LINEAR Dual Output High Ef ciency Converter Produces 3.3V 8.5V Outputs from a 9V to 60V Rail Design Note 479 handbook
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1. DESIGN ECHNOLOGY NO T P S 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 regula tor controllers the LTC3890 stands out because of its unique features This 50A 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 atransformer based topology or external high side drivers to operate from up to 60Vjy 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 develop ment time compared to other design alternatives TG1 BOOST1 SW1 BG1 Feature Rich The LTC3890 is a high performance synchronous buck DC DC controller with integrated N channel MOSFET driv ers Ituses 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 skip ping or Burst Mode operation with a no load quiescent current of only 50uA These features combined with a minimum on time of just 95ns make this controller an AY LI LTC2. 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 LTC3890 SENSEI dnVKFO1 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 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 forthe 3 3V outputwitha 4A load step is less than 50mV as shown in Figure 2 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 50mV DIV 2A DIV dnvk F02 1ms DIV Figure 2 Transient Response of 3 3V Channel Igy71 1A to 5A gt A eT or EFFICIENCY HEE 0 2 3 AMPS A dnvk F03 Figure 3 Efficiency of the Converter in Figure 1 for the Vout2a 8 5V Channel Data Sheet Download www linearcom together For example a 9V to 60V input can be converted toan8 5V outputat6A Figure 4 shows the efficiency of this configuration at input voltages of 10V 30V and 60V Current mode control
3. provides good current balance between the phases Less than 10 mismatch can be achieved as shown in Figure 5 Conclusion Although there are many choices in dual output control lers the LTC3890 brings a new level of performance with its high voltage operation high efficiency conversion and ease of design EFFICIENCY JA ttt ty tt FETT TT IT AMPS A 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 1a KS OA dnvk F05 1us DIV 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 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 www linear com dn479f LT TP 0610 116K PRINTED IN THE USA LS MYR LINEAR TECHNOLOGY CORPORATION 2010
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