User`s guide

Intel
®
StrongARM
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SA-1110 Microprocessor Development Board User’s Guide 4-23
Hardware Considerations
LDO cutout point. The efficiency goes up in sleep mode where the small load results in an even
lower drop out voltage of 20 mV, which allows the battery to discharge to 3.32 V before drop out
starts. With a 3.32 V battery voltage, the 3.3 V LDO linear regulator is over 99% efficient.
In addition, using a LDO linear regulator provides the 3.3V rail results with a very clear power
supply that requires fewer and smaller filter capacitors, saving cost and board space. An added
benefit is less noise in the audio, telephony and display systems.
Unfortunately, two major disadvantages to this scheme are:
• The last 20% of the battery capacity is not usable because the bottom 20% of the charge
terminal voltage goes below the 3.45 V cutout level necessary to maintain the 3.3 V output
from the LDO linear regulator.
• Battery voltage sag due to discharge rates in the 1C to 2C range, which may be typical when
the radio is transmitting, would cause the 3.3 V LDO linear regulator to drop out.
A three part solution to this dilemma also results in solving the two cell Li-ion radio power
requirements as well as allowing the elimination of the backup battery, improved power conversion
efficiency, reducing system cost, and reduced audio and video noise. This unique power system
design is referred to as a linear boost bootstrap with virtual backup battery. The three key elements
of this design are as follows:
• Use of an adjustable boost regulator with a built in adjustable LDO linear post regulator and
LDO track mode feature.
• Use of a separate 3.3 V LDO linear regulator connected directly to the single cell Li-ion
battery in parallel with the boost converter LDO linear post regulator.
• Reservation of the bottom 20% of battery charge when the battery terminal voltage is below
3.45V, to be used as a virtual backup battery. Reserving the last fraction of charge of a system
battery to preserve the memory contents is probably not new, however it is an optional cost
reducing feature that is a natural consequence of the linear boost bootstrap design.
4.11.2 MAX1705 Boost Converter and MIC5219 LDO Linear
Regulator
Note: This section provides a description of how the actual power supply was implemented on the SA-
1110 Development Board, while Section 4.11.1 describes design considerations.
The MAX1705 has three key features that support the linear boost bootstrap SA-1110
Development Board power system design. First, the MAX1705 has a built-in adjustable LDO
linear regulator that is fed from the main boost regulator output. Second, the MAX1705 has a
special track mode that can be set to allow the main boost regulator to be regulated to 0.3 V above
the LDO output instead of the normal 5 V output. This causes the main boost output to track 0.3 V
above the LDO output to keep the LDO from dropping out. This permits efficient boost and post
linear regulation of the 3.3 V supply when the 5 V output is not needed, such as sleep mode. Third,
the MAX1705 supports a low battery sensing circuit that is used to sense the 2.7V bottom of charge
level and shutdown all battery loads in the SA-1110 Development Board system to protect the
Li-ion battery from over-discharge damage.
The SA-1110 Development Board design sets the MAX1705 main boost output to 5.5 V, which is
the maximum the boost output can be set to on the MAX1705. The LDO output is set to 3.2 V.
Given that the MAX1705 LDO power input is from the main boost output rail on the MAX1705,
the resulting conversion efficiency from the battery to the 3.2 V output is
0.9 X 3.2/5.5 X 100 = 52%, which is very poor. However, the MAX1705 3.2 V LDO output is