advertisement High Input Voltage Monolithic Switcher Steps Up and Down Using a Single Inductor – Design Note 330A Jay Celani Introduction Ultrawide input voltage requirements are a common design problem for DC/DC converter applications, but when that range includes voltages both above and below the output voltage, the converter must perform both step-up and step-down functions. The LT®3433 is a high voltage monolithic DC/DC converter that incorporates two switch elements, allowing for a unique topology that accommodates both step-up and step-down conversion using a single inductor. The LT3433 uses a 200kHz constant frequency, current mode architecture and operates with input voltages from 4V to 60V. An internal 1% accurate voltage reference allows programming of precision output voltages up to 20V using an external resistor divider. Burst Mode® operation improves efficiencies during light-load conditions, reducing the device’s quiescent current to 100μA during no-load conditions. A soft-start feature reduces output overshoot and inrush currents during start-up, and both current limit foldback and frequency foldback are employed to control inductor current runaway during start-up and short-circuit conditions. The LT3433 is available in a 16-pin fused TSSOP exposed pad package which provides a small footprint and excellent thermal characteristics. When the converter input voltage is significantly higher than the output voltage, the LT3433 operates as a modified buck converter using a boosted-drive high side switch. If the converter input voltage becomes close enough to the output voltage to require a duty cycle greater than 75% in buck mode, the LT3433 automatically enables a second switch. This second switch pulls the output side of the switched inductor to ground during the “switch on” time, creating a bridged switching configuration. During bridged switching, the LT3433 merges the elements of buck and boost DC/DC converters as shown in Figure 1. In the simplest terms, a buck DC/DC converter switches the VIN side of the inductor, 02/04/330a_conv while a boost converter switches the VOUT side of the inductor. Combining the elements of both topologies achieves both step-up and step-down functionality using a single inductor, so voltage conversion can continue when VIN approaches or is less than VOUT. 4V-60V Input to 5V Output DC/DC Automotive Converter A 4V-60V to 5V DC/DC converter is shown in Figure 2. This converter is well suited for 12V automotive battery applications, maintaining output voltage regulation with battery line voltages from 4V cold crank through 60V load dump. The threshold for bridged mode operation is about 8V, so the converter will operate primarily in buck mode except during a cold crank condition. During buck operation, this converter can provide load currents up to 350mA with input voltages up to 60V. Operating with a nominal 13.8V input, this LT3433 converter accommodates loads of 400mA and produces efficiencies up to 82%. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN SW CIN L VOUT D COUT (1a) Step-Down (VIN > VOUT) L VIN CIN D SW VOUT COUT (1b) Step-Up (VIN < VOUT) VIN SW CIN L D D SW VOUT COUT DN330A F01 (1c) Step-Up/Step-Down (VIN > VOUT or VIN < VOUT) Figure 1. The LT3433 Merges the Elements of Step-Up and Step-Down DC/DC Converters VOUT 5V 4V ≤ VIN ≤ 8.5V: 125mA 8.5V ≤ VIN ≤ 60V: 350mA 100μH CoEv DU1352-101M B160A 1N4148 B120A VBST 0.1μF 10V VIN 4V TO 60V + SW_L SW_H B160A 1N4148 47μF 10V B120A VBST 0.47μF 50V VIN 8V TO 60V PWRGND 330pF 1nF 2.2μF 100V CERAMIC 1N4148 (BURST) BURST_EN VBIAS VC SHDN 0.1μF 10V 68k 330pF 1nF PWRGND VIN VOUT BURST_EN VBIAS VC SHDN 1N4148 (BURST) 0.1μF 10V 68k 20k 100k SS VFB 305k 47μF 25V LT3433 VOUT VIN + SW_L SW_H LT3433 2.2μF 100V CERAMIC VOUT 12V 8V ≤ VIN ≤ 18V: 125mA 18V ≤ VIN ≤ 60V: 380mA 220μH TDK SLF12565T-221M1R0 (NO BURST) SS VFB SGND 0.01μF MODE SWITCH VIN H-L: 7.9V VIN L-H: 8.3V 176k (NO BURST) SGND 0.01μF DN330A F02 MODE SWITCH VIN H-L: 16.6V VIN L-H: 17V DN330A F04 Figure 2. 4V-60V to 5V DC/DC Converter Figure 4. 8V-60V to 12V Converter 100 90 VIN = 13.8V NO BURST 80 VIN = 20V NO BURST 90 80 EFFICIENCY (%) EFFICIENCY (%) VIN = 13.8V BURST 70 60 VIN = 4V NO BURST 50 VIN = 4V BURST 40 70 60 VIN = 20V BURST VIN = 8V NO BURST 50 40 VIN = 8V BURST 30 30 20 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 20 0.1 10 100 1 OUTPUT CURRENT (mA) 1000 DN330A F05 DN330A F03 Figure 3. 4V-60V to 5V Conversion Efficiency Figure 5. 8V-60V to 12V Conversion Efficiency When the input voltage drops below 8V, the converter switches into bridged operation to maintain output voltage regulation. Because the LT3433 switch current limit is fixed, converter load capability is reduced while operating in bridged mode. With an input of 4V, the converter accommodates loads up to 125mA. Not only does this LT3433 converter operate across a large range of DC input voltages, but it also maintains tight output regulation during input transients. When subjected to a 1ms 13.8V to 4V input transition to simulate a cold crank condition, regulation is maintained to 1% with a 125mA load. without cold-crank requirements, as well as many other applications such as those powered by inexpensive wall adapters. This converter operates in buck mode with input voltages above 17V, accommodating loads up to 380mA. This converter accommodates loads up to 435mA and produces efficiencies above 89% at 20V input. 8V-60V Input to 12V Output DC/DC Converter As converter output voltages increase, switch current and duty-cycle limitations prevent operation with VIN at the extreme low end of the LT3433 operational range. The 12V output converter shown in Figure 4 can provide load current up to 125mA with an input voltage as low as 8V. This is suitable for 12V automotive applications Data Sheet Download www.linear.com Linear Technology Corporation Conclusion The LT3433 simplifies ultrawide input range DC/DC voltage conversion, enabling simple and inexpensive solutions to a variety of design problems. Automatic transitioning between buck and bridged modes of operation provides seamless output regulation for wide input voltage ranges and input voltage transients. The use of a small footprint TSSOP package, a single inductor and few external components reduce board space requirements, increase efficiency and improve thermal characteristics. For applications help, call (408) 432-1900 dn330af_conv LT/TP 0204 344K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004