11
LTC1479
OPERATION
U
This scheme allows capacitors and MOSFET switches of
differing sizes and current ratings to be used in the same
system without circuit modifications.
After the transition period has passed, the V
GS
of both
MOSFETs in the selected switch pair rises to approxi-
mately 6V. The gate drive is set at 6V to provide ample
overdrive for logic level MOSFET switches without ex-
ceeding their maximum V
GS
rating.
Internal Power Supplies
Two internal supplies provide power for the control logic
and power source monitoring functions. The V
CCP
logic
supply is approximately 5V and provides power for the
majority of the internal logic circuitry. The V
CC
supply is
approximately 3.60V and provides power for the V
GG
switching regulator control circuitry and the gate drivers.
The V
CC
supply has an undervoltage lockout circuit which
minimizes power consumption in the event of a total loss
of system power; i.e., when all available power sources fall
below approximately 4.5V.
DCIN Voltage Monitoring
The DCIN input is continuously monitored via a two
resistor ladder connected between the DCIN pin and the
DCDIV input. The input threshold is 1.215V (rising edge)
with approximately –35mV hysteresis. The use of a defini-
tive voltage threshold ensures that the DC supply is not
only connected but “healthy” before being attached to the
DC/DC converter input.
Battery Voltage Monitoring
The LTC1479 has the ability to independently monitor both
battery packs. (Because of this, one battery pack may be
discharged as the other is being charged.)
A low-battery detector signals when the selected battery
pack has dropped to the level where a shutdown sequence
should be initiated or the other battery pack engaged.
Battery Charging Management Functions
The LTC1479 directly interfaces with LT1510/LT1511
battery charger circuits. Two gate drive circuits control
the two back-to-back N-channel switch pairs, SW G and
SW H, under logic (CHGSEL) control to connect the
output of the charger to the selected battery pack. Break-
before-make action ensures that current does not pass
from one battery pack to the other during switch-over of
the charger output. The CHGSEL input also simulta-
neously switches the positive terminal of the selected
battery pack to the top of the voltage feedback resistor
ladder in the charger system through the CHGMON pin.
Backup Supply Interface
Power for the LTC1479 is obtained from the backup
supply when power is unavailable from the three main
sources of power.
Interface to Companion Microprocessor
A companion µP must be used in conjunction with the
LTC1479 to provide overall control of the power manage-
ment system. The LTC1479 communicates with the µP by
means of five logic inputs and two logic outputs as
described in Table 1.
Table 1. LTC1479 µP Interface Inputs and Outputs
INPUT ACTION
DCIN/BAT Logic High Required to Connect a Good DC Supply
BATDIS Logic Low Disconnects the Battery from the System
BATSEL Selects Which Battery is Connected to the System
(Logic High Selects BAT1; Logic Low Selects BAT2)
CHGSEL Selects Which Battery is Charged and Monitored
(Logic High Selects BAT1; Logic Low Selects BAT2)
3DM Forces the Main Three Power Path Switches Into
“3-Diode Mode.” See Applications Information Section
OUTPUT ACTION
DCINGOOD Logic High When a Good DC Supply is Present
LOBAT Logic Low When Selected Battery Voltage is Low
