LINEAR Dual Output Step-Down Controller Produces Accurate Ef cient Reliable High Current Rails Design Note 478 Mike Shriver Theo Phillips
Contents
1. DESIGN ECHNOLOGY NO T E S Dual Output Step Down Controller Produces Accurate Efficient and Reliable High Current Rails Mike Shriver and Theo Phillips Introduction The LTC 3855 makes it possible to generate high cur rent rails with the accuracy and efficiency to satisfy the most demanding requirements of today s leading edge network telecommunications and server applications This 2 phase dual output synchronous buck controller includes strong gate drivers that support operation with per phase currents above 20A The accurate 0 6V 0 75 reference and its integrated differential amplifier diff amp allow remote sensing of the output of critical rails This controller has an output voltage range from 0 6V to 12 5V when used without the diff amp and from 0 6V to 3 3V with the diff amp The LTC3855 uses the reliable peak current mode archi tecture to achieve afast and accurate current limitand real time current sharing Its current sense comparators are designed to sense the inductor current with either a sense resistor or with inductor DCR sensing DCR sensing offers 17 4k 17 4k RNTC1 Rntc2 100k 100k 49 9k NEXT 49 9k NEXT TOLI TOL2 L 30 1k LTC3855 Design Note 478 the advantage of reduced conducted power losses since the current is measured using the voltage drop across the already present inductor DC resistance eliminating the losses incurred by adding2. range using its internally compensated phase lock loop High efficiency at light load is achieved by selecting either Burst Mode operation or discontinuous mode operation as opposed to continuous conduction mode The LTC3855 can be used for inputs up to 38V and its 100ns typical minimum on time allows for high step down ratios The LTC3855 has a TK SS pin for programmable soft start or rail tracking and dedicated RUN and PGOOD pins for each channel The LTC3855 comes in either a 6mm x 6mm QFN or a thermally enhanced 38 lead TSSOP package Conclusion The LTC3855 is a high performance dual output buck converter intended for low output voltage high output current supplies It provides the user with the benefits of a precise 0 6V 0 75 reference an accurate current limit and high efficiency For applications help call 408 432 1900 Ext 3720 dn478f LT TP 0510 116K PRINTED IN THE USA LY MYR LINEAR TECHNOLOGY CORPORATION 2010
3. 0 C Another use for the ITEMP pins is to increase the current limit for conventional DCR sense and Reense applications PolyPhase Operation The LTC3855 provides inherently fast cycle by cycle cur rent sharing due to its peak current mode architecture Data Sheet Download www linearcom Linear Technology Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 www linear com lOUT MAX CURRENT LIMIT A DCR SENSING IMPLEMENTED WITH TEMPERATURE COMPENSATION WITHOUT TEMPERATURE COMPENSATION 0 20 40 60 80 100 120 INDUCTOR TEMPERATURE C DN478 F04 Figure 4 Measured Current Limit of the 1 2V Rail Over Temperature with and without Temperature Compensation plus very tight DC current sharing for single output PolyPhase applications Up to 12 phase operation can be achieved by daisy chaining the CLKOUT and MODE PLLIN pins and by programming the phase separation with the PHASMD pins A major advantage of PolyPhase operation is the reduction of the required input and out put capacitance due to ripple current cancellation Also single output PolyPhase applications have a faster load Step response due to a smaller clock delay Other Important Features The switching frequency of the LTC3855 can be pro grammed between 250kHz and 770kHz with a resistor placed from the FREQ pin to ground or synchronized to an external clock in this frequency
4. a sense resistor The trade off is that DCR sensing is less accurate than a dedicated sense resistor because the DCR varies from partto part and over temperature The LTC3855 uses an innovative scheme to improve the accuracy of DCR sensing by compensating for the DCR s variation with temperature 1 5V 20A and 1 2V 20A Buck Converter with Remote Sensing and NTC Compensated DCR Sensing Figure 1 shows a 1 5V 20A and 1 2V 20A dual phase converter with DCR sensing operating at 325kHz High efficiency is achieved with the strong gate drivers 47 LI LTC LTM Burst Mode PolyPhase Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation All other trademarks are the property of their respective owners 22uF 34 0k tL tgoue 14V Ts T ev Mi a RJKO305DPB Voutt 1 5V L1 0474H Couri Courtz 20A DCR 1 0MQ TYP 1 2MQ MAX 6 3V M2 5 x RJK0330DPB x2 L1 L2 VISHAY IHLP5050FD 01 0 47uH Cout1 Cout3 MURATA GRM31CR60J107ME39I Cout2 Cout4 SANYO 2R5TPE330M9 RNTC1 RNTC2 MURATA NCP18WF104J03RB DCR 1 0mQ TYP 1 2MQ MAX M3 L2 RJK0305DPB 0 474H ae 1 2V Cours Courg 20A 100uF 330UF M4 RJKO330DPB x2 4 42k x2 x2 34 0k DN478 F01 Figure 1 Dual 1 5V 20A and 1 2V 20A Converter Operating at fsy 325kHz The Entire Circuit Fits within 1 7in2 with Both Sides of the Board Populated 05 10 478 optimized dead time and DCR sensing The ty
5. pical full load efficiency for the 1 5V and 1 2V rails is 89 5 and 87 8 respectively See Figure 2 The 1 2V output is remotely sensed with the diff amp As a result the 1 2V rail s output accuracy is unaffected by the voltage drops across the Vqyt and GND planes The load step response for the 1 2V rail is shown in Figure 3 6 4 AA m POWER LOSS 4 La a 95 90 EFFICIENCY z oo M SSO1 H4MOd Ul foy 325kHz MODE CCM 5 10 15 20 25 LOAD CURRENT A Figure 2 Efficiency and Power Loss of the 1 5V 20A and 1 2V 20A Converter DN478 F02 one 100mV DIV LOAD STEP 10A DIV DN478 F03 50pus DIV Figure 3 50 to 100 Load Step Response for the 1 2V Rail at Viy 12V The LTC3855 features precise current limit thresholds of 30mV 50mV and 75mV selected via the jy pins The currentlimitthreshold can be raised by biasing the ITEMP pins below 500mV Since the ITEMP pins source 10u A of current the peak current sense voltage can be increased by inserting a resistance of less than 50k from the ITEMP pin to ground By placing an inexpensive NTC thermistor next to the inductor and connecting this thermistor to a linearization network from the ITEMP pin to ground the current limit temperature coefficient can be greatly reduced As Figure 4 illustrates the compensated current limit is 20 higher than the uncompensated current limit at 11
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