User`s guide

4-22 Intel

StrongARM

SA-1110 Microprocessor Development Board

User’s Guide

Hardware Considerations

4.11 Power for the Intel® StrongARM SA-1110

Development Board

The following two sections provide a background on the design considerations for the 3.3 V supply

on the SA-1110 Development Board and the actual implementation used.

4.11.1 3.3 V Main System Power Design Considerations

Note: This section provides a background on design considerations for the power supply on the SA-1110

Development Board, while Section 5.21.2 describes the actual implementation.

The terminal voltage for a single cell Li-ion battery varies between 4.1 V to 2.7 V throughout its

discharge cycle. This presents a problem for the 3.3 V power supply design, because the battery

terminal voltage starts out higher than the 3.3 V rail and ends up lower than the 3.3 V rail.

Switching regulators are generally considered the highest efficiency solution to most battery power

conversion designs. Switching regulator power conversion technology provides buck regulators

that reduce a high voltage to a lower voltage. Boost regulators can convert a low voltage to a higher

voltage. Combination buck/boost regulators can handle the 4.1V to 2.7V input variation of a single

Li-ion cell to produce the 3.3 V on the main power rail.

An alternative solution for 3.3 V generation could use a boost regulator to produce 5 V from the

2.7 V to 4.1 V battery (the LCD requires 5 V anyway) plus a secondary or post regulator to buck

the 5 V down to 3.3 V. This results in reduced conversion efficiency for the 3.3 V rail because the

efficiency of the two regulators are multiplied, 90% of 90% equals 81%.

However, switching regulators are typically less efficient at low-power levels than at high-power

levels. A switching regulator that is 90% efficient at full load may be less than 80% efficient under

very light loads, even when taking advantage of the pulse width modulation (PWM) or pulse

frequency modulation (PFM) selections that the regulators provide to optimize conversion

efficiency for heavy and light loads. This is important because the sleep mode, which is a very low

power mode, must be very efficient to allow the longest possible sleep times. Unfortunately, the 5

V to 3.3 V boost buck combination results in less than 65% (80% of 80%) combined efficiency in

sleep mode.

Buck and boost switching regulators as well as linear regulators are most efficient when the input

voltage is close to the output voltage. This is why the 1.5 V buck regulator is 91% efficient when

at 7.2 V. A key point here is that linear regulators, which are generally considered to be very

inefficient, can be more efficient than switching regulators if the input voltage is only slightly

higher than the output voltage. Low drop out (LDO) linear regulators improve this situation by

allowing the input voltage to be as little as 150 mv higher than the regulated output voltage at the

full rated load.

Given that Li-ion batteries discharge at less then a 0.2C rate (20% of the total mah rating) have a

terminal voltage at or above 3.6 V over 80% of their discharge cycle; the use of a simple LDO

linear regulator connected directly to the battery provides the 3.3 V rail a very power efficient, low-

cost solution. An LDO linear regulator also does not require the bulky coils, capacitors and diodes

that switching regulators do, resulting in even lower cost and less circuit board “real estate”.

At the beginning of a charge with a terminal voltage of 4.1V, the LDO linear regulator for the 3.3 V

rail is over 80% efficient. When the battery is at its nominal voltage of 3.6 V, the LDO regulator is

over 91% efficient and approaches 96% efficiency as the battery discharges towards the 3.45 V