Cellular phones and other portable wireless devices make great demands on their batteries. The battery must source load currents that are capable of jumping virtually instantaneously from a standby current level of 100 to 300 mA to a full-power RF transmission current level of 1 to 3 A. In other words, the load current on the battery can increase rapidly by a factor of up to 1000%. General-purpose power supplies that engineers routinely use to test their prototypes are designed to respond to typical transients of 50% load-current changes. A much larger load-current change puts a tremendous immediate drain on that power supply. Its error-correction circuitry kicks in, causing all sorts of problems that limit the effectiveness of this type of supply.
Keithley Instruments addressed the problem by developing a series of specialized fast-transient power supplies, the 2302 and 2306 models. Their application is to replace or simulate the battery during the testing of cell phones and other portable wireless devices.
"We asked ourselves, 'What happens to a battery when it gets hit with a delta current, a ΔI load change?'" recalls Bob Green, senior market development manager at Keithley. "Effectively, you can model a battery as an ideal source with some internal series resistance. If the load current changes, the battery output voltage is going to drop by ΔI times the internal resistance. We said, let's simulate that. Let's put a programmable output impedance in our unit."
The Keithley power supplies can simulate the real-life sourcing situation of a cellular phone. Because the supply is programmable, designers can program in the typical internal resistance, for example, of a lithium-ion, lithium, nickel-cadmium, or nickel-metal-hydride battery.
"We take pride in the fact that in terms of development or production testing, we can provide a realistic source for reproducing actual operating conditions better than anybody," says Green.
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