LTC1553

The LTC1553 is a 5-bit programmable synchronous switching regulator controller optimized to power present and next generation Pentiumý Pro processors. The LTC1553 operates with efficiencies up to 95% from a 5V or 12V supply and incorporates a 5-bit DAC that digitally controls the output voltage from 1.8V to 3.5V. An internal feedback system provides an output accuracy of ý1.5% at room temperature and ý2% over temperature, load current and line voltage shifts.

The LTC1553 has high current MOSFET drivers capable of switching currents in N-channel MOSFETs in excess of 14A. The LTC1553 senses the output current across the on-resistance of the upper N-channel MOSFET, providing an adjustable current limit without using an external low value sense resistor. These high current N-Channel MOSFETs have lower on-resistance [RDS_ON] than P-Channel MOSFETs, enabling conversion efficiencies to approach 95%. The drivers on the LTC1553 are specially designed to operate with these large N-channel MOSFETs.

"The LTC1553 includes all the inputs and outputs required to implement a power supply or voltage-regulator module (VRM) compliant to Intel's latest specification," says Bob Dobkin, vice president of engineering.

VRM 8.1 DC/DC Converter specifications require the voltage-regulator module to accept a 5-bit voltage ID (VID) code transmitted by the processor. This code is detected by the LTC1553's 5-bit DAC and used to generate the correct output voltage. The supply voltage to the processor requires regulation to within ý5% between 2.1V and 3.5V, depending on variations in the processor's fabrication process and desired clock speed.

Additional features in the LTC1553 include a precision internal reference, over-voltage and over-temperature fault detection and indication. It also features a "power-good" signal to notify a processor when the output voltage is within regulation. Special feedback circuitry speeds response to transient loads, common in Pentium-based systems where current outputs surge from 300mA to 12A in 100%ns. A Soft Start feature allows controlled current ramp up at turn on.

The LTC1553 free-runs at 300kHz and can be synchronized to a faster external clock if desired. Typical designs require the VRM to reside next to the Pentium Pro on the motherboard to minimize impedances and to ensure fast transient response. The motherboard layout may not leave much room for the bulk capacitance typically required to buffer high current power rails. Higher frequency operation of the LTC1553 results in fast transient response and allows smaller inductors and capacitors to be used to save board space.

The whole philosophy of the Pentium power requirements changes almost on a daily basis and the companies involved in power supply solutions are to be commended in keeping up with what have become surrealistic demands; if you follow the future in a crystal ball we will be looking at supplying a score of amps to a processor at 1 V in not too many years. This part from LTC is state-of-the-art for today and covers the needs of the Pentium Pro and, thus, any embedded uses that will come.

The fully programmable 5-bit 1.8 to 3.5 V output voltage, with output currents exceed 14 A, have efficiencies up to 95% with flags for normal, over-temperature and over-voltage faults fit the needs for Pentium Pro, SPARC and ALPHA microprocessors.

The LTC1553 is available from stock in 20-lead SO and SSOP packages and prices start at $3.50 in 1000-piece lots.