DC1708A - Schematic12/15/2011
Demonstration circuit DC1708A is a dual-output regulator focused specifically for DDR memory power applications. It is built based on the LTC3634, a high voltage dualchannel, controlled on-time monolithic synchronous buck regulator. The DC1708A has an input voltage range of 3.6V to 15V. The output voltage of the first channel, VDDQ, of the DC1708A has three fixed voltage settings: 1.5V, 1.8V and 2.5V, and is capable of delivering up to 3A of output current. The second channel, VTT, is set to regulate to half the voltage on the VDDQIN pin, which can be either the channel 1 output or an external reference voltage. It can source or sink a maximum of 3A of output current. The LTC3634 also provides a 10mA buffered output of half VDDQIN – VTTR, which is used to provide the reference voltage needed for DDR applications. With the use of a timing resistor, the DC1708A can have its operating frequency programmed from 500kHz to 4MHz. Additionally the DC1708A can be easily synchronized to an external clock, due to an internal phase-locked loop, within this same frequency range. The DC1708A VDDQ output can operate in either Burst Mode® operation or forced continuous mode. In Burst Mode operation, which is the preferred mode of low load current operation, the DC supply current is typically only 1.3mA (both channels) at no load (sleep mode), and less than 15µA in shutdown. In Burst Mode operation or continuous mode operation, the DC1708A is a very efficient circuit at high load currents: over 80% for either channel. The LTC3634 is also capable of 90° or 180° out-of-phase operation, and can allow its output to track an external voltage, either coincidentally or ratiometrically. The LTC3634 comes in a 28-pin QFN package, featuring an exposed pad on the bottom-side of the IC for better thermal performance. All of these features make the DC1708A an ideal circuit for powering DDR memory applications.