TECHNICAL ARTICLE | | Join Tweet Connect Digital Potentiometers Enable Fast, Linear Adjustment of Switched Mode Power Supplies by Miguel Usach, Analog Devices, Inc. VIN The capability to finely adjust the output voltage in a power supply makes it is possible to remove tolerances and drops in the power path, verify the operation at the system limits, or implement simple dynamic voltage control for microprocessors. This article explores several options for adjusting a switched mode power supply (SMPS) and proposes a solution that uses a switching regulator with a digital potentiometer as the feedback control element, highlighting the design issues and the ways that they can be solved. Finally, the AD5141 single channel, nonvolatile digiPOT is presented as a simple way to overcome common limitations in this application. OUTPUT FILTER Switched mode power supply regulators provide higher efficiency than linear regulators in high current systems, with typical efficiencies of greater than 90% for currents above 100 µA. In a low dropout (LDO) regulator, the efficiency depends on the quiescent current (Iq) and the forward voltage drop, with higher quiescent current causing lower efficiency, as shown in Equation 1. LDEFFICIENCY(%) = VOUT IOUT + 100 VIN (IOUT+Iq ) (1) Today’s LDOs have reasonably low quiescent current, so Iq can be neglected = Vsmall ( R1 + 1)to I . Then, the LDO efficiency is simply FEEDBACK (2) if itVOUT is very compared LOAD R2 (VOUT/VIN) × 100. Because the LDO has no way to store significant amounts of Runused energy, power not delivered to the load is dissipated as heat(3) AW = RAB − RWB within the LDO. Typical LDO efficiencies are less than 83%. AW × R 3 With losses, switching regulators are replacing linear regulators R4 their = Rlower (4) R R3 as ATE, FPGAs, and instrumentation that require high AB + such in applications current dynamic × R3 loads. R5 =orRWB (5) R AB + R3 It is often necessary for the system designer to adjust supply voltages, either to Roptimize their levels or to force them away from nominal values when (6) 7 = R1 + R4 characterizing system performance under extreme conditions. This function RAW = Rperformed (7) AB + RWB during in-circuit test (ICT), when a manufacturer wants is typically to guarantee that a product functions correctly at nominal supplies ± 10%, for example. This procedure, called margining, is done by deliberately changing the supply RAW ≠within RAB − (8) WB voltage theRexpected range. In addition, the capability to finely adjust the output voltage makes it possible to compensate for supply tolerance and voltage drops in the power path. Other applications, such as dynamic voltage control for a microprocessor, must be able to change the voltage on-the-fly, reducing the voltage in low power modes and increasing it in high performance modes. An SMPS works similarly to an LDO, as shown in Figure 1. The output voltage is compared with an internal reference, with the difference connected to the pulse width modulator. VOUT PULSE WIDTH MODULATOR FEEDBACK ERROR AMPLIFIER VREF Figure 1. SMPS voltage control loop. The pulse width modulator compares a ramp with the amplifier output, and generates the PWM signal that controls the switches that deliver energy to the load. Adjusting the output voltage can be done by controlling the voltage at the inverting amplifier pin. This can be done externally, using a DAC, or a digital potentiometer. Some regulators allow internal control of the feedback voltage using a serial interface such as PMBUS, I2C, or SPI. Table 1 compares all three methods in terms of adjustment capability and power dissipation. Table 1. Benchmark Analysis Summary—Adjustable SMPS Method Coarse Adjustment DAC digiPOT Internal Registers Fine Adjustment Medium High High High Medium Low Power Supply Typical Power Rails Consumption VMIN < 2.5 V VMIN < 2.3 V Not applicable >100 µA <20 µA Low Some digital potentiometers are available with nonvolatile memory, so the output supply can be programmed in test. This easy to use feature provides a substantial benefit as compared to the other two methods. Linearizing the Transfer Equation VOUT IOUT LDEFFICIENCY (%) = +of100 Equation 2 describes theVoutput voltage the SMPS based on the ratio of IN (IOUT+Iq ) (1) feedback resistors R1 and R2, VOUT = VFEEDBACK ( R1 + 1) R2 where VFEEDBACK is the internal reference voltage. (2) RAW = RAB − RWB (3) R4 = RAW × R3 RAB + R3 (4) analog.com R5 = RWB × R3 RAB + R3 (5) R7 = R1 + R4 (6) VOUT IOUT LDEFFICIENCY(%) = + 100 (1) ) a digital potentiometer, some issues VINR1(Iand Before directly replacing R2q by OUT+I should be considered. Internally the digital potentiometer has two resistor strings, RAW and RWB. R1 VOUT = VFEEDBACK ( + 1) (2) R2 Both string resistors are complementary, RAW = RAB − RWB DC-TO-DC CONVERTER VIN INPUT R1 (3) where RAB is the end-to-end resistance or nominal value. R4 = RAW × R3 (4) ReplacingRRAB1 and + RR3 2 with RAW and RWB results in a logarithmic transfer function. The nonlinear relation between the digital code and the output voltage ×R R5 = RWB decreases the low 3end resolution. Figure 2 shows an example for a 16-tap(5) RAB + R3 digital potentiometer. R7 = R20 (6) 1 + R4 RAW = RAB + RWB (7) VOUT OUTPUT FEEDBACK digiPOT R3 R2 GND Figure 4. Potentiometer mode. In rheostat mode, the series resistance must be high enough to render the tolerance of the digital potentiometer negligible, that is R2 ≥ 10 × RAB. In potentiometer mode, the parallel resistor must be small enough, that is RAB . 10 16 R3 ≤ 12 Linearizing the potentiometer using a series-parallel combination could be quite complex, as shown in the equivalent circuit of Figure 5, RAW ≠ RAB − RWB VOUT (8) R1 8 RAW 4 R4 0 4 8 RDAC DECIMAL CODE 12 R8 VOUT = VFEEDBACK ( RR12 + 1) R2 This problem can be overcome in several ways; the more common are to use the digital potentiometer in rheostat mode, or to place resistors in series with the potentiometer. (1) R5 15 Figure 2. Logarithmic transfer function. R7 R R3 6 VOUT IOUT + 100 VIN (IOUT+Iq ) RWB LDEFFICIENCY(%) = 0 R1 R2 (2) Figure 5. Final Y-∆ transform. RAW = RAB − RWB (3) R4 = RAW × R3 RAB + R3 (4) Due to the resistor tolerance, using a digital potentiometer in conjunction with external resistors can cause mismatch problems. Precision devices might have 1% resistor tolerance, but the vast majority of digital potentiometers can only achieve 20% resistor tolerance. R5 = RWB × R3 RAB + R3 (5) R7 = R1 + R4 (6) In this case, reducing the mismatch is possible by using a series/parallel resistance combination, as shown in Figure 3 and Figure 4. As a downside, the dynamic range is reduced as well. RAW = RAB + RWB (7) Minimizing the Tolerance DC-TO-DC CONVERTER VIN INPUT VOUT OUTPUT R1 FEEDBACK R3 GND R2 where: the feedback input pin typically has high impedance, so the effect of R6 can be made negligible. Increasing the Bandwidth RAW ≠ RAB − RWB (8) The switching regulator operates at high frequency, typically above 1 MHz, allowing the use of small external components. In worst-case scenarios, it must power dynamic loads, so the feedback resistor network must provide enough bandwidth to accurately track the output voltage. Due to the parasitic internal switch capacitance, the digital potentiometer acts as a low-pass filter. If the feedback network does not have enough bandwidth, the output voltage will oscillate, as shown in Figure 6. Figure 3. Rheostat and serial resistor. | Digital Potentiometers Enable Fast, Linear Adjustment of Switched Mode Power Supplies 2 SW SW VOUT VOUT IOUT IOUT Figure 6. Discrete feedback resistance vs. digital potentiometer with limited bandwidth. A simple way to overcome this limitation is to place a capacitor in parallel VOUTfeedback IOUT between the output and the LDEFFICIENCY (%) = +network 100 (as shown in Figure 7), (1) V IN (IOUT+Iq ) reducing the high frequency impedance, and minimizing the oscillation time. VOUT = VFEEDBACK ( R1 DC-TO-DC + 1) R2CONVERTER VIN RAW = RAB − RWB R4 = RAW × R3 RAB + R3 (2) R7 C1 R8 GND R7 = R1 + R4 A Simpler R = RSolution + R Without Compromise AB (3) IOUT Figure 7. Parallel capacitor reduces high frequency impedance, minimizes oscillation. AW VOUT VOUT INPUT OUTPUT FEEDBACK R5 = RWB × R3 RAB + R3 SW WB (4) (5) (6) (7) ADI’s new AD5141 digiPOT overcomes the problems presented by other digital potentiometers. Its patented linear gain setting mode allows independent control of each string resistor, so RAW ≠ RAB − RWB (8) enabling this mode, no external resistors are needed. The resistor tolerance becomes negligible, and the overall error of the transfer function is only due to the internal string mismatch, which is typically less than 1%. Each string resistor has an associated EEPROM location, so an independent value for each string can be loaded upon power-up. In addition, the device provides up to 3 MHz bandwidth for a fast feedback loop as shown in Figure 8. Figure 8. AD5141 (10 k Ω) version in linear gain setting mode. Conclusion Switched mode power supply regulators are commonly used in high current applications due to their high efficiency. This article describes several ways that can be used to digitally control the output voltage. Due to the inherent benefits obtained by powering up a system in a predefined output state, a solution that uses digital potentiometers with internal nonvolatile memory is desirable. The main trade-offs faced by designers include providing enough resolution, accuracy, and bandwidth to achieve outstanding performance. The AD5141 digiPOT enables designers to provide an optimum solution without compromises. For more information on any of the above mentioned products visit www.analog.com. Online Support Community Engage with the Analog Devices technology experts in our online support community. Ask your tough design questions, browse FAQs, or join a conversation. ez.analog.com. analog.com | 3 Analog Devices, Inc. Worldwide Headquarters Analog Devices, Inc. One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 U.S.A. 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