advertisement 80µA Quiescent Current Synchronous Step-Down Controller Extends Battery Life in Automotive Applications – Design Note 1014 Julian Zhu Introduction Battery powered applications that have a significant amount of time in standby mode, require electrical circuits to operate with a low quiescent current to preserve battery life. The LTC3835 synchronous step-down controller is an excellent solution with its ultralow quiescent current (80µA). Other features make it uniquely qualified to satisfy the needs of automotive applications. A wide 4V to 36V input voltage range protects the supply against high input voltage transients and is compatible with low voltage cold crank conditions. The constant frequency current-mode architecture with high-side inductor current sensing provides excellent line and load regulation and reliable current limiting and sharing. The 0.8V to 10V output voltage range provides flexibility for a variety of applications. Solution size is small due to the 650kHz switching frequency, which reduces external component sizes, and the integrated synchronous FET driver, which keeps component count low. Furthermore, the LTC3835 provides high efficiency over a wide load range up to 20A. Other important features include: CLKOUT • The output voltage can be ramped during start-up with an adjustable soft-start or it can track an external ramp signal. • The controller can be synchronized to an external clock via a phase-locked loop. • The CLKOUT pin and current mode control allow interleaved operation of multiple parallel controllers for delivery of high output current with accurate current sharing. The LTC3835 has overvoltage and overcurrent (shortcircuit) protection, a power good monitor and an EXTVCC option for improved efficiency when VOUT > 4.7V. All these are packed in a small 20-pin exposed pad TSSOP or 4mm × 5mm QFN package. The LTC3835-1 fits in a smaller 16-pin SSOP package or 5mm × 3mm DFN package, by eliminating CLKOUT, EXTVCC and the power good indicator. , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN PLLLPF TRACK/SS ITH LTC3835 1nF 215k BOOST SW 3.3µH VOUT 3.3V 5A 0.012Ω CMDSH-4 68.1k SGND INTVCC PLLIN/MODE EXTVCC VFB VIN 4V TO 36V 0.22µF PGOOD 1200pF 150pF 10k Si4840DY TG RUN 0.1µF 10µF 50V CERAMIC BG + 4.7µF COUT 150µF 10V Si4840DY SENSE– SENSE+ PGND 39pF L: SUMIDA, CDEP105-3R2MC-88 COUT: SANYO, 10TPD150M DN1014 F01 Figure 1. High Efficiency Synchronous Step-Down Converter Consumes only 80µA in Standby Mode 12/06/1014 High Efficiency at Standby The LTC3835 uses synchronous rectification and N-channel MOSFETs to achieve high efficiency over a wide load range. At light load it minimizes the IC supply current with Burst Mode® operation, where the controller switches in fixed duty cycle bursts, as needed, to reduce the IC supply current. When the load current is extremely low, the controller enters sleep mode by turning off both external MOSFETs and most of its internal circuitry, further reduc100000 100 EFFICIENCY VIN = 12V; VOUT = 3.3V 90 80 10000 EFFICIENCY (%) 1000 60 50 100 40 POWER LOSS 30 20 10 POWER LOSS (mW) 70 1 10 0 0.001 0.01 0.1 0.1 1 10 100 1000 10000 LOAD CURRENT (mA) DN1014 F02 Figure 2. Efficiency and Power Loss vs Load Current for the Circuit in Figure 1. 100 VIN = 4V ing quiescent current. To maximize efficiency during Burst Mode operation the bottom MOSFET is turned off when the instantaneous inductor current is close to zero. The IC is also designed with an EXTVCC pin to further increase conversion efficiency. 3.3V/5A Synchronous DC/DC Converter Figure 1 shows an automotive application that converts a 4V to 36V input to a 3.3V/5A output. As Figure 2 shows, the converter achieves high efficiency, up to 92%, at 12V input and maintains light-load efficiency, above 65%, down to 1mA load current. The no-load IC supply quiescent current is kept below 80µA. For low dropout operation the efficiency can reach 96.5% at 4V input as illustrated in Figure 3. The controller uses OPTI-LOOP compensation to optimize the transient response and thus minimize the output capacitance. Figure 4 shows the transient response for the circuit in Figure 1. Conclusion The LTC3835 controller is especially well suited to automotive and other battery-powered applications that demand low quiescent current and light-load efficiency—important for preserving battery life in applications that spend significant time in standby. The LTC3835 also offers a wide input and output voltage range and many other advanced features in either leaded or leadless thermally efficient packages. 95 EFFICIENCY (%) VIN = 12V 90 85 VOUT 100mV/ DIV AC COUPLED 80 75 70 0.1 1 LOAD CURRENT (A) 10 IL 2A/DIV DN1014 F04 DN1014 F03 Figure 3. Efficiency vs Load Current at VIN = 4V and VIN = 12V for the Circuit in Figure 1. 20µs/DIV Figure 4. Load Step Transient with 12V Input for the Circuit in Figure 1. Data Sheet Download For applications help, call (408) 432-1900, Ext. 3598 www.linear.com Linear Technology Corporation dn1014f LT 1206 • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●