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LT3070EUFD

5A, Low Noise, Programmable Output, 85mV Dropout Linear Regulator

Linear Technology 

LT3070

APPLICATIONS INFORMATION

Introduction

Current generation FPGA and ASIC processors place

stringent demands on the power supplies that power the

core, I/O and transceiver channels. These microprocessors

may cycle load current from near zero to amps in tens of

nanoseconds. Output voltage specifications, especially in

the 1V range, require tight tolerances including transient

response as part of the requirement. Some ASIC processors

require only a single output voltage from which the core

and I/O circuitry operate. Some high performance FPGA

processors require separate power supply voltages for the

processor core, the I/O, and the transceivers. Often, these

supply voltages must be low noise and high bandwidth

to achieve the lowest bit-error rates. These requirements

mandate the need for very accurate, low noise, high cur-

rent, very high speed regulator circuits that operate at low

input and output voltages.

The LT3070 is a low voltage, UltraFast transient response

linear regulator. The device supplies up to 5A of output

current with a typical dropout voltage of 85mV. A 0.01μF

reference bypass capacitor decreases output voltage noise

to 25μVRMS (BW = 10Hz to 100kHz). The LT3070’s high

bandwidth provides UltraFast transient response using low

ESR ceramic output capacitors (15μF minimum), saving

bulk capacitance, PCB area and cost.

The LT3070’s features permit state-of-the-art linear regula-

tor performance. The LT3070 is ideal for high performance

FPGAs, microprocessors, sensitive communication sup-

plies, and high current logic applications that also operate

over low input and output voltages.

Output voltage for the LT3070 is digitally selectable in

50mV increments over a 0.8V to 1.8V range. A margining

function allows the user to tolerance system output voltage

in increments of ±1%, ±3% or ±5%.

The IC incorporates a unique tracking function, which if

enabled by the user, controls an upsteam regulator power-

ing the LT3070’s input (see Figure 8). This tracking function

drives the buck regulator to maintain the LT3070’s input

voltage to VOUT + 300mV. This input-to-output voltage

control allows the user to change the regulator output

voltage, and have the switching regulator powering the

LT3070’s input to track to the optimum input voltage with

no component changes.

This combines the efficiency of a switching regulator

with superior linear regulator response. It also permits

thermal management of the system even with a maximum

5A output load.

LT3070 internal protection includes input undervoltage

lockout (UVLO), reverse-current protection, precision cur-

rent limiting with power foldback and thermal shutdown.

The LT3070 regulator is available in a thermally enhanced

28-lead, 4mm × 5mm QFN package.

The LT3070’s architecture drives an internal N-channel

power MOSFET as a source follower. This configuration

permits a user to realize an extremely low dropout, Ultra-

Fast transient response regulator with excellent high fre-

quency PSRR performance. The LT3070 achieves superior

regulator bandwidth and transient load performance by

eliminating expensive bulk tantalum or electrolytic capaci-

tors in the most modern and demanding microprocessor

applications. Users realize significant cost savings as all

additional bulk capacitance is removed. The additional

savings of insertion cost, purchasing/inventory cost and

board space are readily apparent. Precision incremental

output voltage control accommodates legacy and future

microprocessor power supply voltages.

Output capacitor networks simplify to direct parallel com-

binations of ceramic capacitors. Often, the high frequency

ceramic decoupling capacitors required by these various

FPGA and ASIC processors are sufficient to stabilize the

system (see Stability and Output Capacitance section). This

regulator design provides ample bandwidth and responds

to transient load changes in a few hundred nanoseconds

versus regulators that respond in many microseconds.

The LT3070 also incorporates precision current limiting,

enable/disable control of output voltage and integrated

overvoltage and thermal shutdown protection. The

LT3070’s unique design combines the benefits of low

dropout voltage, high functional integration, precision

performance and UltraFast transient response, as well as

providing significant cost savings on the output capacitance

needed in fast load transient applications.

As lower voltage applications become increasingly preva-

lent with higher frequency switching power supplies, the

LT3070 offers superior regulation and an appreciable

3070p

11

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