Easy linear and switching voltage regulators with suggestions networks of the sort proven in Determine 1 are legion. Their output voltages are the reference voltage on the suggestions (FB) pin multiplied by 1 + R_f / R_g. Really helpful values of C_f from 100 pF to 10nF improve the quantity of suggestions at greater frequencies, or at the very least guarantee it isn’t decreased by stray capacitances on the suggestions pin.

Determine 1 The configurations of widespread regulators and their suggestions networks. A linear regulator is proven on the left and a switcher on the appropriate.

Modifying this construction to include PWM management of the output voltage requires some thought, and each Stephen Woodward and I’ve offered a number of Design Concepts (DIs) that tackle this.

Wow the engineering world along with your distinctive design: Design Concepts Submission Information

I’ve steered disconnecting R_g from floor and driving it from a closely filtered (op-amp-based) PWM sign equipped by a 74xx04-type logic inverter. Though this may end up in glorious ripple suppression, it has an obstacle—the necessity for an inverter energy provide, which doesn’t degrade the accuracy of the regulator’s 1% or higher reference voltage.

Stephen has proposed switching the disconnected R_g leg between floor and open with a MOSFET. The fantastic thing about that is that no new reference is required. Though the output voltage is not a linear perform of the PWM obligation cycle, a easy software-based lookup desk renders this a mere inconvenience. (Yup, “we are able to repair it in software program!”)

A normal scheme to mitigate PWM controller-induced ripple needs to be versatile sufficient to accommodate completely different regulators, regulator reference voltages, output voltage ranges, and PWM frequencies. In deciding on one, listed here are some attainable traps to pay attention to:

• Nulling by including an out-of-phase model of the ripple sign is on the mercy of element tolerances.
• Low-cost ceramics, similar to the ever-present X7R, have DC voltage and temperature-sensitive capacitances. If used, the circuit should tolerate these undesirable traits.
• Schemes which join capacitors between floor and the suggestions pin will scale back loop suggestions at greater frequencies. The end result could possibly be degradation of line and cargo transient responses.

Circuit

With this in thoughts, contemplate the circuit of Determine 2, able to operation from 0.8 V to slightly greater than 5 V.

Determine 2 A particular occasion of a PWM-controlled regulator with ripple suppression. Solely a linear regulator is proven, however the adaptation for switcher operation entails solely the addition of an inductor and a filter capacitor.

The low capacitance MOSFET has a most on-resistance of beneath 2 Ω at a V_GS of two.5 V or extra. C_g1 and C_g2 see most DC voltages of 0.8 V (as much as 1.25 V in some regulators). Their capacitive accuracies are usually not crucial, and at these low voltages, they barely budge when 10-V or higher-rated X7R capacitors are employed.

C_f can see a big DC voltage, nevertheless. Right here, you would possibly get away with an X7R, however a 10-nF (voltage-insensitive) C0G is affordable. The worth of Cf was chosen to assist in ripple administration. If it weren’t current, the ripple could be bigger and proportional to the worth of R_f. With a 10-nF C_f, bigger values of R_f for greater output voltages would don’t have any impact on the PWM-induced ripple; smaller ones might solely scale back it. The most important peak-to-peak ripple happens at obligation cycles from 30 to 40%.

The filtering equipped by the three capacitors produces a sinusoidal ripple waveform of amplitude 5.7 µV peak-to-peak. For a 16-bit ADC with a full scale of 5 V, the peak-to-peak amplitude is lower than 1 LSbit.

Flexibility

You may need a requirement for a wider or narrower vary of output voltages. Be happy to switch R_f accordingly with no penalty in ripple amplitude.

Ripple amplitude will scale in proportion to the regulator’s reference voltage. The design assumes a regulator whose optimum FB-to-ground resistance is 10 kΩ. If it’s crucial to vary this for the regulator of your selection, scale the three Rg resistors by the identical issue Z. As a result of the resistors and three capacitors implement a 3^rd order filter, the ripple will scale in accordance with Z^-3. To maintain the identical ripple amplitude, scale the three capacitors by 1/Z. You would possibly wish to scale the capacitors’ values for another motive, even when the resistors are unchanged.

Altering the PWM frequency by an element F will change the ripple amplitude by an element of F^-3. However too excessive a frequency might encounter accuracy issues because of the parasitic capacitances and unequal turn-on/turn-off instances of the MOSFET.

Some regulators won’t tolerate a C_f of a price massive sufficient to assist in ripple suppression. Often, these will tolerate a resistor R_cf in sequence with C_f. In such circumstances, ripple might be elevated by an element Okay equal to the sq. root of ( 1 + R_cf · 2π · f_PWM · C_f ), and the waveform would possibly not be sinusoidal. However growing C_g1 and C_g2 by the sq. root of Okay will compensate to yield roughly the identical suppression as provided by the design with R_cf equal to 0. If all else fails, there may be all the time the opportunity of including an R_g4 and a C_g3 to offer one other stage of filtering.

Tying all of it collectively

 A versatile strategy has been launched for the suppression of PWM control-induced ripple in linear and switching regulators. Easy guidelines have been offered for the use and modification of the Determine 2 circuit for operation over completely different output voltage ranges, PWM frequencies, most well-liked resistances between floor and the regulator’s suggestions pin, and tolerances for reasonably massive capacitances between the FB pins and the output.

The restrictions of capacitors with sensitivities to DC voltages are acknowledged. These parts are used appropriately and judiciously. Dependency on element matching is prevented. Normal suggestions community constructions are maintained or, at worst, subjected to minor modifications solely; particularly, suggestions at greater frequencies is just not decreased from that advisable by the regulator producer. This maintains the required line and cargo transient responses.

Addendum

As soon as once more, the Feedback part of DIs has proven its price. And it’s Deja vu yet again; worth was supplied by the redoubtable Stephen Woodward. In an earlier DI, he identified that regulators typically don’t tolerate adverse voltages at their suggestions pins. But when there’s a capacitor C_f of various hundred picofarads linked from the output to this pin, as I’ve advisable on this DI, and the output is shorted or quickly discharged, this capacitor might couple a adverse voltage to that pin and injury the half. To guard towards this, add the parts proven within the following determine.

Determine 3 Add these parts to guard the FB pin from output speedy adverse voltage modifications.

In regular operation and through startup, the CUS10S30 Schottky diode seems like an open circuit and it, C_c, and the 1 MΩ resistor have a negligible impact on circuit operation. C_c prevents the stream of diode reverse present, which might in any other case produce output voltage errors. If V_out transitions to floor quickly, C_c and the diode stop any adverse voltage from showing on the junction of the capacitors. R_c offers an inexpensive “simply in case” restrict of the present into the FB pin from that voltage transient if it someway noticed a adverse voltage. (Verify the utmost FB pin present to make sure that no important error-inducing voltages develop throughout R_c.) When the circuit has settled, the voltage throughout C_c is discharged, and the circuit is able to restart usually.

Christopher Paul has labored in numerous engineering positions within the communications trade for over 40 years.

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