Power analysis of the iPad
With a 25 Watt-hour battery, the Apple iPad touts 10+ hours of battery life. This is an important product differentiator and invites investigation. Of particular interest is how much power the A4 processor consumes, since Apple’s acquisition of both Intrinsity and PA Semi had the potential to improve processor performance and power consumption.
By performing functional tests and measuring power consumption, we were able to find out more about the power consumption characteristics of the iPad. As shown in Figure 2, excerpted from our full product teardown, the LCD module consumes around 1W to 3.5W depending on the brightness of the panel. Web browsing, which is one of typical usages of the iPad, dissipates a little over 2W when run at a moderate level of screen brightness.
Combining the results and cross referencing numbers, the main board was found to consume about 1W of power. Assuming that 50 to 80 percent of the main board power consumption is due to the A4 processor, we concluded that the power consumption of the A4 would be between 500mW and 800mW during Web browsing when using Wi-Fi connectivity.
For operations such as playing music or video, the iPad consumes 1.5W ~ 1.7W when the LCD is set to minimum brightness. After removing the contribution of the LCD power, main board power consumption would be 450mW ~ 650mW during this activity. Again assigning a 50-80 percent usage to the CPU, the A4 power consumption is estimated to be 250mW ~ 520mW.
Overall the numbers reflect an efficient CPU design, consistent with the specifications published on similar ARM-based devices with advanced process nodes and some innovation in critical path optimization. Apple is the first to market with a production device in this class, however.
The power measurements also show that the LCD backlight remains the more power-hungry activity in the iPad, at least for light browsing and media activities. More details of our power measurements can be found in our iPad product teardown.