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LTC1479

APPLICATIONS INFORMATION

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-channelswitches

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

Figure 4. LTC1479 PowerPath Switches in “3-Diode” Mode

BAT1

BAT2

SW A

SW C

SW E

HIGH

EFFICIENCY

DC/DC

SWITCHING

REGULATOR

3.3V

12V

CIN

DCIN

1479 F04

SW B

SW F

SW D

RSENSE

POWER

MANAGEMENT

µP

LTC1479

PowerPath CONTROLLER

OFF

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

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