Design Note 258: High Efficiency I/O Power Generation for Mobile Pentium III Microprocessors

The demand for higher performance notebook computers has fueled the development of faster and more power-hogging microprocessors. These microprocessors also require a faster input/output (I/O) bus and a faster clock. From a power management perspective, this means that core, I/O and clock power supplies should be able to handle more power. This requires that core and I/O DC/DC converters operate more efficiently and be as small as possible. Linear Technology recommends the LTC1778 to provide I/O power for the next generation Pentium III microprocessors. The I/O input voltage requirement is 1.25V; transient (AC) tolerance is ±9% and static (DC) tolerance is ±5%. Load current requirements are as follows:

Processor VTT: 2.7A
830M chipset VTT: 0.7A

The 830M chipset core has the following two possibilities:

1. Internal graphics using the 830M engine: 3.6A
2. External AGP graphics: 1.6A
Total maximum I/O current: 7.0A

The LTC1778 is a synchronous step-down switching regulator controller that provides synchronous drive for two external N-channel MOSFET switches. The true current mode control architecture has an adjustable current limit, can be easily compensated, is stable with ceramic output capacitors and does not require a power-wasting sense resistor. An optional discontinuous mode of operation increases efficiency at light loads. The LTC1778 operates over a wide range of input voltages from 4V to 36V and provides output voltages from 0.8 to 0.9 • VIN. Switching frequencies up to nearly 2MHz can be chosen, allowing wide latitude in trading off efficiency for component size. Fault protection features include a power-good output, current limit foldback, optional short-circuit shutdown timer and an overvoltage soft latch. The LTC1778 is available in a 16-Lead narrow SSOP package.

Figure 1 shows a typical LTC1778 application schematic for a mobile Pentium III I/O supply. This circuit is optimized for small size and high efficiency from an input supply that varies from 5V to 24V. In order to reduce board space, it uses a single, dual N-channel FDS6982S MOSFET and only one 180μF (Panasonic SP) output capacitor. The superfast internal gate drivers with a typical rise time of 20ns help to minimize switching losses and the strong gate drivers help minimize conductive losses.

Figure 1. LTC1778 Mobile Pentium III I/O Supply

Figure 2 shows the transient response for a 0A to 5.5A load step. It can be seen that the LTC1778 can easily meet the I/O transient and static specification with only one output capacitor. The LTC1778 allows the use of many different kinds of output capacitors such as aluminum electrolytic, tantalum, POSCAP, NEOCAP, SPs and ceramic, because of OPTI-LOOP® compensation that allows the feedback loop to be compensated externally.

Figure 2. Output Voltage Transient Response for IL = 0A to 5.5A with One SP Output Capacitor

Figure 3 shows the same output voltage transient with two 150μF POSCAP output capacitors. Note that the equivalent series resistance (ESR) of a POSCAP is 40mΩ, which is approximately twice that of an SP capacitor. Therefore, two POSCAPs are required to achieve the same output voltage transient response as one SP capacitor.

Figure 3. Output Voltage Transient Response for IL = 0A to 5.5A with Two POSCAP Output Capacitors

Figure 4 shows typical efficiency curves for VIN = 5V and VIN = 15V, VOUT = 1.25V, ILOAD = 10mA to 7A. It can be seen that the efficiency is better than 85% for load currents up to 5A. The measured MOSFET case temperature is only 70°C for VIN = 12V at ILOAD = 7A. This circuit can be implemented on a 0.5" × 1" board space.

Figure 4. Efficiency Curves for Figure 1

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Wei Chen