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Power supplies

Power Tip 44: Handling high dI/dt load transients, Part 1

February 24, 2012 | Robert Kollman | 222904181
Power Tip 44: Handling high dI/dt load transients, Part 1 Robert Kollman, Texas Instruments discusses why high di/dt loads require careful bypassing to preserve power supply dynamic regulation.
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(Editor's note: Power Tips is an ongoing series; to see a linked list of all entries from #1 to the latest one, click here.)

With many central processing units (CPUs), specifications require that the power supply must be capable of providing large, rapidly changing output currents, typically as the processor changes operating modes.

For instance, in a 1-volt system, the requirement may be to stabilize the supply voltage within three percent for a 100 A/sec load transient. The key to attacking this problem is to realize that this is not just a power supply problem but a power distribution system problem as well, and the two become intertwined in the solution.

The implication of these high di/dt requirements is that the voltage source must have very low inductance. Rearranging the following expression and solving for the allowable source inductance:

There can be only 0.3 nH of inductance in the path of the rapid load-current transient. For comparison, the inductance of a 0.1 inch-wide (0.25 cm) circuit-board trace on a four-layer board has an inductance of about 0.7 nH/inch (0.3 nH/cm). The typical inductance of a wire bond within an IC package is in the 1 nH range, and vias in a printed circuit board are in the 0.2 nH range.

There also is a series inductance associated with bypass capacitors as illustrated in Figure 1. The top curve is the impedance of a single 22 F, X5R, 16V, 1210 ceramic capacitor mounted on a four-layer circuit board.

Figure 1: Parasitics in parallel capacitors impedance

diminish effectiveness.

(Click here for enlarged image)

As expected, below 100 kHz, the impedance drops with increasing frequency. However, there is a series resonance at 800 kHz where the capacitor begins to turn inductive. The inductance, which can be calculated from the value of the capacitor and the resonant frequency, is equal to 1.7 nH which is well above our goal of 0.3 nH. Luckily, you can parallel capacitors to reduce the effective ESL.

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