design features 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 Shuo Chen and Terry Groom Electrical conditions once considered extreme are now the norm. Modern electronic systems demand high currents and very low voltages that can appear to a DC/DC converter as an intermittent electrical short. It is not uncommon for sub-0.9V power supply rails to demand 25A or more. In this environment, tight total differential regulation accuracy is critical to achieve the demanding voltage tolerances required to power core processors and large ASICs. In addition, PCB and component size constraints have driven up converter operating frequencies to enable the use of smaller components. VIN 4.5V TO 38V + CIN1 100µF CIN2 10µF ×3 LTspice IV circuits.linear.com/546 VIN 0.1µF 3.57k + COUT2 330µF ×2 SENSE1– SENSE2– SENSE1+ SENSE2+ BOOST1 BOOST2 0.1µF TG1 MT1 3.57k TG2 MT2 DB2 SW1 1µF 15k 0.1µF DB1 2.2Ω COUT1 100µF ×2 LTC3838 0.1µF 15k VOUT1 1.2V 15A In addition to architectural advantages, the proprietary detect transient release (DTR) feature improves the transient 2.2Ω 1µF L1 0.56µH The LTC3838 and LTC3839 controllers are designed to meet the needs of the most demanding low output voltage, high load current applications. Both feature superior differential regulation accuracy and fast transient response. The controlled on-time architecture yields minimum on-times as low as 30ns and is capable of switching frequencies from 200kHz to 2MHz with synchronization to an external clock. 4.7µF DRVCC1 INTVCC DRVCC2 EXTVCC BG2 MB2 PGND 10k VOUTSENSE1+ PGOOD1 0.01µF Figure 1. 4.5V to 38V input, 1.2/15A, 1.5V/15A dual output, 350MHz step-down converter. With the output sensed directly through a resistor divider network, the remote sensing scheme in channel 1 mimics the traditional feedback used in channel 2. The LTC3838’s novel remote sensing scheme eliminates the diff amp output pin required in other parts. 40.2k 220pF 115k COUT3 100µF ×2 10k VOUTSENSE1– PGOOD1 PGOOD2 PGOOD2 ITH1 DTR1 VRNG1 RT SGND RUN1 ITH2 DTR2 VRNG2 100k 0.01µF TRACK/SS1 TRACK/SS2 22pF COUT4 + 330µF ×2 15k VFB2 10k 100k VOUT2 1.5V 15A SW2 BG1 MB1 L2 0.56µH 22pF 220pF 40.2k CIN1: NICHICON UCJ1H101MCL165 CIN2: MURATA GRM32ER71H106K COUT2, COUT4: SANYO 2R5TPE330M9 COUT1, COUT3: MURATA GRM31CR60J107ME39L DB1, DB2: DIODES INC. SDM10K45 L1, L2: TOKO FDA1055-R56M MT1, MT2: INFINEON BSC093N04LSG MB1, MB2: INFINEON BSC035N04LSG PHASMD MODE/PLLIN CLKOUT RUN2 April 2012 : LT Journal of Analog Innovation | 19 The LTC3838 and LTC3839 controllers are designed to meet the needs of the most demanding low output voltage, high load current applications. Both feature superior differential regulation accuracy and fast transient response. ILOAD 10A/DIV PLLIN 5V/DIV ILOAD 10A/DIV VOUT 50mV/DIV AC-COUPLED VOUT 50mV/DIV AC-COUPLED SW1 10V/DIV IL 10A/DIV SW2 10V/DIV IL 10A/DIV 0° 180° CLKOUT 5V/DIV 5µs/DIV LOAD STEP = 0A TO 15A VIN = 12V VOUT = 1.2V FORCED CONTINUOUS MODE 5µs/DIV LOAD RELEASE = 15A TO 0A VIN = 12V VOUT = 1.2V FORCED CONTINUOUS MODE 60° 500ns/DIV VIN = 12V VOUT1 = 5V, VOUT2 = 3.3V LOAD = 0A MODE/PLLIN = 333kHz EXTERNAL CLOCK PHASMD = GND Figure 2. Switching frequency is constant and phase locked during steady state, but fast transient performance is achieved by momentarily adjusting the switching frequency: increasing it on a load step; decreasing it on a load release. performance in high step-down ratio, low output voltage applications. This enables the LTC3838/LTC3839 to maintain accuracy and respond to load transients faster than other topologies. In high output current supplies applications, it is important that overall regulation accuracy is well understood. To this end, the LTC3838 and LTC3839 internally combine the output differential amplifier and error amplifier and specify DC, line and load regulation output voltage errors as a single lumped parameter. This allows the LTC3838 and LTC3839 to achieve a level of total differential accuracy unavailable in other controllers. The LTC3838 and LTC3839 make high frequency switching practical in a high input voltage, low output voltage converter. Both devices can produce high step-down ratios at high switching frequencies while maintaining high efficiency at heavy load 20 | April 2012 : LT Journal of Analog Innovation currents—previously challenging due to greater switching losses and limitations inherent in other architectures. For instance, in the typical 12V input to 3.3V/25A output application shown in Figure 3, the LTC3838/LTC3839 delivers a peak efficiency of 93% at 2MHz. FLEXIBLE DUAL/SINGLE OUTPUT, HIGH ACCURACY REMOTE SENSE The LTC3838’s dual channels can be configured for either dual- or singleoutput applications, whereas the LTC3839 is dedicated for single-output applications. Both convert an input of 4.5V to 38V (40V abs max) down to outputs of 0.6V to 5.5V (6V abs max) in applications with per-channel currents up to 25A. Their remotely sensed differential feedback has a voltage regulation accuracy of ±0.67%—where the remote power ground can deviate as much as ±500mV. The LTC3838’s second channel can provide an independent ±1% output, or together with the first channel, serve as one of the PolyPhase® channels for a single-output, higher current application. For higher load currents, or to maximize efficiency, multiple LTC3838s and LTC3839s can be paralleled for up to 12-phases. FAST TRANSIENT PERFORMANCE, CONSTANT FREQUENCY The LTC3838 and LTC3839 employ the new controlled on-time, valley current mode architecture, primed for fast transient performance. This architecture retains the benefits of a constant on-time controller: it responds to sudden load increases by a sequence of consecutive on-time pulses with a very short 90ns off-time in between, without having to wait until the next switching cycle like that of a fixed frequency controller. During a load release, the LTC3838/LTC3839 delays the turn-on of the top FET until inductor current drops design features The controlled on-time architecture yields minimum on-times as low as 30ns and makes high frequency switching practical in a high input voltage, low output voltage converter, while maintaining high efficiency at heavy load currents. CIN2 22µF ×4 2.2Ω 90 LTC3839 VIN 10Ω SENSE1– 1nF 10Ω 1nF SENSE1+ 0.1µF SENSE2+ BOOST1 VOUT RS1 0.004Ω MT1 L1 0.3µH 1µF MB1 TG2 SW1 SW2 DRVCC1 INTVCC BG2 VOUT 3.3V 25A 8 6 80 FORCED CONTINUOUS MODE 70 60 RS2 0.004Ω DISCONTINUOUS MODE 50 LOSS FORCED CM 0.1 4 2 LOSS DCM 0 100 1 10 LOAD CURRENT (A) MB2 PGND 45.3k 100 VOUTSENSE+ 10k VOUTSENSE– PGOOD 0.01µF PHASMD TRACK/SS MODE/PLLIN 150pF ITH DTR 18.7k LTspice IV CIN1: SANYO 16SVP180MX CIN2: MURATA GRM32ER61C226KE20L COUT1, COUT2: MURATA GRM31CR60J107ME39L DB1, DB2: CENTRAL CMDSH-3 L1, L2: WÜRTH 7443340030 MT1, MT2: INFINEON BSC050NE2LS MB1, MB2: INFINEON BSC032NE2LS VRNG CLKOUT SGND RUN 10 VIN = 5V 8 FORCED CONTINUOUS MODE 80 6 4 70 60 50 RT circuits.linear.com/547 90 DISCONTINUOUS MODE LOSS FORCED CM 0.1 1 10 LOAD CURRENT (A) POWER LOSS (W) PGOOD 33.2k L2 0.3µH DRVCC2 EXTVCC BG1 100k MT2 DB2 4.7µF 10Ω 0.1µF BOOST2 TG1 DB1 2.2Ω COUT1 100µF ×6 10Ω SENSE2– 10 VIN = 12V POWER LOSS (W) 1µF EFFICIENCY (%) CIN1 180µF 100 + EFFICIENCY (%) VIN 4.5V TO 14V 2 LOSS DCM 0 100 Figure 3. A 2MHz, 3.3V/25A step-down converter. The LTC3838/LTC3839 can operate at switching frequencies above the AM radio band (fSW > 1.8MHz). The high switching frequency permits the use of inductors of very small footprint, so that the entire circuit can fit within a 0.9in2 area with both sides populated. The peak efficiency is 95%, and full load efficiency well above 90% at 25A, even at a frequency of 2MHz. to desired value, preventing overcharging the output capacitor. Once the transient condition subsides, the switching frequency quickly returns to the programmed nominal or external clock frequency. Meanwhile, the on-time is adjusted (hence controlled on-time) so that the switching frequency is constant during steady-state operation, synchronized to its internal programmable or an external clock, to mimic a fixed frequency controller with predictable switching noise. HIGH AND WIDE STEP-DOWN RATIO, SWITCHING FREQUENCY The LTC3838/LTC3839’s 30ns minimum on-time (60ns effective on-time with dead-time delays) enables low duty cycles for high VIN to low VOUT applications, even while the part operates at high frequency. The 90ns minimum off-time helps achieve high duty cycle operation and avoid output dropout when VIN is only slightly above the regulated VOUT. The LTC3838 and LTC3839 are capable of a full decade programmable switching frequency from 200kHz to 2MHz. They can be synchronized to external clocks of ±30% of the programmed frequency. April 2012 : LT Journal of Analog Innovation | 21 In addition to the LTC3838/LTC3839’s architectural advantages, the proprietary detect transient release (DTR) feature improves the transient performance in low output voltage applications. This enables these parts to maintain accuracy and respond to load transients faster than other topologies. LTC3838/LTC3839 EA VREF VFB SW 5V/DIV INTVCC 1/2 INTVCC + – ITH + – DTR LOAD RELEASE DETECTION TO LOGIC CONTROL DTR 1V/DIV CITH2 (OPTIONAL) BOTTOM MOSFET GATE TURNS BACK ON, INDUCTOR CURRENT (IL) GOES NEGATIVE IL 10A/DIV INTVCC CITH1 BG 5V/DIV RITH2 RITH1 DTR DETECTS LOAD RELEASE, TURNS OFF THE BOTTOM MOSFET GATE FOR FASTER INDUCTOR CURRENT (IL) DECAY 5µs/DIV Figure 4. Transient detection is done through the detect-transient (DTR) pin, which is DC-biased slightly above ½ INTVCC, and AC-coupled to ITH pin through the compensation capacitor CITH1. The equivalent compensation resistance RITH = RITH1 || RITH2 . NOVEL TRANSIENT DETECTION REDUCES LOAD-RELEASE VOUT OVERSHOOT As the output voltage becomes lower and the VIN -to-VOUT step-down ratio increases, a major challenge is to limit the overshoot in VOUT during a fast load current drop. An innovative feature of the LTC3838/LTC3839 is to detect “load-release” transients indirectly by monitoring the ITH negative slew rate. The detection is done through the detecttransient (DTR) pin that is coupled to ITH pin through the compensation capacitor. At steady state, the DTR pin remains slightly higher than the detection threshold (half of the voltage on INTVCC pin) with a voltage divider of the compensation resistors from INTVCC to SGND. In the event of a sudden drop of load current, the output voltage overshoots and ITH slews down quickly. If the DTR pin drops below half of INTVCC , the 22 | April 2012 : LT Journal of Analog Innovation LTC3838/LTC3839 temporarily turns off the bottom MOSFET, and the inductor current flows through the body diode of the bottom MOSFET. This increases the reverse voltage drop across the inductor, allowing the inductor current to drop to zero faster, lowering the VOUT overshoot by reducing overcharging of the output capacitor. Once the inductor current reaches zero, the bottom MOSFET turns back on to pull the inductor current to negative, discharging the output capacitor to recover regulation. Figure 5. Load-release detect transient (DTR) feature significantly reduces VOUT overshoot and time to recover regulation. (Shades are obtained with infinite persistence on oscilloscope triggered at load release steps.) VSW 3V/DIV VSW 3V/DIV VOUT 50mV/DIV AC-COUPLED VOUT 50mV/DIV AC-COUPLED ITH 1V/DIV ITH 1V/DIV IL 10A/DIV IL 10A/DIV 5µs/DIV FIGURE 1 CIRCUIT, CHANNEL 1 MODIFIED: • RFB2 = 0Ω, VRNG2 = SGND, CITH1 = 120pF, CITH2 = 0pF, • FROM DTR1 PIN: RITH1/2 = 46.4k TO SGND, 42.2k TO INTVCC VIN = 5V, LOAD RELEASE = 15A TO 5A, VOUT = 0.6V 5µs/DIV • CONNECTION FROM RITH1/2 AND CITH1 TO DTR1 PIN REMOVED • DTR1 PIN TIED TO INTVCC design features The LTC3838 and LTC3839 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). Download at cds.linear.com/docs/LT%20 Journal/LTJournal-V21N3-2011-10.pdf For More Information VIN 4.5V TO 14V + CIN2 22µF ×4 CIN1 180µF 2.2Ω 1µF VIN LTC3839 SENSE1– SENSE2– SENSE1+ SENSE2+ BOOST1 BOOST2 0.1µF 0.1µF 2.55k L1 0.33µH VOUT 1.2V 50A MT1 TG1 + COUT2 330µF ×2 4.7µF MB1 BG2 PGOOD 0.01µF The LTC3838 and LTC3839 are high performance, feature-rich, 2-phase, synchronous step-down DC/DC controllers that excel at meeting the performance demands of high current, low voltage loads, in either dual or single output applications. Their controlled on-time architecture retains the fast response and low on-time of traditional constant on-time controllers, and allows for constant frequency and external clock synchronization. Other unique features include novel remote output sensing, which VOUTSENSE– PGOOD PHASMD TRACK/SS 47pF CONCLUSION MB2 COUT3 + 330µF ×2 COUT4 100µF ×2 VOUTSENSE+ 10k 47.5k VOUT DRVCC2 EXTVCC PGND 10k Figure 6. The LTC3839 in a single 1.2V/50A output, 2-phase, 300kHz, DCR sense, step-down converter, with the detect transient load-release (DTR) feature enabled for VOUT overshoot reduction. The LTC3838 can also be used here. The LTC3838/LTC3839 is ideal for powering low voltage, high current, fast slew rate loads such as with a microprocessor. L2 0.33µH SW2 DRVCC1 INTVCC BG1 100k 2.55k MT2 DB2 SW1 1µF 0.1µF TG2 DB1 2.2Ω COUT1 100µF ×2 0.1µF 470pF 41.2k 137k ITH MODE/PLLIN DTR VRNG CIN1: SANYO 16SVP180MX CIN2: MURATA GRM32ER61C226KE20L COUT1, COUT4: MURATA GRM31CR60J107ME39L COUT2, COUT3: SANYO 2R5TPE330M9 DB1, DB2: CENTRAL SEMI CMDSH-4ETR L1, L2: VISHAY IHLP5050CEERR33M01 MT1, MT2: INFINEON BSC050NE2LS MB1, MB2: INFINEON BSC010NE2LS CLKOUT RT SGND RUN allows for a ±500mV remote ground, and load-release transient detection for overshoot reduction. In addition, LTC3838 and LTC3839 include popular features, such as: •external VCC power pin for loss reduction in the controller •continuously programmable range of current limits for flexibility with either RSENSE or inductor DCR sensing •selectable light load operating modes: discontinuous operation (similar to Burst Mode® operation) for higher efficiency, or forced continuous operation for constant frequency •overvoltage protection and current limit foldback •soft-start/rail tracking, PGOOD, and RUN pins for each output. The LTC3838 is offered in 38-pin QFN (5mm × 7mm) and TSSOP packages. The LTC3839 is offered in a 32-pin QFN (5mm × 5mm). All packages have exposed pads for enhanced thermal performance. n April 2012 : LT Journal of Analog Innovation | 23