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PXA250 and PXA210 Applications Processors Design Guide 8-23
Power and Clocking
8.7.4 I/O 3.3 V Power
A simple LDO linear regulator supplies the 3.3V rail. The Analog Devices ADP3335 is chosen for
its very low drop-out – 200 mV at 500 mA and 110 mV at 200 mA. So typically, the input cut-off
voltage for this device is about 3.3 V + 0.11 V = 3.41 V. The power is drawn directly from the Li+
battery. For a 3.6 V battery, this device has a 82% efficiency. There are four zones of operation for
the Li+ battery:
• 4.1 – 3.8 V zone 10% of the time;
• 3.7 – 3.6 V zone at 70% of the time;
• 3.5 – 3.4 V at 10% of the time; and
• 3.4 – 3.1 V at 10% of the time.
The ADP3335 operates in zone 1,2, 3, and cutoffs in zone 4.
The overall efficiency is:
0.1(3.3/4.0) + 0.7(3.3/3.6) + 0.1(3.3/3.4) = 0.0825 + 0.642 + 0.097 = 0.82
To access the energy in zone 4 use the second LDO linear regulator in a parallel configuration with
the ADP3335 and set it to output 3.2 V. Input to this regulator is 5.5 V from the boost converter.
When the battery voltage drops below 3.5 V, the ADP3335 drops-out and the second regulator
takes over.
8.7.5 Peripheral 5.5 V Power
The example form factor reference design provides a 5.5 V rail to supply power to LCD, Audio
amplifier and Radio modules. An LT1308A boost converter is used. This device supplies up to 1 A
at 5.5 V while operating at 600 kHz with up to 90% efficiency at rated load and 3.6 V input.
In addition, a low battery voltage detect circuit has an open-drain output. The detect voltage is set
at 3.45 V by a resistor divider circuit. When the battery drops below 3.45 V the output transitions
to a logic low. This output signal is used as a processor interrupt.