Analog Devices LTC3877

Description

The current drawn by FPGAs, ASICs and processorsfor high performance servers, network and computingsystems continues to rise, and load currents of 100Aor greater are becoming common. Meanwhile, thechip's operating voltages are dropping to 0.9V andbelow with tighter voltage regulation requirements.For many of these applications, the core voltage mayneed to be adjusted for optimum performance witha VID (voltage identification) interface. A significantchallenge is clearly placed on the power supplydesigner to meet the demands of high efficiency andtight output voltage regulation with a small amountof board space.One way to meet these demands is to use the LTC3877and LTC3874 chipset. The LTC3877 is a peak currentmode, VID-controlled dual output synchronousstep-down controller. The output of phase 1 can beprogrammed from 0.6V to 1.23V in 10mV incrementswith a six-bit parallel VID interface. Phase 2 providesan output of 0.6V to 5V, which is set by an externaldivider. The two phases can be paralleled togetheror with phases from another LTC3877 or an LTC3874for higher output current.The LTC3874 is a peak current mode phase extenderchip. It does not have an error amplifier and insteadregulates its phase current to the ITH signal fromthe LTC3877 master. The elegant design of theLTC3874 reduces trace count and board space.The LTC3877 comes in a 44-lead, 7mm × 7mm QFNpackage; the LTC3874 slave controller comes in a28-lead, 4mm × 5mm QFN package.

Features

The LTC3877 and LTC3874 both have a phase-lockablefrequency range of 250kHz to 1MHz and a FREQ pinto set the internal frequency if synchronization isnot required. The LTC3877 offers three light loadoperating modes: Burst Mode® operation, forcedcontinuous mode and pulse-skipping mode. TheLTC3874 operates in either forced continuous modeor pulse-skipping mode.The minimum on-time of the LTC3877 is 40nsectypical, ideal for high step-down converters or smallfootprint or high bandwidth converters operatingat switching frequencies of 500kHz to 1MHz. Theminimum on-time of the LTC3874 is 90nsec, typical.Phase 1 of the LTC3877 provides VID control. Ifthe FPGA, ASIC or processor is not awake or VIDprogramming is not required, then the VID section canbe disabled by pulling the VIDEN pin low and settingthe output voltage with a divider at the output of thedifferential amplifier. Both phases of the LTC3877have differential remote sense amplifiers for precisecontrol of the output voltage. The input voltage rangeof both chips is 4.5V to 38V.Other features of the LTC3877 include PGOOD pins,RUN pins and TK/SS pins for each rail. The LTC3874has its own RUN pins and FAULT pins for quickresponse to fault conditions.

Typical applications

• FPGAs and Processor Power
• Servers and Computing

More information

• Datasheet
• IC product information