High Voltage Current Mode Step-Down Converter with Low Power Standby Capability – Design Note 383 Jay Celani Introduction Low power standby requirements are typically associated with battery-powered systems. Automotive systems, for example, commonly require power supplies to maintain output voltage regulation even under no-load conditions—while drawing minimal quiescent current to preserve battery life. Rising energy costs, however, have extended the need for low current standby operation to line-powered systems, such as small plugged-in appliances for home and business. converter requires few external components, maintains high conversion efficiencies over a wide load range and supports low power standby operation for compliance with system power-management requirements. High Efficiency at Standby The LT3800 is a 4VIN to 60VIN, 200kHz fixed-frequency controller that uses synchronous operation and N-channel MOSFETs to maximize high current efficiency. Current mode operation with continuous high-side inductor current sensing yields fast transient response and excellent line regulation. Low current standby requirements are met using Burst Mode® operation. A reverse inductor current inhibit feature also increases efficiency during light-load conditions. The LT3800 runs directly from the converter input supply, so there are no local supplies required to power the IC. The IC is also designed for easy use of output-derived power which further increases conversion efficiency. Cycle-by-cycle current limiting maintains the Designing a power supply that is very efficient at light loads is particularly difficult in systems where high input voltages and substantial load currents are required. A common approach in such high power systems is to add a secondary power path for low current operation—a potentially significant increase in the cost, board space and complexity of the power supply. A better solution is to use the LT® 3800 as the core of a single-supply synchronous DC/DC converter. The resulting power supply is simple and efficient. An LT3800-based VIN 20V TO 55V + 120mF 63V 2.2mF 100V ´2 , LTC, LT and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 0.22mF 16V 0.1mF 100V VIN BOOST 1M NC TG SHDN SW Si7850DP 82.5k CSS 130k 1% 1500pF 120pF VC 82.5k 680pF LT3800 BURST_EN VFB 14.7k 1% 15mH BAS19 200k VCC BG 1N4148 1mF 16V 470pF Si7370DP PGND SENSE+ SENSE– SGND 0.015W 22mF 16V ´2 + Figure 1. LT3800 12V, 75W Buck DC/DC Converter with High Efficiency at Light Loads 02/06/383 VOUT 12V 270mF 75W 16V DN383 F01 programmed current limit, even during instantaneous short circuit fault conditions. 12V/75W Synchronous Buck DC/DC Converter Figure 1 shows a 12V, 75W DC/DC converter that can operate with input voltages from 20V to 55V. The 20V minimum input is set by a programmable UVLO function implemented using the precision hysteretic threshold of the LT3800 SHDN pin. The 55V upper bound is limited by switch FET margin. This converter provides full-load efficiencies above 95%, as shown in Figure 2, and can maintain a no-load output voltage with only 0.1mA of input supply quiescent current. The light load efficiency enhancement features of the LT3800 produce a supply that maintains high conversion efficiency across a 4-decade range. This DC/DC converter also features excellent heavy-load efficiency, producing peak conversion efficiencies as high as 96%. 8 100 95 7 VIN = 20V 6 90 85 5 VIN = 48V 4 80 3 75 VIN = 48V 70 2 65 1 POWER LOSS (W) EFFICIENCY (%) This DC/DC converter incorporates a controlled dV/dt soft-start function which servos the converter output voltage to a programmed rising rate during start-up—in this case 1.3V/ms, yielding a start-up rise time of just under 10ms. The LT3800 automatically resets the softstart function if the output drops out of regulation, so recovery from a short circuit or brownout event is graceful and controlled. VOUT 100mV/DIV IOUT 2A/DIV VIN = 20V 60 1 100 1000 10 OUTPUT CURRENT (mA) 0 10000 DN383 F02 Figure 2. The DC/DC Converter in Figure 1 Exhibits High Efficiency Across a Wide Load Range and Produces Peak Efficiencies Above 95% VIN = 48V 100µs/DIV DN383 F04 Figure 4. 1A to 5A Load Step Generates <2% VOUT Transient VOUT 10V/DIV IOUT 10A/DIV VIN = 48V 2.5ms/DIV DN383 F03 Figure 3. Current Limit is Maintained During Instantaneous Short-Circuit and Auto-Reset Soft-Start Yields a Graceful Recovery from the Fault Event Figure 5. LT3800 12V, 75W DC/DC Converter Layout Data Sheet Download For applications help, call (408) 432-1900, Ext. 2593 www.linear.com Linear Technology Corporation dn383f LT/TP 0206 305K • 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 ●