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LT1579

APPLICATIONS INFORMATION

Five milestones are noted on the timing diagram. Time A

is where the primary input voltage drops enough to trip the

low-battery detector LB1. The trip threshold for LB1 is set

at set at 5.5V, slightly above the dropout voltage of the

primary input. At time B, the BACKUP flag goes low,

signaling the beginning of the transition from the primary

source to the secondary source. Between times B and C,

the input current makes a smooth transition from VIN1 to

VIN2. By time C, the primary battery has dropped below the

point where it can deliver useful current to the output. The

primary input will still deliver a small amount of current to

the load, diminishing as the primary input voltage drops.

By time D, the secondary battery has dropped to a low

enough voltage to trip the second low-battery detector,

LB2. The trip threshold for LB2 is also set at 5.5V, slightly

above where the secondary input reaches dropout. At time

E, both inputs are low enough to cause the LT1579 to enter

dropout, with the DROPOUT flag signaling the impending

loss of output regulation. After time E, the output voltage

drops out of regulation.

Some interesting things can be noted on the timing

diagram. The amount of current available from a given

input is determined by the input/output voltage differen-

tial. As the differential voltage drops, the amount of

current drawn from the input also drops, which slows the

discharge of the battery. Dropout detection circuitry will

maintain the maximum current draw from the input for the

given input/output voltage differential. In the case shown,

this causes the current drawn from the primary input to

approach zero, though never actually dropping to zero.

Note that the primary begins to supply significant current

again when the output drops out of regulation. This occurs

because the input/output voltage differential of the pri-

mary input increases as the output voltage drops. The

LT1579 will automatically maximize the power drawn

from the inputs to maintain the highest possible output

voltage.

Thermal Considerations

The power handling capability of the LT1579 is limited by

the maximum rated junction temperature (125

°C). Power

dissipated is made up of two components:

1. The output current from each input multiplied by the

respective input to output voltage differential:

(IOUT)(VIN – VOUT) and

2. Ground pin current from the associated inputs multi-

plied by the respective input voltage: (IGND)(VIN).

If the primary input is not in dropout, all significant power

dissipation is from the primary input. Conversely, if SS has

been asserted to minimize power draw from the primary,

all significant power dissipation will be from the second-

ary. When the primary input enters dropout, calculation of

power dissipation requires consideration of power dissi-

pation from both inputs. Worst-case power dissipation is

found using the worst-case input voltage from either input

and the worst-case load current.

Ground pin current is found by examining the Ground Pin

Current curves in the Typical Performance Characteris-

tics. Power dissipation will be equal to the sum of the two

components above for the input supplying power to the

load. Power dissipation from the other input is negligible.

The LT1579 has internal thermal limiting designed to

protect the device during overload conditions. For con-

tinuous normal load conditions, the maximum junction

temperature rating of 125

°C must not be exceeded. It is

important to give careful consideration to all sources of

thermal resistance from junction to ambient. Additional

heat sources nearby must also be considered.

Heating sinking for the device is accomplished by using

the heat spreading capabilities of the PC board and its

copper traces. Copper board stiffeners and plated through-

holes can also be used to spread the heat. All ground pins

on the LT1579 are fused to the die paddle for improved

heat spreading capabilities.

The following tables list thermal resistances for each pack-

age. Measured values of thermal resistance for several

different board sizes and copper areas are listed for each

package. All measurements were taken in still air on 3/32”

FR-4 board with one ounce copper. All ground leads were

connected to the ground plane. All packages for the

LT1579 have all ground leads fused to the die attach

paddle to lower thermal resistance. Typical thermal

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