Dual 2-Phase, No R Synchronous Controller for DDR/QDR Memory Termination

The LTC3776 is a 2-phase dual output synchronous stepdown switching regulator controller for DDR/QDR memory termination applications. The second controller regulates its output voltage to 1/2 VREF while providing symmetrical source and sink output current capability.

The No RSENSE constant frequency current mode architecture eliminates the need for sense resistors and improves efficiency. Power loss and noise due to the ESR of the input capacitance are minimized by operating the two controllers out of phase.

The switching frequency can be programmed up to 750kHz, allowing the use of small surface mount inductors and capacitors. For noise sensitive applications, the LTC3776 switching frequency can be externally synchronized from 250kHz to 850kHz, or can be enabled for spread spectrum operation. Forced continuous operation reduces noise and RF interference. Soft-start for VOUT1 is provided internally and can be extended using an external capacitor.

The LTC3776 is available in the tiny thermally enhanced (4mm × 4mm) QFN package or 24-lead SSOP narrow package.

Protected by U.S. Patents including 5481178, 5929620, 6144194, 6580258, 6304066, 6611131, 6498466, patent pending on Spread Spectrum.

Applications

* DDR, DDR II and QDR Memory

* SSTL, HSTL Termination Supplies

* Servers, RAID Systems

* Distributed DC Power Systems

* No Current Sense Resistors Required

* Out-of-Phase Controllers Reduce Required Input Capacitance

* VOUT2 Tracks 1/2 VREF

* Symmetrical Source/Sink Output Current Capability (VOUT2)

* Spread Spectrum Operation (When Enabled)

* Wide VIN Range: 2.75V to 9.8V

* Constant Frequency Current Mode Operation

* 0.6V ±1.5% Voltage Reference (VOUT1)

* Low Dropout Operation: 100% Duty Cycle

* True PLL for Frequency Locking or Adjustment

* Internal Soft-Start Circuitry

* Power Good Output Voltage Monitor

* Output Overvoltage Protection

* Micropower Shutdown: IQ = 9µA

* Tiny Low Profile (4mm × 4mm) QFN and Narrow SSOP Packages