GET A CONSTANT +5V OUTPUT BY SWITCHING BETWEEN A +5V INPUT AND A SINGLE-CELL LI+ RECHARGEABLE CELL




Abstract: Design provides a simple method for maintaining an uninterrupted +5V even while switching
between the external +5V supply and a rechargeable single-cell Li+ battery.
Some portable applications need to be powered up from an external +5V wall adapter supply and still
require a +5V system voltage when in battery-backup mode. This design provides a simple method of
switching between the external +5V supply and a rechargeable single-cell Lithium-ion (Li+) battery. The
design will still maintain uninterrupted +5V to the application circuitry.
GET A CONSTANT +5V OUTPUT BY SWITCHING BETWEEN A +5V INPUT AND A SINGLE-CELL LI+ RECHARGEABLE CELL
Three key circuit functions are needed:
1.
An efficient Li+ battery charger
2.
An efficient boost converter to convert the battery voltage to +5V
3.
A low-loss switchover circuit that automatically switches between the main DC power and the backup
Li+ battery
As shown in
Figure 1
, U1 is a high-performance battery charger (
MAX8903
) capable of supplying up to 2A
to charge a single-cell battery. Resistors RIDC and RISET set the DC-input current limit and charge-current
set points, respectively. Since the SYS output (pins 23 and 24) is not used, the charge current will be
constant and not dictated by the load current.
Q1 and Q2 are dual PNP transistors, available in an SC74 package. These transistors are used with the p-
channel FETs, Q4a and Q4b, to form ideal low-loss ORing diodes. The output of the ORing diode
configuration is then fed to a high-efficiency boost converter, U2
For more detail:




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