Datasheet LTC1704, LTC1704B (Analog Devices) - 10

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