Dual Output High Efficiency Converter Produces 3.3V and 8.5V Outputs from a 9V to 60V Rail – Design Note 479 Victor Khasiev Introduction Among the many step-down DC/DC switching regulator controllers, the LTC3890 stands out because of its unique features. This 50μA quiescent current device can produce two output voltages ranging from 0.8V to 24V when powered from an input voltage of 4V to 60V. Many high input voltage step-down DC/DC converter designs use a transformer-based topology or external high side drivers to operate from up to 60VIN. Others use an intermediate bus converter requiring an additional power stage. However, the LTC3890 simplifies design, with its smaller solution size, reduced cost and shorter development time compared to other design alternatives. Feature Rich The LTC3890 is a high performance synchronous buck DC/DC controller with integrated N-channel MOSFET drivers. It uses a current mode architecture and operates from a phase-lockable fixed frequency from 50kHz to 900kHz. The device features up to 99% duty cycle capability for low voltage dropout applications, adjustable soft-start or voltage tracking and selectable continuous, pulse-skipping or Burst Mode ® operation with a no-load quiescent current of only 50μA. These features, combined with a minimum on-time of just 95ns, make this controller an L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 22μF 50V 1μF VIN INTVCC TG1 0.1μF TG2 BOOST1 4.7μH BOOST2 SW1 0.1μF 8μH SW2 BG1 BG2 LTC3890 PGND SENSE1+ SENSE2+ SENSE1– VFB1 ITH1 SENSE2– 0.01Ω 0.01Ω VOUT1 3.3V 5A + VIN 9V TO 60V 100k 470μF 1000pF 31.6k 34.8k VFB2 ITH2 TRACK/SS1 SGND TRACK/SS2 0.1μF 0.1μF 100k 470pF 10.5k + VOUT2 8.5V 3A 330μF 34.8k dnVK F01 Figure 1. High Efficiency Dual 8.5V/3.3V Output Step-Down Converter 06/10/DN479 ideal choice for high step-down ratio applications. Power loss and supply noise are minimized by operating the two output stages out-of-phase. together. For example, a 9V to 60V input can be converted to an 8.5V output at 6A. Figure 4 shows the efficiency of this configuration at input voltages of 10V, 30V and 60V. Dual Output Application Figure 1 shows the LTC3890 operating in an application that converts a 9V to 60V input into 3.5V/5A and 8.5V/3A outputs. The transient response for the 3.3V output with a 4A load step is less than 50mV (as shown in Figure 2). Current mode control provides good current balance between the phases. Less than 10% mismatch can be achieved, as shown in Figure 5. Figure 3 shows the efficiency of the 8.5V channel with a 36V input voltage. Single Output Application The LTC3890 can also be configured as a 2-phase single output converter by simply connecting the two channels Conclusion Although there are many choices in dual-output controllers, the LTC3890 brings a new level of performance with its high voltage operation, high efficiency conversion and ease of design. 100 98 10V EFFICIENCY (%) 96 50mV/DIV 2A/DIV 30V 94 92 60V 90 dnvk F02 1ms/DIV Figure 2. Transient Response of 3.3V Channel (IOUT1: 1A to 5A) 88 86 1 2 3 4 AMPS (A) 5 6 dnvk F04 Figure 4. Efficiency of the LTC3890 Configured as a 2-Phase Single Output of 8.5V at Up to 6A from a 10V to 60V Input 95 VIN = 36V 94 EFFICIENCY (%) 93 92 91 1A/DIV 90 89 0A 88 87 1μs/DIV 0 1 2 dnvk F05 3 AMPS (A) dnvk F03 Figure 3. Efficiency of the Converter in Figure 1 for the VOUT2A 8.5V Channel Data Sheet Download www.linear.com Figure 5. The Inductor Current in a 2-Phase Single Output Converter. Currents in Both Inductors Shown with a 24V Input and 8.5V at 6A Output. For applications help, call (408) 432-1900, Ext. 3161 Linear Technology Corporation dn479f LT/TP 0610 116K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2010 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●