Keeping Parts Cost Down in Mobile CPU Power Supplies

Oct 1, 2006 12:00 PM
By Tod Schiff, Senior Field Applications Engineer, Computing Segment, Analog Devices, Beaverton, Ore

New processors for notebook computers demand more from their power supplies: higher currents, faster response to load steps and faster output voltage changes in response to an updated voltage-identification (VID) code. Reusing an existing power-supply design in a new system would be preferable if it could meet the latest load-step specifications, offer low ripple and guarantee high efficiency in all

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New processors for notebook computers demand more from their power supplies: higher currents, faster response to load steps and faster output voltage changes in response to an updated voltage-identification (VID) code. Reusing an existing power-supply design in a new system would be preferable if it could meet the latest load-step specifications, offer low ripple and guarantee high efficiency in all operating modes, especially during standby.

Unfortunately, older controllers can't provide fast load steps directly through existing output inductors, so they need additional bulk capacitors to smooth transients. The space available for the new power-supply design is the same as that available for the older design, however, so additional capacitors won't fit. What are the alternatives?

For most notebook applications, a 2-phase design keeps inductor currents at or below 20 A per phase, gives the fastest response to load steps and yields the lowest cost. The switching frequency must be set high enough to respond to load transients at the required slew rate. The R_DSON of the MOSFETs must be kept low to minimize high-frequency switching losses, and the bandwidth of the controller's feedback loops must be high enough to guarantee fast response.

However, older controllers have limited bandwidth. Raising the switching frequency doesn't help, because the low bandwidth limits the loop response. The inductors can't supply large current steps, so more bulk capacitors will be needed. This is expensive in cost and size, and limits the response time for on-the-fly output voltage steps.

New multiphase synchronous buck controllers solve these problems. Their stable, high-speed feedback loops permit smaller designs at lower cost. Some controllers also offer single-phase operation at lower switching frequencies, greatly improving efficiency for low or intermittent current demands.

Properly compensated, a high-bandwidth controller handles maximum load steps without oscillation. The controller provides more current, faster from the inductors, so less charge is required from the bulk capacitors. New controllers respond quickly to current transients, simultaneously turning on multiple phases, increasing the available load current without additional bulk capacitance. The controller handles the big load steps, making inductor, capacitor and MOSFET choices fairly straightforward.

Inductor First

A switching frequency (F_SW) of a few hundred kilohertz per phase provides a good tradeoff between switching losses, ripple and output filter size, though many controllers will go higher. The value of the inductor used in the output filter depends on ripple requirements, not output voltage.

where V_VID is the programmed output voltage, R₀ is the load resistance, D_MIN is the minimum duty cycle and V_RIPPLE is the allowed ripple voltage due to inductor ripple current. The peak-to-peak ripple current in the inductor should be less than half of its maximum dc current. An 8-A ripple current gives a 20-mV_PK-PK ripple voltage with a 2.5-mΩ load. For a 2-phase supply, a V_VID output voltage of 1.1150 V and F_SW = 280 kHz, Eq. 2 gives L ≥ 423 nH.

The inductor should not saturate at the per-phase peak current and should handle the power dissipation from core loss and average winding current. Using the smallest possible inductor reduces the number of output capacitors. The dc resistance (DCR) of the inductor affects current sensing in many controller designs, with its value providing a tradeoff between power loss and measurement accuracy.