powerZONE Products for the week of December 9, 2002
Linear Technology Says . . .
LTC3730: 3-Phase 60A DC/DC Controller with Onboard
Drivers for Pentium 4 Processors
Linear Technology Corporation introduced the LTC3730, a 3-phase synchronous step-down switching controller with integrated MOSFET drivers dedicated to power Intel's Pentium 4 IMVP-III mobile processors. Housed in the industries smallest (5.3mm wide) 36-lead SSOP, the LTC3730 is a single-IC solution capable of delivering up to 60A to the microprocessor core. Competing solutions require a total of four power ICs, which include one controller and three MOSFET drivers. In addition to a simpler solution, the LTC3730's precision ±5% output current matching simplifies thermal management of the power supply by spreading the heat evenly among its three output stages. Competing solutions which incorporate MOSFET or inductor sensing introduce as much as 35% error in power dissipation between the output stages resulting in an unbalanced power handling and overheating of one or two stages. To cope with this excess heat and to prevent thermal stress, designers are forced to over-size the MOSFETs and inductors, which add to the cost and size of the solution. Moreover, the LTC3730 incorporates a patented power-saving technique called Stage Shedding to optimize efficiency and improve battery life at light loads by eliminating the gate charging losses and switching losses of two of its output stages. Applications vary from high-end notebook PCs to embedded systems and single-board computers for industrial, instrumentation, telecom and medical systems.
The LTC3730 drives external N-channel MOSFETs in three stages with 120
degree phase separation. The 3-phase operation reduces input ripple current
and improves transient response as well as minimizing the requirement for
input and output capacitors. The IC operates from 250kHz to 600kHz per phase
to achieve optimal frequency for efficiency, ease of thermal design and
use of small inductors and capacitors. The LTC3730's output voltage is adjustable
from 0.6V to 1.75V with a 5-bit VID interface. To protect the load and MOSFETs,
the LTC3730 is equipped with short-circuit and overload protection as well
as soft-start to protect the system from inrush current.
analogZONE Says . . .
The LTC3730 is not the only controller designed for the Pentium 4, and it's not the cheapest - but I think it is the best solution overall that I have seen so far with the lowest number and cost of the external components needed.
Six or seven years ago, when the talk about lower processor voltages started to heat up, it became obvious that the digital side of the industry fully expected that the analog semiconductor vendors would come through with solutions to save themselves from themselves. And that is just what has happened: Our industry has come up with elegant, affordable supply solutions for power delivery down to 0.600 V with up to 60 A of available current.
The LTC3730 uses three phases of current delivery, so reducing the ripple on the input rail and dramatically reducing the sizes of the capacitors needed at the input and output. Operation can be a nominal 225 kHz to 680 kHz per phase with the frequency set by the voltage applied to the PLL filter between 0 and 2.4 V. The part also does not require any external drivers for the external MOSFETs as they are on-chip and the 5% output current matching reduces any load imbalance between output stages - which with some other solutions has created the need to oversize components, just in case.
There is nothing "extra-clever" about LTC's solution; it is a straightforward version of its family of polyphase controllers with current sense feedback on each channel. In a current "shedding" mode at 10% of the of the maximum-designated load current two of the three stages are shut down optimizing efficiency by removing the gate charging and switching losses of the other two stages. At very light loads this shedding mode is not as efficient as burst mode operation but it does provide lower noise. The on-chip reference is accurate to 1% and the input voltage can range from 4 V to 36 V. The quiescent of the controller itself is a typical 2.3 mA and 20 µA in shutdown. Power good output is provided as is current foldback for short-circuit/overload conditions, and there is adjustable soft-start current ramping.
As specified in IMVP III the output voltage is adjusted by 5-bit programming with voltage steps of 0.025 V from 0.600 V to 1.000 V and then 0.050 V steps from 1.000 V to 1.750 V.
This is an elegant solution with simplified external needs compared to others and with tighter control of the separate phases. The premium in price can be easily justified by a BOM comparison.
The LTC3730 is in production in a 5.3-mm high SSOP-36 and is priced at $4.35 in 1000-piece lots.