Dual Controller Provides 2µs Step Response and 92% Efficiency for 1.5V Rails Design Note 505 Mike Shriver Introduction The LTC®3838 is a dual output, dual phase buck controller that employs a controlled constant on-time, valley current mode architecture to provide fast load step response, high switching frequency and low duty cycle capability. The switching frequency range is 200kHz to 2MHz—its phase-locked loop keeps the frequency constant during steady-state operation and can be synchronized to an external clock. The LTC3838 accepts a wide input range, 4.5V to 38V, and can produce 0.6V to 5.5V outputs. VIN 4.5V TO 14V + CIN1 180μF CIN2 22μF w4 The remotely sensed VOUT1 has a voltage regulation accuracy of 0.67%, from 0°C to 85°C, even with a voltage difference of ±0.5V between local ground and remote ground. The current sense comparators are designed to sense the inductor current with either a sense resistor for high accuracy or with the inductor DCR directly for reduced power losses and circuit size. L, LT, LTC, LTM, Linear Technology, the Linear logo and PolyPhase are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 2.2Ω LTC3838 1μF VIN SENSE1– SENSE2– SENSE1+ SENSE2+ BOOST1 BOOST2 0.22μF 7.5k 0.22μF 0.1μF 0.1μF 2.21k L1 0.4μH VOUT1 1.2V 25A MT1 + COUT2 330μF w3 TG1 TG2 SW1 SW2 2.21k MT2 DB1 DB2 2.2Ω COUT1 100μF w2 7.5k DRVCC1 INTVCC 1μF DRVCC2 EXTVCC 4.7μF BG1 MB1 BG2 MB2 L2 0.4μH VOUT2 1.5V 25A COUT3 + 330μF w3 COUT4 100μF w2 PGND 10k VOUTSENSE1+ 15k VFB2 10k 100k PGOOD1 0.01μF PGOOD1 PGOOD2 470pF 137k ITH1 DTR1 VRNG1 RT SGND RUN1 ITH2 100k PGOOD2 0.01μF TRACK/SS1 TRACK/SS2 47pF 18.2k 10k VOUTSENSE1– 220pF DTR2 VRNG2 PHASMD MODE/PLLIN CLKOUT RUN2 CIN1: SANYO 16SVP180MX CIN2: MURATA GRM32ER61C226K COUT2, COUT3: SANYO 2R5TPE330M9 COUT1, COUT4: MURATA GRM31CR60J107ME39L DB1, DB2: CENTRAL CMDSH-3 L1, L2: VISHAY IHLP5050FDER0R4M01 MT1, MT2: INFINEON BSC050NE2LS MB1, MB2: INFINEON BSC010NE2LS 47pF 40.2k DN505 F01 300kHz Figure 1. Dual Output, 1.5V/25A and 1.2V/25A Buck Converter Operating at FSW = 300kHz 08/12/505 1.5V/25A and 1.2V/25A Buck Converter Figure 1 shows a dual 25A output buck converter synchronized to an external 300kHz clock. The controlled constant on-time valley current mode architecture allows the switch node pulses to temporarily compress when a 5A to 25A load step is applied to the 1.2V rail, resulting in a voltage undershoot of only 58mV (see Figure 2). The full load efficiency for the 1.5V and 1.2V rails is 91.8% and 90.8%, respectively, as shown in Figure 3. The high efficiency is realized by the strong gate drivers, optimized dead time and DCR sensing. VO(AC) 50mV/DIV 58mV VSW 10V/DIV IL 20A/DIV 25A LOAD STEP 20A/DIV 5A 5μs/DIV DN505 F02 Figure 2. 20% to 100% Step Load Response of the 1.2V Rail at VIN = 12V, FSW = 300kHz, Mode = FCM Detect Transient Feature Further Speeds Up Transient Response An innovative feature of the LTC3838 is the load release transient detect feature. The DTR pin indirectly monitors the output voltage by looking at the AC-coupled ITH signal. If the inferred overshoot exceeds a user set value, the bottom FET turns off. This allows the inductor current to slew down at a faster rate, which in turn reduces the overshoot. Per Figure 4, a 32% reduction in the overshoot is realized on the 1.2V rail. Greater improvements occur at lower output voltages. Conclusion The LTC3838 is a dual output buck controller ideal for applications that require a fast load step response, high switching frequency, high efficiency and accurate output voltages. Other features include selectable operating modes: forced continuous mode (FCM) for fixed frequency operation or discontinuous mode (DCM) for higher efficiency at light load, programmable current limit thresholds, soft-start, rail tracking and individual PGOOD and RUN pins. The LTC3838 comes in a 5mm × 7mm QFN package or a thermally enhanced 38-lead TSSOP package. LTC3838 INTVCC 6 95 1.5V 90 470pF 3 80 1.2V 2 75 POWER LOSS (W) 4 1.5V POWER LOSS 70 65 0 5 34.8k DTR1 47pF EFFICIENCY 85 EFFICIENCY ITH1 5 1.2V 10 15 20 LOAD CURRENT (A) VIN = 12V 1 FSW = 300kHz MODE = CCM 0 25 30 DN505 F03 Figure 3. Efficiency and Power Loss of the 1.5V/25A and 1.2V/25A Converter The two channels operate 180° out-of-phase, which permits the use of fewer input capacitors due to input capacitor ripple current cancellation. For higher current applications, two or more phases can be tied together to form a single output, PolyPhase® converter. The benefits include a faster load step response, reduced input and output capacitance and reduced thermal dissipation. Data Sheet Download www.linear.com DN505 F04a 38.3k Figure 4a. Implementation of the Detect Transient Feature on the 1.2V Rail 90mV VO(AC) 100mV/DIV WITH DTR IL 50A/DIV VO(AC) 100mV/DIV 119mV ORIGINAL CIRCUIT IL 50A/DIV 5μs/DIV DN505 F04 Figure 4b. 100% to 20% Step Load Response of the 1.2V Rail with and without the Detect Transient Feature, VIN = 12V, FSW = 300kHz, Mode = FCM For applications help, call (408) 432-1900, Ext. 3720 Linear Technology Corporation dn505f LT/AP 0812 196K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2012 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●