芯片LTC3759英文概述

Many of today’s electronic devices require an inverting or noninverting converter or sometimes both. They also need to operate from a variety of power sources includ-ing USB, wall adapters, alkaline and lithium batteries. To produce various polarity outputs from variable input voltages, power supply designers often use a variety of regulator ICs, which makes for a long inventory list. Wide Input Voltage Range with Internal LDO

The LT3759’s wide input range simplifies the design of power supplies that must be compatible with a wide array of power sources. There is no need to add an external regulator or a slow-charge hysteretic start scheme because the LT3759 includes two internal low dropout (LDO) voltage regulators powered from VIN and DRIVE respectively, allowing simple start-up and biasing. The LT3759’s internal INTVCC current limit function protects The LT®3759 operates over an input voltage range from the IC from excessive on-chip power dissipation.1.6V to 42V and controls either positive or negative outputs using the same feedback pin, thus shortening Sensing Output Voltage Made Easier

the inventory list and simplifying design. It also packs LT3759 features a novel FBX pin architecture that simpli-many popular features such as soft-start, adjustable fies the design of inverting and noninverting converters. frequency and synchronization into a small footprint. It contains two internal error amplifiers—one senses The LT3759 comes in a 5mm × 4mm 12-pin MSE pack-positive outputs and the other negative—allowing the age and can be used in multiple configurations such as FBX pin to be connected directly to a divider from either boost, SEPIC, flyback and Cuk topologies.a positive output or a negative output, eliminating any confusion associated with positive or negative output 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 sensing and simplifying the board layout. Simply de-respective owners.cide the output polarity and topology and the LT3759 does the rest.VIN2.5V TO 15VCIN22µF×4348kVINEN_UVLO887kSYNCRT27.4k300kHz0.1µFSSVC1.4k22nFDRIVEPGOODVSWGATESENSE0.02ΩFBXGNDINTVCC15.8k1%1.8mΩ0.5WM1IL2L2102k1%COUT147µF×4100kL1IL1CDC47µF ×416V1009080

D1EFFICIENCY (%)70605040302010000.51.51LOAD CURRENT (A)

22.5VIN = 12VLT3759VOUT

12V, 2.5A4.7µF10VX5RM1: BSC027N04LSL1, L2: XAL-1010-682-EMBD1: PMEG4030

Figure 2. Efficiency for the Converter in Figure 1Figure 1. SEPIC Converter Produces 12V Output from 2.5V to 15V Inputs03/12/500

Adjustable/Synchronizable Switching FrequencyIt is often necessary to operate a converter at a par-ticular frequency, especially if the converter is used in an RF communications product that is sensitive to spectral noise in certain frequency bands. Also, if the area available for a converter is limited, operating at higher frequencies allows the use of smaller compo-nent sizes, reducing the real estate required and the output ripple. If power loss is a concern, switching at a lower frequency reduces switching losses, improving efficiency. The switching frequency can be set from 100kHz to 1MHz via a single resistor from the RT pin to ground. The device can also be synchronized to an external clock via the SYNC pin. A 2.5V to 15V to 12V SEPIC Converter

Figure 1 shows a 2.5V to 15V input, 12V/2.5A output SEPIC power supply using the LT3759. The typical ef-ficiency for this converter is shown in Figure 2. Figure 3 shows the switch waveform during an output short-circuit event. Notice how the switching frequency folds back to one-third of the regular frequency as soon as the output voltage is shorted to ground. This feature enhances short-circuit performance of both Cuk and SEPIC converters.A 1.8V to 4.5V to 5V/2A Boost Converter

Figure 4 shows a 5V, 2A output converter that takes an input of 4.5V to as low as 1.8V. The LT3759 is config-ured as a boost converter for this application, where Precision UVLO and Soft-Startthe converter output voltage is higher than the input Input supply undervoltage (UVLO) for sequencing or voltage. The 500kHz operating frequency allows the start-up overcurrent protection is easily achieved by use of small inductor and output capacitors. driving the UVLO with a resistor divider from the VIN Conclusion

supply. The divider output produces 1.22V at the UVLO pin when VIN is at the desired UVLO rising threshold The LT3759 is a versatile IC that integrates a rich set of voltage. The UVLO pin has an adjustable input hysteresis, unique features in a tiny 5mm × 4mm 12-pin MSE pack-which allows the IC to ignore a settable input supply age. It accepts wide input voltage range from 1.6V to droop before disabling the converter. During a UVLO 42V, with low shutdown current and frequency foldback event, the IC is disabled and VIN quiescent current at light load. The LT3759 is ideal for wide input voltage applications from single-cell, lithium-ion powered sys-drops to 1µA or lower. tems to automotive, industrial and telecommunications The SS pin provides access to the soft-start feature, power supplies. The high level of integration yields a which reduces the peak input current and prevents simple, low parts-count solution for boost, SEPIC and output voltage overshoot during start-up or recovery Inverting converters.from a fault condition. The SS pin reduces the inrush current by lowering the switch peak current. In this way soft-start allows the output capacitor to charge gradually toward its final value.VIN1.8V TO 4.5VCIN47µF6.3VX5RR359kR4124kIL1 + IL210A/DIVVSW20V/DIVVOUT10V/DIV

20µs/DIV

VINEN_UVLODRIVEPGOODR510kL12.2µHD1VOUT5V2AR234k1%SYNCRTRT16.9k500kHzSSVCCC2100pFCSS0.1µFLT3759GATESENSEM1FBXGNDINTVCCRC7.5kCC122nFCVCC4.7µF10VX5RRS5mΩ0.5WR115.8k1%Figure 3. Short-Circuit Event for the Converter in Figure 1COUT2100µF6.3VX5R×2M1: Si7858ADPL1: WÜRTH ELEKTRONIK 7443552200D1: SBR3U20SA

Figure 4. Boost Converter Produces 5V/2A Output from 1.8V to 4.5V Inputwww.linear.comFor applications help, call (408) 432-1900, Ext. 3798

dn500f LT/AP 0312 196K • PRINTED IN THE USA

Linear Technology Corporation(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com1630 McCarthy Blvd., Milpitas, CA 95035-7417  LINEAR TECHNOLOGY CORPORATION 2012