Cell phones and other hand-held devices generally require testing that simulates operation with a rechargeable battery in various states of discharge. Conventional power supplies cannot adequately simulate the increasing internal resistance of a battery as it discharges. Also, accurate simulation requires a supply with wide bandwidth to minimize voltage drop at the phone due to large current transients produced by pulsed transmissions.
Battery internal resistance depends on its chemistry and discharge state. Therefore, the power supply should have programmable output resistance over the appropriate range of discharge conditions and the battery types used.
Similarly, it is desirable to test a battery’s response to the cell phone’s charger system. The ideal power supply would also need a second “charger” channel, thereby eliminating a separate supply and providing accurate simulation during testing of switching from a low battery to the charger, without turning off the phone.
The power supply should have readback features that allow it to accurately measure a wide range of currents and perform dynamic power consumption measurements. Measurement resolution should be at least 0.1A for measuring sleep-mode load current. During pulse loading, the unit must supply currents up to 5A. And voltage readback should take place over a separate, remote-sense cable.
Avoiding instabilities. Freedom from oscillation, undershoot and overshoot in the cell phone supply are necessary for accurate test results. Typically, a conventional power supply does not meet these criteria: When connected to a cell phone with a few meters of cable, it may cause the phone to reset when it is placed in the transmit mode. One way to solve this pulse current/voltage drop problem is to add a capacitor at the phone’s power connection. Unfortunately, with a capacitor it is impossible to perform accurate transient current measurements during the transmit, receive, and standby modes.
A power supply featuring either a linear or switching approach can provide the fast response and variable output impedance of a battery. In a regulated linear supply, the output impedance is lowered by the gain of the voltage regulator at dc and low frequencies. However, linear regulators do not have sufficient bandwidth for the fast rise time of cell phone pulse loads. Standard switching supplies have improved bandwidth, but are still not good enough to prevent significant voltage drop. Also, some switching supplies are noisy enough to reset digital circuitry in cell phones.
Power supplies that simulate batteries by utilizing a fast feedback loop concept, which can be built on either a linear or switching design, overcome these problems. With appropriate feedback loop design, adequate bandwidth(>1 MHz) can be achieved and local decoupling capacitors are not required. Voltage overshoot and undershoot are minimized, and accurate pulse current measurement is possible. The only disadvantage in using a high bandwidth supply is that the load impedance, including the cell phone and interconnects, must be minimized to achieve the best transient performance. Keeping cable length to a minimum and using high quality twisted pair cables helps to minimize additional inductance and capacitance that may cause oscillations and other instabilities. To achieve maximum performance, a cable with 10-ohm characteristic impedance has been developed.
For more information on battery simulation, a white paper titled “Optimizing Test Performance of Digital Cellular Products With Ultra-High Speed Power Supplies” can be downloaded from the Keithley Instruments Web site at http://www.keithley.com. To speak with a Keithley applications engineer, call (888) 534-8453, fax (440) 248-6168.