L DESIGN IDEAS 12A Monolithic Synchronous Buck Regulator Accepts Inputs up to 24V by Stephanie Dai and Theo Phillips Introduction Flexible Control The LTC3610 is a high power monolithic synchronous buck regulator capable of providing up to 12A from inputs as high as 24V in a complete solution that takes little space (Figure 1). It integrates the step-down controller and power MOSFETs into a single, compact 9mm × 9mm QFN package. Its high step-down ratio, wide input and output voltage range and high current capability present a single IC solution for many applications previously requiring separate FETs and controller ICs. Its very low profile (0.9mm max) allows mounting on the back of a circuit board, freeing up valuable front-side board space. High step-down ratios (Figure 2) are possible because of the LTC3610’s constant on-time operation and valley current control architecture, which allow a minimum on-time of less than 100ns. Output voltages approaching VIN are also possible (Figure 5). In either case, efficiency is very high—up to 97% (Figures 4 and 6). Synchronous operation affords high efficiency at low duty cycles, whereas a non-synchronous converter would conduct current through the forward drop of a Schottky diode most of the time. Transient response (Figure 3) is fast because the LTC3610 reacts immediately to a load increase. It does Figure 1. Who says a lot of space is needed for a complete high power density stepdown regulator? The LTC3610 is capable of providing up to 12A from inputs as high as 28V. Its low 0.9mm profile allows it to be mounted on the back of the board too. INTVCC CVCC 4.7µF 6.3V GND CF 0.1µF 25V SW RF1 1Ω VIN VIN VIN 5V TO 24V CIN 10µF 35V 3× C6 10µF 35V + (OPTIONAL) GND 12 13 14 15 16 SGND SVIN SGND SVIN INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND PGND PGND SGND SW ION LTC3610 SW SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN RX1 0Ω 47 46 R1 9.5k 1% 45 44 43 41 40 (OPTIONAL) CON 0.01µF 39 38 DB CMDSH-3 SW C2 VOUT R5 31.84k VIN CC1 470pF 36 35 RPG1 100k 34 33 RVON PGOOD 0Ω RSS1 510k CB1 0.22µF (OPTIONAL) 37 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 INTVCC R2 30.1k 1% RON 182k 1% 42 SW CIN: TAIYO YUDEN GMK325BJ106MM-B COUT: SANYO 10TPE220ML L1: CDEP85NP-R80MC-50 C5: MURATA GRM31CR60J226KE19 EXTVCC C4 0.01µF 48 SGND 11 SW SGND 10 VFB RUN/SS 9 SW BOOST 8 EXTVCC SGND 7 (OPTIONAL) GND SGND SW NC 6 SW SW 5 PVIN L1 0.8µH PVIN + PVIN COUT1 220µF 2× SGND PVIN C5 22µF 6.3V PGND PVIN 4 SGND PVIN VOUT 2.5V AT 12A SGND PGND PVIN 3 PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 CSS 0.1µF CC2 100pF INTVCC VOUT VIN (OPTIONAL) RUN/SS Figure 2. This converter runs at 550kHz and delivers 2.5V at 12A from an extremely wide 5V–24V input. 36 Linear Technology Magazine • June 2007 DESIGN IDEAS L 100 VOUT 100mV/DIV EFFICIENCY (%) 90 IL 5A/DIV ILOAD 5A/DIV VIN = 12V 80 70 60 40µs/DIV LOAD STEP 0A TO 8A VIN = 12V VOUT = 2.5V FCB = 0V FIGURE 6 CIRCUIT (the ITH pin) rises, initiating another cycle. As the load current rises, so does the average inductor current. Eventually, the interval between constant on-time pulses ends before the inductor current can reach zero, at which point the inductor continuously conducts current. This point is determined by duty cycle, inductance value, and the interval between constant on-time pulses. By using single on-time pulses of fixed width, this mode provides well-controlled output ripple at any supported load. This process also prevents reverse inductor current, which minimizes power loss at light loads. The on-time is set by the current into the ION pin and the voltage at the VON pin according to a simple equation VIN = 5V VOUT=2.5V EXT VCC=5V 50 0.01 0.1 1 10 LOAD CURRENT (A) 100 Figure 3. The LTC3610 responds quickly to an 8A transient (circuit of Figure 2). Figure 4. Efficiency vs load current for the circuit of Figure 2 not wait for the beginning of the next clock cycle to respond, so there is no clock latency. The LTC3610 can be programmed for two kinds of light-load operation: forced continuous mode or discontinuous mode. Forced continuous operation offers the lowest possible noise and output ripple. The top MOSFET turns on for the programmed on-time and the bottom MOSFET turns on for the (remaining) off-time. Inductor current is allowed to reverse, even at no load. In discontinuous mode, the top MOSFET turns on for a preset ontime. Then (after a brief non-overlap period) the bottom MOSFET turns on until the current comparator senses reverse inductor current. When the error amplifier senses a small decrease at the feedback node VFB, its output CVCC 4.7µF 6.3V INTVCC CF 0.1µF 25V SW GND TON = VVON IION • 10pF Tying a resistor RON from VIN to the ION pin yields an on-time inversely proportional to VIN. RF1 1Ω VIN 11 VIN VIN 24V CIN 10µF 25V 3× C6 10µF 35V + (OPTIONAL) GND 12 13 14 15 16 SGND SVIN SGND SVIN INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND PGND PGND SGND SW ION LTC3610 SW SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN SGND RX1 0Ω CIN: TAIYO YUDEN TMK432BJ106MM COUT: SANYO 16SVP180MX L1: SUMIDA CDEP1055R7 48 EXTVCC C4 0.01µF 47 46 R1 1.58k 1% 45 44 43 41 40 (OPTIONAL) R2 30.1k 1% C2 VOUT RON 3.4M 1% 42 (OPTIONAL) CON 0.01µF 39 38 VIN CC1 560pF R5 20k 37 36 35 34 33 SGND 10 SW SGND 9 VFB RUN/SS 8 SW BOOST (OPTIONAL) GND EXTVCC SGND 7 SGND SW NC 6 SW SW 5 PVIN L1 5.7µH PVIN + PVIN COUT 180µF 16V SGND PVIN C5 22µF 25V PGND PVIN 4 SGND PVIN VOUT 12V AT 5A SGND PGND PVIN 3 PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 CC2 100pF RPG1 100k INTVCC PGOOD (OPTIONAL) RVON 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 RSS1 510k CB1 0.22µF INTVCC DB CMDSH-3 CSS 0.1µF CVON VOUT (OPTIONAL) VIN (OPTIONAL) RUN/SS Figure 5. Although the LTC3610 is optimized for high step-down ratios, it can also regulate output voltages beyond the range of many DC/DC buck converters. For example, this schematic shows a 500kHz regulator delivering 12V at up to 5A, with high efficiency and low output ripple. Linear Technology Magazine • June 2007 37 L DESIGN IDEAS 100 Adjustable current limit is also builtin. The inductor current of LTC3610 is determined by measuring the voltage across the sense resistance between the PGND and SW pins, where RDS(ON) of the bottom MOSFET is about 6.5mΩ. The current limit is set by applying a voltage to the VRNG pin, which sets the relative maximum voltage across the sense resistance. An external resistive divider from the internal bias, INTVCC, can be used to set the voltage of the VRNG pin between 0.5V and 1V resulting in a typical current limit of 16A to 19A. Tying VRNG to SGND defaults the current limit to 19A. The LTC3610 also has soft-start and latch off functions enabled by the Run/SS pin. Pulling the Run/SS below 0.8V puts the LTC3610 into a low quiescent current shut down state, whereas releasing the pin allows a 1.2µA current source to charge up the external soft-start capacitor. When the voltage on Run/SS reaches 1.5V, the LTC3610 begins operating with an initial clamp on ITH of approximately 0.9V. This prevents current overshoot during start up. As the soft-start capacitor charges, the ITH clamp increases, allowing normal operation at full load current. If the output voltage falls below 75% of the LTC4067, continued from page 34 Conclusion OUT voltage rises above the BAT voltage, the charge cycle restarts where it left off. At any time, the user may monitor both instantaneous charge current and instantaneous USB current by observing the PROG pin and CLPROG pin voltages respectively. LTC2355/56, continued from page 21 power, and small package makes the LTC2356-14 ideal for high speed, portable applications including data acquisition, communications, and medical instrumentation. The LTC2356-14 achieves 72.3dB SINAD and –82dB SFDR with a 1.4MHz input frequency. While measuring ±1.25V bipolar inputs differentially, the LTC2356-14’s 80dB common mode rejection ratio allows users to eliminate ground loops and common mode noise. When the ADC is not converting, power dissipation can be reduced to 4mW in nap mode, with the internal 2.5V reference remaining active, and 13µW with all analog circuitry powered down in sleep mode. 38 VIN = 24V EFFICIENCY (%) 80 60 40 20 VOUT=12V 1 10 100 1000 LOAD CURRENT (mA) 10000 Figure 6. Efficiency vs load current for the circuit of Figure 4 regulated voltage, then a short-circuit fault is assumed. At this point, a 1.8µA current discharges capacitor CSS. If the fault condition persists until Run/SS drops to 3.5V, the controller’s overcurrent latch off turns off the MOSFETS until Run/SS is grounded and released. If latch off is not desired, a pull-up current source at Run/SS defeats this feature. Conclusion Few synchronous monolithic DC/DC converters are versatile enough to use in low power portable devices such as notebook and palmtop computers, as well as high power industrial distributed power systems. The LTC3610’s broad input and output ranges, efficiency greater than 90% and high current capability make it a superior alternative to many solutions requiring separate power switches. L The LTC4067 satisfies the needs of voltage sensitive battery operated devices, replacing as many as three separate devices. With accuracy better than ±0.4% on the battery float voltage, the LTC4067 is ideally suited for demanding highprecision applications. The LTC4067 offers both a power management strategy that complies with USB port specifications as well as providing an advanced battery charger. The LTC4067 also offers overvoltage protection up to 13V, to protect itself as well as system devices in the event that an incorrect wall adapter is attached. L For applications requiring a unipolar measurement, the LTC2355-14 measures 0V to 2.5V input signals, but is otherwise identical to the LTC235614. For lower resolution applications, the LTC2356-12 and LTC2355-12 are pin- and software-compatible 12bit versions of the LTC2356-14 and LTC2355-14. The LTC2355-14/LTC2356-14/ LTC2355-12/LTC2356-12 ADCs are pin- and software-compatible with the LTC1403 2.8Msps ADC family, allowing users to easily upgrade their design for a 25% faster sample rate. Table 2 details these fast single-channel unipolar and bipolar ADCs. Summary With PCB real estate getting tighter and designers always searching for lower power ICs, fast data acquisition can be a challenge. Linear Technology’s families of simultaneous sampling ADCs and fast single-channel ADCs make it possible to optimize solution size, power and cost. The pin- and software-compatible families of 6-channel, 2-channel and single-channel ADCs offer flexibility to upgrade from 12bit resolution to 14-bit resolution. Whatever your motor control, power monitoring, or data acquisition system requires, Linear Technology has a fast SAR ADC to do the job. L Linear Technology Magazine • June 2007