LINEAR LTC3866 Manual
Contents
1. M2 BSCO10NE2LS L1 M1 14H DCR 1 3mQ CONCLUSION The LTc3866 delivers an outsized set of features for its small 4mm x 4mm 24 pin QEN package The unique ultralow DCR current sensing with current mode control makes the LTc3866 a good fit for low voltage high current applications with high efficiency and high reliability Tracking strong on chip drivers multichip operation and external sync capability fill out its menu of features The LTc3866 is ideal for computer and telecom sys tems industrial and medical instruments and Dc power distribution systems m CK BOTTOM OF BOARD 1 45 Tae HT 4658 E 2500m Bi SOONSSMFNLT1 April 2012 LT Journal of Analog Innovation 272. phase high efficiency 1 5v 80A power supply based on a 2x parallel LTc3866 power block scheme Although the pcr of the inductor in the power block is only o 53m0 the current sharing performance is excellent in both pc and transient conditions as shown in Figure 8 In applications where higher value DCR inductor Or Regngz 1s used the LTC3866 can be used like any typical current mode controller by disabling the sNsD pin shorting it to ground An R gysg resis tor or a RC filter can be used to sense the LTspice IV circuits linear com 548 Figure 9 High efficiency power supply 12V input to 5V 25A output output inductor signal and connect to the SNSA pin If the rc filter is used its time constant R C is set equal to L DcR of the output inductor In these applica tions the current limit Vsensemax 1S five times larger for the specified ILIM and the operating voltage range of SNSA and SNS is OV to 5 25V Without using the internal differential amplifier the output voltage of 5v can be generated as shown in Figure 9 The thermal test shows that the hot spot the inductor temperature is about 57 3 c at full load without any airflow as shown in Figure 10 where the ambient temperature is 25 C Figure 10 Thermal test of the circuit in Figure 9 TOP OF BOARD lg ae 13V INPUT 5V 25A OUTPUT NO AIRFLOW 2 2Q VIN 10uF 1804F 12V nE E D1 CMDSH 3 M1 BSCO24NE2LS
3. 38v input range remote output voltage sensing with accurate 0 5 reference programmable and temperature compensated current limit when using inductor DCR sens ing short circuit soft recovery without overshoot and chip thermal shutdown The Lrc3866 facilitates the design of high efficiency high power density and high reliability solutions for telecom sys tems industrial and medical instruments and Dc power distribution systems The controller is available in a low thermal 24 April 2012 LT Journal of Analog Innovation Figure 1 LTC3866 current sensing scheme with ultralow inductor DCR V High current paths are shown with i thicker lines pm INDUCTOR l L DCRI VouT r a 7b I I I I I I i 3 e l l l l i l PLACE C1 C2 NEXT TO IC PLACE R1 R2 NEXT TO INDUCTOR R1C1 5 R2C2 Figure 2 High efficiency 1 5V 30A step down converter with very low 100k y IN DCR sensing 4 5V TO 20V 220uF VouT 1 5V A 220pF 20k 10k 1 5nF design features Burst Mode NCAT j m EFFICIENCY HWHH M E TAA TT s LLA Wia isw Sot 0 0 01 0 1 1 10 100 lLoap A Figure 3 Efficiency of the circuit in Figure 2 impedance 24 lead 4mm x 4mm QFN and 24 lead exposed pad FE packages FEATURES The LTC3866 uses a constant frequency peak current mode control archi tecture guaranteeing cycle by cycle peak current limit and current sharing between diff
4. ST O Current Mode Switching Supply with Ultralow Inductor DCR sensing for High Efficiency and High Reliability Jian Li Haoran Wu and Gina Le Current mode switching supplies have several advantages over voltage mode switching supplies 1 high reliability with fast cycle by cycle current sensing and protection 2 simple and reliable loop compensation stable with all ceramic output capacitors 3 easy and accurate current sharing in high current PolyPhase supplies In high current applications power losses in the current sensing component are a concern so the resistance of the sense component must be as low as possible The problem is that low resistance sensing elements produce reduced signal to noise ratios such that switching jitter becomes an issue in high current high density applications The L1c3866 solves this problem by mak ing it possible to build reliable current mode switching supplies with lt o 5m cur rent sensing resistance This single phase synchronous buck controller drives all N channel power MOSFET switches with onboard gate drivers It employs a unique architecture that enhances the signal to noise ratio of the current sense signal allowing the use of a very low DC resis tance DCR power inductor or low value current sensing resistor to maximize efficiency in high current applications This feature reduces the switching jitter commonly found in low DCR applications The controller has a wide 4 5v
5. erent power supplies The LTC3866 employs a unique architecture that enhances the signal to noise ratio of the current sense signal allowing the use of a very low DC resistance DCR power inductor or low value current sensing resistor to maximize efficiency in high current applications TOP OF BOARD 12V INPUT 1 5V 30A OUTPUT NO AIRFLOW Figure 4 Thermal test of the circuit in Figure 2 It is especially well suited to low volt age high current supplies because of a unique architecture that enhances the signal to noise ratio of the current sense circuit This allows it to operate with the small sense signals produced by very low DCR 1m or less inductors which improve power efficiency in high current supplies The improved sNR minimizes Figure 5 Switching node jitter comparison at 12V input 1 5V 25A output STANDARD DCR SENSING 160ns lt lt gt Vsw 10V DIV 100ns DIV Vin 12V Vout 1 9V LOAD 20A LTC3866 ENHANCED DCR SENSING Vsw 10V DIV Vin 12V Vout 1 9V ILOAD 20A BOTTOM OF BOARD I LTC3868 4839 t f 2501 Kt jitter due to switching noise which could corrupt the signal The LTc3866 can sense a DCR value as low as o 2mQ with care ful pcs layout though in this extreme situation the additional PCB and sol der resistance should be considered As shown in Figure 1 the LTC3866 com prises two positive sense pins SNSD and SNSA to acquire signals and
6. hows a high efficiency 1 5V 30A step down converter with very low DCR sensing An induc tor with DCR 0 32mQ is used in this design to maximize efficiency The efficiency in different operation modes is shown in Figure 3 The full load effi ciency is as high as 90 3 at 12V input voltage It is about 1 4 improvement over the supply using a 1mQ sense resis tor with the same power stage design The hot spot bottom MOSFET tem perature rise is only 39 6 c without any VIN 10pF VOUT 1 5V 80A 330uF 330uF GND wg 7 VIN 10uF VOUT 100pF 330uF 330UF 26 April 2012 LT Journal of Analog Innovation design features l1 amp ILo TOA DIV W t i OUT 500mV DIV s ILOAD 50A DIV 10us DIV Figure 8 Current sharing performance of the 1 5V 80A supply in Figure 7 airflow as shown in Figure 4 where the ambient temperature is about 23 8 c The unique design improves the effi ciency as well as the noise sensitivity The worst case switching node jitter is reduced by 60 as shown in Figure 5 with a very low o 32m0 inductor DCR Another unique feature of LTC3866 is short circuit soft recovery The internal soft recovery circuit guarantees that there is no overshoot when the power supply recovers from a short circuit condition as shown in Figure 6 The Lrc3866 can be used with a power block for a more compact design and very high current Figure 7 shows a dual
7. processes Figure 6 Short circuit test 10A DIV 500us DIV April 2012 LT Journal of Analog Innovation 25 Ooo O them internally to provide a 14dB 5x sig nal to noise ratio improvement in response to low voltage sense signals The current limit threshold is still a function of the inductor peak current and its DCR value and can be accurately set from 10mV to 30mV in a 5mV steps with the ILIM pin The part to part current limit error is only about 1mvV over the full temperature range The filter time constant R1 C1 of the SNSD path should equal the L DCR of the output inductor while the filter at SNSA path should have a bandwidth five times larger than sNsSD namely R2 C2 R1 C1 5 An additional optional temperature compensation circuit guarantees the accurate current limit over a wide temperature range especially important in DCR sensing The LTc3866 also features a precise o 6v reference with a guaranteed limit of 0 5 that provides an accurate output voltage from o 6v to 3 5V Its differential remote Voyr sensing ampli fier makes the LTC3866 ideal for low voltage high current applications Figure 7 A high efficiency 1 5V 80A power supply based on parallel LTC3866s and power blocks 0 1p1F 1500pF 5 36k 220pF 100k INTVec4 470uF ACBEL POWER BLOCK VRA001 401G 470uF ACBEL POWER BLOCK VRA001 401G APPLICATIONS Figure 2 s
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LINEAR LTC3866 Manual