Advanced processors and FPGAs are becoming more sophisticated and powerful. At the moment, there are new FPGAs using process nodes as small as 20 micrometers! As semiconductor process technology advances, the core voltage decreases and the tolerance becomes tighter. New FPGAs offered in deep sub-micron process technology can have a total core voltage tolerance as little as 3%, which includes reference voltage, line voltage, temperature and even load transient variations. When considering whether a DC/DC switching regulator has suitable voltage accuracy, a quick scan of the converter’s datasheet or the evaluation module can reveal its performance characteristics before the design is even started.
Reference Accuracy
It is a good idea to initially inspect the regulator’s reference accuracy. Poor reference accuracy will make the design more difficult to meet the voltage requirements when considering dynamic changes. Note the front-page of the datasheet may only show the initial accuracy. To get a better idea of the true accuracy, the specification table will shed more light.
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SWIFT TPS54020 ELECTRICAL CHARACTERISTICS: |
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TJ = –40°C to 150°C, VIN = 4.5V to 17V, PVIN = 4.5V to 17V (unless otherwise noted) |
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Parameter |
Conditions |
Min |
Typ |
Max |
Unit |
|
Voltage Reference |
0 A ≤ IOUT ≤ 10 A, –40°C ≤ TA ≤ 150°C |
0.594 |
0.6 |
0.606 |
V |
The example shows the 0.6V reference is 1% accurate over the entire input voltage range, output current range and temperature range. A temperature reference accuracy of 1% indicates very good performance. Choosing tighter tolerance resistor dividers is also believed to help improve the accuracy of the regulator. Unfortunately, 0.1% and 0.5% resistors will cost more than 1% resistors. However, designing a high accuracy, low voltage is not difficult, since low divider ratios are inherently accurate. Click here to view Power Tip #18 and learn more about voltage divider considerations pertaining to voltage accuracy.
Transient Considerations
A voltage can also vary during load transients. More output capacitance will help lower these voltage peaks and overshoots during a load transient since the peaks and overshoots are related to the capacitor’s equivalent series resistance. However, certain non-linear control modes can achieve a fast transient response with minimal capacitance. TI’s new D-CAP3™ control-mode offers a fast transient response with ceramic capacitors, and integrates circuitry to remove voltage offsets to improve total regulation accuracy. Check out this short video to view the advantages of D-CAP3 for powering advanced processors.
The best way to approach voltage accuracy is to find a device with the good transient, ripple and voltage reference performance. Datasheets with an application section or an evaluation module’s user guide will usually reveal these details without having to build up a test circuit. When selecting DC/DC converters, the newer DC/DC converters on the market have a much better chance of meeting these tighter specifications versus older DC/DC converters that we often reuse.
Please share your experiences when designing for high accuracy, low output voltage applications. In the next section, handling inrush currents with performance FPGAs will be discussed.
