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.

When system design engineers are in search of power for mixed-signal electronics, they must satisfy performance, cost, efficiency and space requirements. Electronic equipment such as rack-mount servers, communication equipment, laptops and many consumer electronic goods must conserve space to maintain the form factor of the product. Certain features and specifications of power controller ICs can help satisfy system specifications and enhance the performance of the power supply and the load.

Key controller features and specs include adjustable switching frequency, feedback voltage accuracy, startup tracking, power sequencing, prebiased startup, the ability to work with an external reference and the ability to synchronize to an external clock. When optimized, these features can reduce electromagnetic interference (EMI), transient response times, transient voltage amplitudes, solution size, output capacitance requirements and overall bill-of-materials costs.

User-Adjustable Features

A user-adjustable switching frequency allows power designer to set the oscillator switching frequency to achieve the desired filter component size and, consequently, the solution size. A high switching frequency reduces the power-solution footprint by decreasing the size of the charge storage components. This includes input/output capacitors, inductors and other filtering components.

By moving from a 100-kHz to a 1-MHz switching frequency, the typical inductance required decreases tenfold while the volume of the inductor decreases fivefold. (The comparison was done with two shielded drum-core inductors of the same series, with saturation levels 15% apart from each other, with the inductor current ripple set to 30% of the maximum load current and with the following application power parameters: V_IN = 12 V, V_OUT = 3.3 V, I_LOAD = 5 A, as shown in Fig. 1.)

Shielded drum-core inductors are a good choice for switch-mode power supplies (SMPS) that require an inductance value between 0.33 μH and 1 mH. These inductors are appropriate for high-frequency, low-EMI and low-cost applications.

The output capacitor also decreases in size as the switching frequency increases. Assume that we have selected multilayer ceramic capacitors (MLCCs) for the output filter and that the equivalent series resistance (ESR) is low enough that the output-voltage ripple is capacitive. A SMPS design with a 100-kHz switching frequency, a 1.5-A peak-to-peak ac current and a 50-mV output-voltage ripple requires 37.5-μF capacitance. The typical capacitance requirement decreases tenfold when operating at a 1-MHz switching frequency, and the case-size transitions from a 1210 MLCC to a 0603 case representing almost a 20-times reduction in volume.

In this example, the design for the output filter components only considered voltage and current ripple without examination of load and line transients. The availability of external compensation gives the power-supply designer the flexibility to optimize the feedback loop without oversizing the output capacitance.

For example, to decrease the output-voltage transient, designers can take advantage of the wide gain-bandwidth product of the error amplifier. A high closed-loop-bandwidth frequency decreases the time needed for the error amplifier to react to load and line transients. As a typical rule of thumb, a bandwidth designed at one-tenth to one-fifth of the switching frequency results in a high performance loop response.

Fig. 2 and Fig. 3 show the result of increasing the bandwidth. The SMPSs underwent the same test conditions: equal slew rates, 350-mA to 8-A load transient step, 440-μF output capacitance and equal feedback-loop-gain phase margin (48 degrees).

In the example, the output-voltage transient decreased by approximately ±80 mV. The external compensation provides the flexibility to fine-tune the speed of the loop response while maintaining the same output capacitance.

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