order that the meant most constructive op-amp output voltage multiplied by R4 / (R4 + R5) is at most barely larger than Vfb. This enables a regulator output of Vfb. This ratio might be smaller if the minimal desired regulator output is bigger than Vfb. The resistors’ parallel mixture must be the worth specified by the regulator for the only resistor related between Vfb and floor, sometimes 10 kΩ. R6 must be set in accordance with the specified vary of output voltages.
The allowed vary of PWM obligation cycles ought to exclude extremes for a minimum of two causes. First, the op-amp output is barely assured to swing inside 70 mV of every rail (with a 10k load related to half of the provision voltage.) Second, the processor, GPIO particularly (but additionally the gate to some extent), seemingly has unequal rise and fall instances and delays. Though these variations are small, they’ve their best results on accuracy at obligation cycle extremes, which subsequently must be prevented. Happily, accommodating these limitations has a negligible impact on performance.
On this design, the output voltage is linear with the obligation cycle. The regulator’s loop acquire is unchanged from that of ordinary operation. With the regulator disabled till the PWM filter output settles, there are not any startup points. Lastly, there’s negligible inherent injection of noise into the suggestions pin from an exterior provide.
Christopher Paul has labored in numerous engineering positions within the communications business for over 40 years.
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