AN-9720 Datasheet, PDF (1/7 Pages) Fairchild Semiconductor – Power Path Implementation Tradeoffs

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AN-9720
Power Path Implementation Tradeoffs, Featuring the
FAN5400 Family of PWM Battery Chargers
Overview
Many battery-powered systems, such as smart phones, must be
fully functional and their electronics up and running shortly
after the user plugs a charger into the phone. The system
electronics need to be functional regardless of the state of the
battery, even if the battery is fully discharged or absent. The
FAN5400 family charger IC allows power to be delivered to
the system when the charger is plugged in, whether the battery
is absent or present, and allows the system to power up and be
functional quickly after the charger is plugged in. Other
approaches, such as power path, add additional impedance in
series with the battery.
This application note describes how the FAN5400 family of
battery chargers distributes power between the system and
battery to achieve similar results of power path, often without
the overhead and power loss of an additional switch element.
The block diagram in Figure 1 is a typical implementation of
power path using an “ideal” diode. Current flow is indicated
by the arrows, which reveal that the “ideal” diode (whether
internal or external) helps to steer current appropriately.
While this implementation meets the criteria for power path,
an ideal diode is never truly ideal. For example, the internal
ideal diode of one such IC is actually a PMOS with a typical
value of 180mΩ. This means that there is a permanent 180mΩ
power dissipating series element between battery and system
load that creates significant additional power loss during
periods of high-current drain from the battery, such as GSM
pulses. A parallel diode PMOS switch can reduce this
resistance; however, this increases the solution size and cost.
The implementation in Figure 1 differs from the FAN540X
approach in the block diagram in Figure 2. Although on the
surface it may seem like FAN5400 does not have power path
functionality; it serves almost all the same needs and, in
addition, can provide the benefit of having no power
dissipating series element between system load and battery.
Figure 1. Typical Implementation of Power Path using an “Ideal” Diode
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/23/10
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