Datasheet LT1575, LT1577 (Analog Devices) - 10

LT1575/LT1577
U U W U APPLICATIONS INFORMATION Introduction
nificant cost as all additional bulk capacitance is removed. The additional savings of insertion cost, purchasing/in- The current generation of microprocessors place strin- ventory cost and board space are readily apparent. gent demands on the power supply that powers the processor core. These microprocessors cycle load cur- Precision-trimmed adjustable and fixed output voltage rent from near zero to amps in tens of nanoseconds. versions accommodate any required microprocessor Output voltage tolerances as low as ±100mV include power supply voltage. Proper selection of the N-channel transient response as part of the specification. Some MOSFET RDS(ON) allows user-settable dropout voltage microprocessors require only a single output voltage from performance. The only output capacitors required are the which the core and I/O circuitry operate. Other higher high frequency ceramic decoupling capacitors. This regu- performance processors require a separate power supply lator design provides ample bandwidth and responds to voltage for the processor core and the I/O circuitry. These transient load changes in a few hundred nanoseconds requirements mandate the need for very accurate, very versus regulators that respond in many microseconds. high speed regulator circuits. The ceramic capacitor network generally consists of 10 to 24 1uF capacitors for individual microprocessor require- Previously employed solutions included monolithic ments. The LT1575/LT1577 family also incorporates cur- 3-terminal linear regulators, PNP transistors driven by low rent limiting for no additional system cost, provides on/off cost control circuits and simple buck converter switching control and overvoltage protection or thermal shutdown regulators. The 3-terminal regulator achieves a high level with simple external components. of integration, the PNP driven regulator achieves very low dropout performance and the switching regulator achieves Therefore, the unique design of these new ICs combines high electrical efficiency. the benefits of low dropout voltage, high functional inte- gration, precision performance and ultrafast transient However, the common trait manifested by these solutions response, as well as providing significant cost savings on is that transient response is measured in many microsec- the output capacitance needed in fast load transient appli- onds. This fact translates to a regulator output decoupling cations. As lower input/output differential voltage applica- capacitor scheme that requires several hundred microfar- tions become increasingly prevalent, an LT1575-based ads of very low ESR bulk capacitance using multiple solution achieves comparable efficiency performance with capacitors surrounding the CPU. This required bulk ca- a switching regulator at an appreciable cost savings. pacitance is in addition to the ceramic decoupling capaci- tor network that handles the transient load response The new LT1575/LT1577 family of low dropout regulator during the first few hundred nanoseconds as well as controller ICs step to the next level of performance re- providing microprocessor clock frequency noise immu- quired by system designers for the latest generation nity. The combined cost of all capacitors is a significant motherboards and microprocessors. The simple versatil- percentage of the total power supply cost. ity and benefits derived from these circuits allow the power supply needs of today’s high performance micro- The LT1575/LT1577 family of single/dual controller ICs processors to be met with ease. are unique, easy to use devices that drive external N-channel MOSFETs as source followers and permit a user
Block Diagram Operation
to realize an extremely low dropout, ultrafast transient response regulator. These circuits achieve superior regu- The primary block diagram elements consist of a simple lator bandwidth and transient load performance by com- feedback control loop and the secondary block diagram pletely eliminating expensive tantalum or bulk electrolytic elements consist of multiple protection functions. Exam- capacitors in the most modern and demanding micropro- ining the block diagram for the LT1575, a start-up circuit cessor applications. For example, a 200MHz Pentium provides controlled start-up for the IC, including the processor can operate with only the recommended 24 1µF precision-trimmed bandgap reference, and establishes all ceramic capacitors. Users benefit directly by saving sig- internal current and voltage biasing. 10