LTC1479IG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1479IG' PDF : 24 Pages View PDF
LTC1479
APPLICATIONS INFORMATION
In the “3-diode” mode, only the first half of each power
path switch pair, i.e., SW A, SW C and SW E, is turned on;
and the second half, i.e., SW B, SW D and SW F, is turned
off. These three switch pairs now act simply as three
diodes connected to the three main input power sources
as illustrated in Figure 4. The power ‘diode’ with the
highest input voltage passes current through to the input
of the DC/DC converter to ensure that the power manage-
ment µP is powered at start-up or under abnormal oper-
ating conditions. (An undervoltage lockout circuit defeats
this mode when the V + pin drops below approximately
4.5V).
“Cold Start” Initial Condition
The LTC1479 is designed to start in the “3- diode” mode
when all five logic inputs are low— when no power is
available (including the backup system). A 100k resistor
from the 3DM input to ground ensures that this input is low
during a “cold start.” This will cause the main PowerPath
switches to pass the highest voltage available to the input
of the DC/DC converter. Normal operation will then
resume after a good power source is identified.
Recovery from Uncertain Power Conditions
The “3-diode” mode can also be asserted (by applying an
active low to the 3DM input) when abnormal conditions
exist in the system, i.e., when all power sources are
deemed not “good” or are depleted, or the management
system µP is being reset or not functioning properly. (See
the Power Management µP Interface section for additional
information on when to invoke “3-diode” mode.)
COMPONENT SELECTION
N-Channel Switches
The LTC1479 adaptive inrush limiting circuitry permits the
use of a wide range of logic-level N-channel MOSFET
switches. A number of dual low RDS(ON) N-channel switches
in 8-lead surface mount packages are available that are
well suited for LTC1479 applications.
The maximum allowable drain source voltage, VDS(MAX),
of the three main switch pairs, SW A/B, SW C/D and SW
E/F, must be high enough to withstand the maximum DC
supply voltage. If the DC supply is in the 20V to 28V range,
use 30V MOSFET switches. If the DC supply is in the 10V
to 18V range, and is well regulated, then use 20V MOSFET
switches.
As a general rule, select the switch with the lowest RDS(ON)
at the maximum allowable VDS. This will minimize the heat
dissipated in the switches while increasing the overall
system efficiency. Higher switch resistances can be toler-
ated in some systems with lower current requirements,
but care should be taken to ensure that the power dissi-
pated in the switches is never allowed to rise above the
manufacturer’s recommended levels.
The maximum allowable drain-source voltage, VDS(MAX),
of the two charger switch pairs, SW G and SW H, need only
DCIN
BAT1
BAT2
SW A
SW B
ON
OFF
SW D
SW C
SW F
ON
OFF SW E
ON
OFF
RSENSE
+
CIN
12V
HIGH
EFFICIENCY
DC/DC
5V
SWITCHING
REGULATOR
3.3V
LTC1479
PowerPath CONTROLLER
POWER
MANAGEMENT
µP
1479 F04
Figure 4. LTC1479 PowerPath Switches in “3-Diode” Mode
13
Share Link: 
