design features Dual, Fast, Step-Down Controller’s External Reference Input Enables Dynamic Voltage Scaling from 0.4V to 5.5V and 0.3% Total Combined Regulation Accuracy Shuo Chen and Terry Groom Low voltage, high current systems require accurate differential regulation. It is not uncommon for power supply rails at or below 0.9V to demand 25A or more, with fast transients that look like intermittent electrical shorts to the power supply rail. Such systems typically require power supply regulation accuracy of less than 1% regulated DC and 3% in the face of input transients. Increasingly, core processors and other large scale digital ICs (such as ASICs and FPGAs) require dynamic voltage scaling—either multiple fixed levels, or a continuously adjusted reference voltage using a servo loop—to deliver power based on the processor demand. The goal is that the system can keep the applied power supply at the minimum voltage necessary for proper operation based on processing demand to conserve Figure 1. Channel 2 of the LTC3838-2 regulates to an external reference; channel 1 to an internal reference. VOUT1 and VOUT2 allow remote grounds up to ±500mV and ±200mV, respectively. VREF2 is also differentially sensed, but no separate pin for the remote ground of external reference is required. PART NUMBER LTC3838-1/-2 Ch.1 and LTC3838-1 Ch.2 LTC3838-2 Ch.2 (e.g., with ±0.1% Linear Technology Voltage References) REFERENCE VOLTAGE OUTPUT VOLTAGE TOTAL COMBINED ACCURACY (GROUND, LINE, LOAD & TEMP)* 0.6V Internal 0.6V to 5.5V 0.6V External 0.6V to 5.5V < ±0.67% (-40ºC ≤ T A ≤125ºC) 1.5V External 1.5V to 5.5V < ±0.4% (-40ºC ≤ T A ≤125ºC) 2.5V External 2.5V to 5.5V < ±0.3% < ±0.75% (0ºC ≤ T A ≤85ºC) < ±1% (-40ºC ≤ TA ≤125ºC) *external resistor divider error not included Table 1. Output voltage regulation accuracy over remote power ground deviation (up to ±200mV), input voltage (4.5V to 38V), output current, and temperature energy. One example is LSI’s adaptive voltage scaling & optimization (AVSO). stage. The problem is that soft-start and many common fault control features such a overvoltage protection might be sacrificed, depending on the technique used. The LTC3838-2 is designed to meet the extreme accuracy requirements through precision differential output sensing, and offer dynamic output voltage scaling using the differential external reference voltage input. DUAL DIFFERENTIAL V OUT ACCURACY THAT MATTERS To obtain superior regulation accuracy, power supply designers sometimes bypass a controller’s internal error amplifier and instead use a discrete precision reference and external op amps to control the power The LTC3838-2 avoids this trade-off by allowing the use of an external reference for accuracy while preserving valuable fault and protection features. With a precision voltage reference (such as LTC6652) from Linear Technology, or a DAC for programmability, the channel 2 output of the LTC3838-2 can be tightly regulated from 0.4V to 5.5V in applications with currents up to 25A per channel. At a very low 0.6V, the LTC3838-2 is able to achieve a total VOUT2+ VOUT1+ RFB2 COUT1 LTC3838-2 RDFB2 VOUTSENSE1+ VDFB2+ RDFB1 RFB1 VOUT1– REMOTELY-SENSED POWER GROUND 1, ±500mV MAX vs SGND VOUTSENSE1– VDFB2– EXTVREF2 VREF2+ + – VREF2– RDFB3 = RDFB1//RDFB2 TO PROGRAM VOUT2 = VREF2, COUT2 REMOVE RDFB1AND USE RDFB3 = RDFB2 VOUT2– REMOTELY SENSED POWER GROUND 2, ±200mV MAX vs SGND REMOTELY SENSED EXTERNAL REFERENCE GROUND April 2013 : LT Journal of Analog Innovation | 15 For differential external reference sensing, the LTC3838-2 has only one pin for external reference input. Channel 2 features a unique feedback amplifier configuration, which eliminates the need for a separate pin to sense the external reference’s remote ground. Instead, one additional resistor, equal to the parallel of the two feedback resistors, is used to connect to the remote ground externally. combined accuracy of ±4mV, or ±0.67%, over all operating conditions including line, load, extreme temperature and remote ground deviation up to ±200mV. TRACKING DYNAMIC DIFFERENTIAL EXTERNAL REFERENCE by scaling feedback with respect to a fixed lower reference voltage, where the percentage error does not change. For example, with an external reference of 2.5V, the total relative tolerance is less than ±0.3%. The LTC3838-2’s dual channels can be configured to singleoutput applications using channel 2’s external reference at such accuracy. Relative accuracy improves as the reference increases because the absolute error is a smaller percentage out of a larger reference voltage. This contrasts with programming the output voltage For differential external reference sensing, the LTC3838-2 has only one pin for external reference input. Channel 2 features a unique feedback amplifier configuration, which eliminates the need for a separate pin to sense the external reference’s remote ground. Instead, one Figure 2. A LTC3838-2, 300kHz, 2-phase single-output step-down converter with inductor-DCR sense. This application converts a 4.5V to 14V input to a dynamic 0.4V to 2.5V output at 50A. VIN 4.5V TO 14V + CIN2 22µF ×4 CIN1 180µF 2.2Ω 1µF LTspice IV 0.1µF 4.02k L1 0.4µH MT1 + COUT2 330µF ×2 SENSE1– SENSE2– SENSE1+ SENSE2+ BOOST1 BOOST2 0.1µF TG1 DB2 4.7µF MB1 DRVCC2 EXTVCC DRVCC1 INTVCC BG1 BG2 EXTVREF2 VOUTSENSE1+ VDFB2+ VREF2+ 0.4V TO 2.5V 10k 10k VREF2– VOUTSENSE1– PGOOD1 VDFB2– PGOOD2 TRACK/SS1 TRACK/SS2 ITH1 16 | April 2013 : LT Journal of Analog Innovation 137k DTR1 RT SGND RUN1 L2 0.4µH VOUT 0.4V TO 2.5V 50A SW2 PGND CIN1: SANYO 16SVP180MX CIN2: MURATA GRM32ER61C226KE20L COUT1, COUT4: MURATA GRM31CR60J107ME39L COUT2, COUT3: SANYO 2R5TPE330M9 DB1, DB2: CENTRAL SEMI CMDSH-4ETR L1, L2: VISHAY IHLP5050FDERR40M01 MT1, MT2: INFINEON BSC050NE2LS MB1, MB2: INFINEON BSC010NE2LS 4.02k MT2 TG2 SW1 1µF 16.2k 0.1µF DB1 2.2Ω COUT1 100µF ×2 LTC3838-2 0.1µF 16.2k circuits.linear.com/624 VIN ITH2 DTR2 PHASMD MODE/PLLIN CLKOUT RUN2 100k PGOOD 0.01µF 100pF 1000pF 7.5k MB2 COUT3 + 330µF ×2 COUT4 100µF ×2 design features In addition to regulation accuracy, the LTC3838-2 offers widebandwidth tracking to a dynamic external reference. Tracking bandwidth is important in applications like dynamic voltage scaling because bandwidth determines how quickly the supply can respond to changes to the programmed external reference. additional resistor equal to the parallel of the two feedback resistors is used to connect to the remote ground externally. See the LTC3838-2 data sheet for an explanation of how this configuration works. LT3838-1 CONTROLLER: INTERNAL REFERENCE ON BOTH CHANNELS quickly the supply can respond to changes to the programmed external reference. The LTC3838-1 shares the same functions as LTC3838-2, except channel 2 of the LTC3838-1 uses a 0.6V internal reference. Like its predecessors, the LTC3838 and LTC3839, both the LTC3838-1 and -2 use the controlled on-time, valley current mode architecture, which offers superior regulation during fast load transients without the typical switching period response delay of fixed frequency controllers, while still capable of constant frequency switching locked to an external 200kHz to 2MHz clock. They retain all features of the LTC3838, including the proprietary detect transient release (DTR), which improves the transient performance in low output voltage applications. Like the LTC3838, both LTC3838-1 and -2 include a full set of popular features, such as an external VCC power pin, RSENSE or inductor-DCR current sensing, selectable light load operating modes, overvoltage protection and current Figure 3 shows Bode plots from a 350kHz LTC3838-2 step-down converter compensated to an aggressive bandwidth close to 1/3 of the switching frequency without sacrificing stability. This allows the LTC3838-2 to track an external sine wave of 3.5kHz or 1/100 switching frequency at full power, without any noticeable distortion even at the sine wave’s very high bandwidth start and stop instants (Figure 4). Careful attention should be paid to the bandwidth requirements for any dynamic system. The wide-bandwidth external-reference-tacking capability, in addition to high speed transient performance, makes the LTC3838-2 ideally suited for the most dynamic supply applications. Figure 2 shows a typical LTC3838-2 application with external reference input. This 2-phase converter is capable of producing 50A over a wide ranging output from 0.4V to 2.5V. For example, at 1.5V this application can achieve 0.4% total combined accuracy for all operating conditions. The high accuracy and superior transient performance make the LTC3838-2 well suited for the most demanding processor applications. In addition to regulation accuracy, the LTC3838-2 offers wide-bandwidth tracking to a dynamic external reference. Tracking bandwidth is important in applications like dynamic voltage scaling because bandwidth determines how Figure 3. Loop gain and closed-loop Bode plots taken with an OMICRON Lab network analyzer on VOUT2 of a 350kHz LTC3838-2 step-down converter with external reference (EXTVREF2). 60 90 50 75 PHASE 45 30 10 15 0 –15 –20 –30 10 PHASE GAIN VOUT2 1V/DIV –5 –60 –10 –120 0 –10 –30 0 0 PHASE (deg) GAIN (dB) 60 20 GAIN EXTVREF2 1V/DIV PHASE (deg) 30 60 5 GAIN (dB) 40 Figure 4. The LTC3838-2 tracks a 1V peak-to-peak, 3.5kHz sine wave external reference. 100 FREQUENCY (kHz) –45 1000 –15 1 10 100 FREQUENCY (kHz) –180 1000 SW2 10V/DIV PGOOD2 5V/DIV 100µs/DIV April 2013 : LT Journal of Analog Innovation | 17 The LTC3838-1/-2 is the ideal choice for power in applications requiring fast transient performance, dual accurate differential output regulation, and external references for increased VOUT accuracy and programmability down to 0.4V. limit foldback, soft-start/rail tracking, and PGOOD and RUN pins for each channel. In addition to the differential remote output sensing on both channels, a significant improvement of the LTC3838-1/-2 over the original LTC3838 is the maximum current sense threshold voltage (i.e., current limit) accuracy. Unlike the LTC3838, which has a continuously variable and two fixed current limit ranges (VRNG), the LTC3838-2 has a fixed VRNG of 30mV (typical) and its tolerance over temperature is ±20%, POWER VIN 3.3V TO 14V which is much improved. The LTC3838-1 has the same 30mV and an additional 60mV (typical) VRNG setting whose tolerance is also significantly tighter. Refer to Table 2 for the comparison on the current limit tolerances and VRNG controls of the LTC3838-series 2-channel controllers. VBIAS SUPPLY 5V TO 5.5V + CIN1 180µF LTspice IV circuits.linear.com/625 CIN2 22µF ×4 2.2Ω 1µF VIN L1 0.47µH M1 SENSE2– SENSE1+ SENSE2+ BOOST1 BOOST2 TG1 COUT2 330µF ×2 4.7µF DRVCC1 INTVCC DRVCC2 EXTVCC 0.01µF VOUTSENSE1– VDFB2– PGOOD2 470pF 137k ITH1 DTR1 VRNG RT SGND RUN1 ITH2 DTR2 PHASMD MODE/PLLIN CLKOUT RUN2 + COUT4 100µF ×2 20k //10k SGND PGOOD2 100k 0.01µF TRACK/SS1 TRACK/SS2 47pF 23.2k 20k PGOOD1 VOUT2 0.9V 20A 10k VDFB2+ 10k PGOOD1 COUT3 330µF ×2 BG2 VOUTSENSE1+ 100k RS2 L2 0.47µH 0.0015Ω SW2 BG1 10k 18 | April 2013 : LT Journal of Analog Innovation M2 DB2 SW1 1µF 100Ω 0.1µF TG2 PGND Figure 5. When an external 5V rail is commonly available to bias up the controller, this LTC3838-1 application converts a dynamic 3.3V to 14V power input to 20A dual outputs of 1.2V and 0.9V. 100Ω DB1 2.2Ω + SENSE1– 1nF 0.1µF RS1 0.0015Ω LTC3838-1 1nF 100Ω COUT1 100µF ×2 Figure 5 shows the VIN pin connected via diode-OR to the VBIAS 5V rail and to power VIN, 3.3V–14V, rail. This allows the power VIN rail to dynamically switch between a higher voltage and a minimum of 3.3V. When operating with the power VIN supply below 5.5V, this application requires the VBIAS supply to be present at EXTVCC in order to maintain The LTC3838-1/-2 controllers require a minimum VIN pin voltage of 4.5V, but this does not limit the power input to 4.5V. For example, many digital systems have an available regulated 100Ω VOUT1 1.2V 20A 5V rail, which can be used to bias the VIN pin and gate drivers, and to efficiently step down inputs less than 4.5V. 47pF 470pF 17.4k CIN1: SANYO 16SVP180MX CIN2: MURATA GRM32ER61C226KE20L COUT1, COUT4: MURATA GRM31CR60J107ME39L COUT2, COUT3: SANYO 2R5TPE330M9 DB1, DB2: CENTRAL SEMI CMDSH-4ETR L1, L2: WÜRTH 7443330047 M1, M2: INFINEON BSC0911ND design features Using an external reference, the LTC3838-2 can achieve total accuracy levels as low as 0.3% under all operating conditions. The external reference feature is designed to accommodate dynamic voltage scaling and track fast external reference inputs with minimum distortion. Table 2. Maximum Current Sense Threshold Voltage Specifications and Range Controls PART V RNG = SGND V RNG = INTV CC V RNG CONTROL V RNG PIN(s) LTC3838 and LTC3839 21mV to 40mV 39mV to 61mV 30mV–200mV continuous & 30mV/50mV fixed each per channel LTC3838-1 24mV to 36mV 54mV to 69mV 30mV/60mV fixed single LTC3838-2 24mV to 36mV 30mV fixed only no DRVCC , INTVCC and VIN pin voltages needed for the IC to function properly. The EXTVCC supply is optional when the power VIN supply is at or above 5.5V. Note that the power input voltage range of this application cannot be generalized for other frequencies and output voltages, and each application that needs a power input voltage different from the VIN pin voltage should be tested individually for margin of range in which the switching nodes (SW1, SW2) phaselock to the clock output (CLKOUT). SUMMARY The LTC3838-1/-2 is the ideal choice for power in applications requiring fast transient performance, dual accurate differential output regulation, and external references for increased VOUT accuracy and programmability down to 0.4V. Compared to the original LTC3838, the LTC3838-1/ LTC3838-2 offers differential output sensing on both channels, improved current limit accuracy, and the choice of internal/ external reference. Using an external reference, the LTC3838-2 can achieve accuracy levels as low as 0.3% under all operating conditions. The external reference feature is designed to accommodate dynamic voltage scaling and track fast external reference inputs with minimum distortion. The LTC3838-1 and -2 are offered in 38-pin QFN (5mm × 7mm) packages with exposed pads for enhanced thermal performance. Visit www.linear.com/LTC3838-1 and /LTC3838-2 for data sheets, demo boards, a variety of applications designs, and for more information about how: •a 30ns minimum on-time enables high step-down ratios, e.g., from 38V to 0.8V at 350kHz •2MHz switching frequency enables applications with tiny power components For More Information… THE LTC3838/LTC3839, PREDECESSOR TO THE LTC3838-1/-2: See the article: • 2MHz Dual DC/DC Controller Halves Settling Time of Load Release Transients, Features 0.67% Differential VOUT Accuracy and is Primed for High Step-Down Ratios in the LT Journal of Analog Innovation, April 2012 (Volume 22, Number 1). THE LTC3833 SINGLE-CHANNEL CONTROLLER The LTC3838 series of dual controllers are based on and have all features of the single-channel controller LTC3833. For a full discussion of the features shared with LTC3833, refer to the cover article: • Fast, Accurate Step-Down DC/DC Controller Converts 24V Directly to 1.8V at 2MHz in the LT Journal of Analog Innovation, October 2011 (Volume 21, Number 3). •25A output becomes practical at 2MHz, with 95% peak efficiency (2V-5V VOUT). n April 2013 : LT Journal of Analog Innovation | 19