Exploit Controller Features to Optimize Power Designs

Jun 25, 2008 3:07 PM
By Ricardo Capetillo, Applications Engineer, Linear and Low Voltage, National Semiconductor, Santa Clara, Calif.

External Reference Feature

Regulators with an external reference feature can be advantageous in specific applications. One example is in meeting DDR and DDR2 SDRAM termination specifications. DDR and DDR2 memory require a single power source for primary supply voltages (V_DD) to ensure that all voltage levels track each other, which is especially critical during powerup.

At initial powerup, all supply power should be stable and meet specification timing. The external reference voltage is expected to equal one-half V_DD and must track variations in the dc voltage level. Fig. 8 shows a typical application circuit for a DDR2 solution.

Frequency Synchronization

Data communication and high-quality audio and video electronics require low noise to maintain uncompromised signal integrity. Typically, a single-rail bus supplies power to multiple SMPS, which subsequently provide voltage rails to a number of electronic components in a system.

Beat noise occurs when two or more SMPS are connected to the same input supply or when the input of one SMPS connects to the output of another SMPS. The sum and difference between switching frequencies of nonsynchronized SMPS will develop beat frequencies. The power supplies will reflect beat noise to the input bus and conduct into any electrical circuit connected to it, such as other SMPS.

Techniques used to attenuate the beat noise include:

1. Additional filtering at the front end of each switching regulator
2. Increasing the loop gain sufficiently at the beat frequency in order to reject the input to output-noise transfer
3. Setting the switching frequency of one of the SMPS to switch at two times greater than the second SMPS.

These techniques increase the solution size and/or increase design time. A better alternative involves synchronizing the SMPS oscillators. Synchronizing two or more internal oscillators will theoretically eliminate beat noise and relieve conducted noise interaction with other loads. Synchronizing also keeps the generated EMI to a predictable set of frequencies, as illustrated in Fig. 9 and Fig. 10.

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