LTC1479 datasheet(18/24 Pages) LINER

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LTC1479

APPLICATIONS INFORMATION

The LT1511 has a third control loop that regulates the

current drawn from the AC adapter. Therefore, the DC

input to the LTC1479 and the input to the host system

through SW A/B, is obtained from the “output” of the

LT1511 adapter sense resistor, RS4, and not directly from

the DC input connector as with the LT1510. This allows

simultaneous operation of the host system while charging

a battery without overloading the AC adapter. Charging

current is reduced to keep the adapter current within

specified levels.

However, as with the LT1510 , the output of the LT1511 is

directed to the charging battery through either SW G or SW

H, and the charging battery voltage is connected to the top

of the voltage resistor divider, R6 and R7, for constant

voltage charging. (See the LT1511 data sheet for further

detail on battery charging techniques and applications

hints.)

LT1620/LTC1435 Battery Charger Interface

The LTC1479 also interfaces with the LT1620/LTC1435

synchronous high efficiency low dropout battery charger.

The circuit shown in Figure 12 is a constant-current/

constant-voltage battery charger specifically designed for

lithium-ion applications having thermal, output current, or

input voltage headroom constraints which preclude the

use of other high performance chargers such as the

LT1510 or LT1511.

This circuit can charge batteries at up to 4A. The precision

current sensing of the LT1620 combined with the high

efficiency and low dropout characteristics of the LTC1435

provide a battery charger with over 96% efficiency requir-

ing only 0.5V input-to-output differential at 3A charging

current.

Charge current programming is achieved by applying a

µA to 100µA current from the LT1620 PROG pin to

ground, which can be derived from a resistor or DAC

output controlled by the power management

µP. (See the

LT1620 data sheet for further details on this circuit.)

Capacitive Loading on the CHGMON Output

In most applications, there is virtually no capacitive load-

ing on the CHGMON output—just a simple resistor

divider. Care should be taken to restrict the amount of

capacitance to ground on the CHGMON output to less than

100pF. If more capacitance is required, it may become

necessary to “mask” the LOBAT output when the charge

monitor is switched between batteries. (Internal resis-

tance between the BAT1 and BAT2 inputs and the charge

monitor switch may create a transient voltage drop at the

VBAT output during transitions which could be falsely

interpreted by the

µP as a low battery condition.)

THE POWER MANAGEMENT MICROPROCESSOR

Interfacing to the LTC1479

The LTC1479 can be thought as a “real world” interface to

the power management

µP. It takes logic level commands

directly from the

µP, and makes changes at high current

and high voltage levels in the power path. Further, it

provides information directly to the

µP on the status of the

AC adapter, the batteries and the charging system.

The LTC1479 logic inputs are TTL level compatible and

therefore interface directly with standard power manage-

ment

µPs. Further, because of the direct interface via the

five logic inputs and the two logic outputs, there is virtually

no latency (i.e. time delay) between the

µPandtheLTC1479.

In this way, time critical decisions can be made by the

µP

without the inherent delays associated with bus protocols,

etc. These delays are acceptable in certain portions of the

power management system, but it is vital that the power

path switching control be made through a direct connec-

tion to the power management

µP. The remainder of the

power management system can be easily interfaced to the

µP through a serial interface.

Selecting a Power Management Microprocessor

The power management

µP provides intelligence for the

entire power system, is programmed to accommodate the

custom requirements of each individual system and allow

performance updates without resorting to costly hard-

ware changes.

The power management

µP must meet the requirements

of the total power management system, including the

LTC1479 controller, the batteries (and interface), the

backup system, the charging system and the host proces-

sor. A number of inexpensive processors are available

which can easily fulfill these requirements.

LTC1479 Datasheet(PDF) 18 Page - Linear Technology

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