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LTC1704B Datasheet(PDF) 10 Page - Linear Technology |
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LTC1704B Datasheet(HTML) 10 Page - Linear Technology |
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10 / 28 page  LTC1704/LTC1704B 10 1704bfa OVERVIEW The LTC1704 includes a step-down (buck), voltage mode feedback switching regulator controller and a linear regu- lator controller. The switching regulator controller em- ploys a synchronous switching architecture with two external N-channel MOSFETs. The chip operates from a low voltage input supply (6V maximum) and provides high power, high efficiency, precisely regulated output voltage. The switcher output regulation is extremely tight, with initial accuracy and DC line and load regulation and better than 1.5%. Total regulation, including transient response, is inside of 3.5% with a properly designed circuit. The 550kHz switching frequency allows the use of physically small, low value external components without compro- mising performance. The LTC1704’s internal feedback amplifier is a 20MHz gain bandwidth op amp, allowing the use of complex multipole/ zero compensation networks. This allows the feedback loop to maintain acceptable phase margin at higher fre- quencies than traditional switching regulator controllers, improving stability and maximizing transient response. The 800mV internal reference allows regulated output voltages as low as 800mV without external level shifting amplifiers. The LTC1704’s synchronous switching logic transitions automatically into Burst Mode operation, maxi- mizing efficiency with light loads. The linear regulator controller drives an external NPN pass transistor to provide a programmable output voltage up to 2A of current. An external pull-up resistor programs the current limit threshold for the linear regulator. Under short-circuit condition, the foldback current limit circuitry prevents excessive pass transistor heating. The switcher and the linear regulator can be individually disabled. When both controllers are disabled, the LTC1704 enters shut- down mode and the supply current reduces to 75 µA. An onboard power good (PGOOD) flag goes high when both outputs are regulating. Small Footprint The LTC1704 switcher supply operates at a 550kHz switch- ing frequency, allowing it to use low value inductors without generating excessive ripple currents. Because the inductor stores less energy per cycle, the physical size of the inductor can be reduced without risking core satura- tion, saving PCB board space. The high operating fre- quency also means less energy is stored in the output capacitors between cycles, minimizing their required value and size. The remaining components, including the LTC1704, are tiny, allowing an entire power convertor to be constructed in 1.5in2 of PCB space. Fast Transient Response The LTC1704 switcher supply uses a fast 20MHz GBW op amp as an error amplifier. This allows the compensation network to be designed with several poles and zeros in a more flexible configuration than with a typical gm feedback amplifier. The high bandwidth of the amplifier, coupled with the high switching frequency and the low values of the external inductor and output capacitor, allow very high loop crossover frequencies. The low inductor value is the other half of the equation—with a typical value on the order of 1 µH, the inductor allows very fast di/dt slew rates. The result is superior transient response compared with conventional solutions. High Efficiency The LTC1704 switcher supply uses a synchronous step- down (buck) architecture, with two external N-channel MOSFETs. A floating topside driver and a simple external charge pump provide full gate drive to the upper MOSFET. The voltage mode feedback loop and MOSFET VDS current limit sensing remove the need for an external current sense resistor, eliminating an external component and a source of power loss in the high current path. Properly designed circuits using low gate charge MOSFETs are capable of efficiencies exceeding 90% over a wide range of output voltages. Linear Regulator Controller The LTC1704 linear regulator controller drives an exter- nal NPN pass transistor in emitter-follower configuration to provide an externally adjustable output voltage. The con- troller senses the output voltage via the REGFB pin, drives the base of the NPN through the REGDR pin to regulate the REGFB pin to 0.8V. REGDR is capable of sourcing more than 30mA of base current to the external NPN. APPLICATIO S I FOR ATIO |
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Similar Description - LTC1704B |
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