Datasheet LT3150 (Analog Devices) - 9

LT3150
U U W U APPLICATIO S I FOR ATIO INTRODUCTION
The LT3150 controller IC is a unique, easy-to-use device that drives an external N-channel MOSFET as a source With each new generation of computing systems, total follower and realizes an extremely low dropout, ultrafast power increases while system voltages fall. CPU core, transient response regulator. The circuit achieves supe- logic and termination supplies below 1.8V are now com- rior regulator bandwidth and transient load performance mon. Power supplies must not only regulate low output by eliminating expensive special polymer, tantalum or voltages, but must also operate from low input voltages. bulk electrolytic capacitors in the most demanding appli- A low voltage, very low dropout linear regulator is an cations. Performance is optimized around the latest gen- attractive conversion option for applications with output eration of low cost, low ESR, readily available ceramic current in the range of several amperes. Component count capacitors. Users benefit directly by saving significant and cost are low in comparison with switching regulator cost as all bulk capacitance is removed. Additional savings solutions and with low input-to-output differential volt- include insertion cost, purchasing/inventory cost and ages, efficiencies are comparable. board space. In addition to low input-to-output voltage conversion, The precision-trimmed adjustable voltage LT3150 ac- these systems require stringent output voltage regulation. commodates most power supply voltages. Proper selec- The output voltage specification includes input voltage tion of the N-channel MOSFET R change, output load current change, temperature change DS(ON) allows user-settable dropout voltage performance. Transient load step perfor- and output load current transient response. Total toler- mance is optimized for ceramic output capacitor networks ances as low as ±2% are now required. For a 1.5V output allowing the regulator to respond to transient load changes voltage, this amounts to a mere ±30mV. Transient load in a few hundred nanoseconds. The output capacitor current response is the most critical component as output network typically consists of multiple 1µF to 10µF ceramic current can cycle from zero to amps in tens of nanosec- capacitors in parallel depending on the power supply onds. These requirements mandate the need for a very requirements. The LT3150 also incorporates current lim- accurate, very high speed regulator. iting, on/off control for power supply sequencing and Historically employed solutions include monolithic overvoltage protection or thermal shutdown with simple 3-terminal linear regulators, PNP transistors driven by external components. low cost control circuits and simple buck converter The LT3150 combines the benefits of low input voltage switching regulators. The 3-terminal regulator provides operation, very low dropout voltage performance, preci- high integration, the PNP driven regulator provides low sion regulation and fast transient response. With low dropout performance and the switching regulator pro- input/output differential voltage applications becoming vides high electrical efficiency. the norm, an LT3150-based solution is a practical alterna- However, these solutions manifest a common trait of tive to switching regulators providing comparable effi- transient response measured in many microseconds. This ciency performance at an appreciable cost savings. fact translates to a regulator output decoupling capacitor scheme requiring several hundred microfarads of very low
BLOCK DIAGRAM OPERATION
ESR bulk capacitance using multiple capacitors in parallel. This required bulk capacitance is in addition to the ceramic Gate drive for the external N-channel MOSFET in the linear decoupling capacitor network that handles the transient regulator loop is provided by a current mode, internally load response during the first few hundred nanoseconds compensated, fixed frequency step-up switching regula- as well as providing high frequency noise immunity. The tor. Referring to the Block Diagram, Q1 and Q2 form a combined cost of all capacitors is a significant percentage bandgap reference core whose loop is closed around the of the total power supply cost. output of the regulator. The voltage drop across R5 and R6 3150f 9