In the last decade mobile device users have seen unprecedented changes in technology. These changes have brought more functionality and lower cost to the overall handset market. Users are demanding more and more functionality in mobile devices, which comes at the cost of talk time. As more functionality is integrated into mobile devices, designers are faced with ever-increasing requirements for overall system efficiency and lower current consumption. In the face of these changes, there has been a push for designers to add DC-DC converters to power RF front ends. This is driving the need to evaluate RF components from a slightly different angle compared to the way it has been done in the past. In this paper, the authors would like to introduce a new approach to evaluating RF front end performance for current consumption when DC-DC converters are present.

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The approach of benchmarking at a fixed battery voltage comes into question when one considers the discharge of a real battery. Voltage changes as a battery discharges. Figure 1 illustrates the nominal battery discharge curve for a Lithium Ion battery, which is commonly found in many mobile devices today.

As Figure 1 depicts, the battery voltage can range from 4.2 V for a fully charged battery to less than 3.0 V for a completely depleted battery. Most current generation mobile devices power down their systems around the 3.0 to 3.2 V range, limiting the usage at these lower battery voltages.

Benchmarking RF front end current consumption to a fixed battery voltage can continue to be a valid approach when current does not vary by much across battery voltage. This case typically applies when there is no DC-DC converter present in the RF front end, as depicted in Figure 2. Measured results are shown in Figure 3. The current consumption for this GSM amplifier at 29 dBm varies slightly from 1.05 to 1.08 amps across a lithium-ion battery discharge. The current draw benchmark at 3.6 V provides a good estimate and is found to be 1.06 amps.

When a DC-DC converter is introduced to an RF front end, as illustrated in Figure 4, battery current varies much more as battery voltage changes. Measured performance of such a solution is shown in Figure 5. The current consumption for this GSM amplifier at 29 dBm varies from 600 mA to more than 800 mA as a lithium-ion battery discharges.

This large current change can be explained by taking a look at Equation 1. Battery current increases as battery voltage decreases when a DC-DC converter is present.

Equation 1 |

Table 1. Captured Ibatt Data |

Average current can then be calculated according to Equation 2, where L is the battery charge level.

Average current is then found by adding all the geometric area contributions and plugging the results into Equation 2.

Ibatt

_{ave}=0.666A, Geometric Fit with Vbatt Stepped in 0.1 V Increments.

Table 2 |

click to enlarge Equation 3 |

Average Ibatt is then found by plugging Equation 4 into Equation 2. L0 is the battery level of 100 percent, and L1 is the battery level at the shutdown voltage of 2.9 V.

Ibatt

_{ave}=0.665A, Curve Fit, Tenth Order Polynomial Equation

Results fall within 1 mA of the geometric approach.

click to enlarge Equation 4 |

Jackie Johnson has been employed with RFMD for 10 years. He is currently the Applications Engineering Manager within the Cellular Products Group. Jackie can be reached by email at jjohnson@rfmd.com.

Keith Adkins received his BSEE from the University of Colorado at Boulder and has been working in the RF and microwave communication industry since 1995. Currently Keith is a senior RF systems engineer at RFMD. Keith can be reached by email at kadkins@rfmd.com

##### Acknowledgements

The authors would like to thank many RFMD colleagues for their contributions to this article, especially Scott Yoder and Ray Arkiszewski, for their technical insights on this concept.Advertisement