18
LTC1479
APPLICATIONS INFORMATION
WUU
U
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, R
S4
, 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
0µA 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
V
BAT
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 µP and the LTC1479.
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.
