LTC1479 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1479' PDF : 24 Pages View PDF
LTC1479
APPLICATIONS INFORMATION
POWER PATH SWITCHING CONCEPTS
Power Source Selection
The LTC1479 drives low-loss switches to direct power in
the main power path of a dual rechargeable battery system
— the type found in most notebook computers and other
portable equipment.
Figure 3 is a conceptual block diagram which illustrates
the main features of an LTC1479 dual battery power
management system, starting with the three main power
sources and ending at the system DC/DC regulator.
Switches SW A/B, SW C/D and SW E/F direct power from
either the AC adapter (DCIN) or one of the two battery
packs (BAT1 and BAT2) to the input of the DC/DC switch-
ing regulator. Switches SW G and SW H connect the
desired battery pack to the battery charger.
Each of the five switches is intelligently controlled by the
LTC1479 which interfaces directly with a power manage-
ment system µP.
Using Tantalum Capacitors
The inrush and “outrush” current of the system DC/DC
regulator input capacitor is limited by the LTC1479. i.e.,
the current flowing both in and out of the capacitor during
transitions from one input power source to another is
limited. In many applications, this inrush current limiting
makes it feasible to use lower cost/size tantalum surface
mount capacitors in place of more expensive/larger alumi-
num electrolytics at the input of the DC/DC converter.
Note: The capacitor manufacturer should be consulted for
specific inrush current specifications and limitations and
some experimentation may be required to ensure compli-
ance with these limitations under all possible operating
conditions.
Back-to-Back Switch Topology
The simple SPST switches shown in Figure 3 actually
consist of two back-to-back N-channel switches. These
low-loss, N-channel switch pairs are housed in 8-pin SO
and SSOP packaging and are available from a number of
manufacturers. The back-to-back topology eliminates the
problems associated with the inherent body diodes in
power MOSFET switches and allows each switch pair to
block current flow in either direction when the two switches
are turned off.
The back-to-back topology also allows for independent
control of each half of the switch pair which facilitates
bidirectional inrush current limiting and the so called “3-
diode” mode described in the following section.
The “3-Diode” Mode
Under normal operating conditions, both halves of each
switch pair are turned on and off simultaneously. For
example, when the input power source is switched from a
good DC input (AC adapter) to a good battery pack, BAT1,
both gates of switch pair SW A/B are turned off and both
gates of switch pair SW C/D are turned on. The back-to-
back body diodes in switch pair, SW A/B, block current
flow in or out of the DC input connector.
DCIN
BAT1
BAT2
BATTERY
CHARGER
SW A/B
SW C/D
SW E/F
SW G
SW H
LTC1479
PowerPath CONTROLLER
RSENSE
+
CIN
12V
HIGH
EFFICIENCY
DC/DC
5V
SWITCHING
REGULATOR
3.3V
POWER
MANAGEMENT
µP
1479 F03
Figure 3. LTC1479 PowerPath Conceptual Diagram
12
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