LTC4098/LTC4098-1
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Introduction
The LTC4098/LTC4098-1 are high efficiency power manage-
ment and Li-Ion charger solutions designed to make optimal
use of the power available from a variety of sources, while
minimizing power dissipation and easing thermal budgeting
constraints. The innovative PowerPath architecture ensures
that the application is powered immediately after external
voltage is applied, even with a completely dead battery by
prioritizing power to the application over the battery.
The LTC4098/LTC4098-1 include a Bat-Track monolithic
step-down switching regulator for USB, wall adapters and
other 5V sources. Designed specifically for USB applica-
tions, the switching regulator incorporates a precision
average input current limit for USB compatibility. Because
power is conserved, the LTC4098/LTC4098-1 allow the load
current on V
OUT
to exceed the current drawn by the USB
port, making maximum use of the allowable USB power
for battery charging.
The switching regulator and battery charger communicate
to ensure that the average input current never exceeds the
USB specifications.
For automotive, Firewire, and other high voltage applica-
tions, the LTC4098/LTC4098-1 provide Bat-Track control
of an external LTC step-down switching regulator to
maximize battery charger efficiency and minimize heat
production.
When power is available from both the USB and high
voltage inputs, the high voltage input is prioritized and
the USB input is automatically disabled.
The LTC4098/LTC4098-1 feature an overvoltage protec-
tion circuit which is designed to work with an external
N-channel FET to prevent damage to its inputs caused by
accidental application of high voltage.
The LTC4098/LTC4098-1 contain both an internal 180mΩ
ideal diode and an ideal diode controller designed for use
with an external P-channel FET. The ideal diodes from BAT
to V
OUT
guarantee that ample power is always available
to V
OUT
even if there is insufficient or absent power at
V
BUS
or WALL.
Finally, to prevent battery drain when a device is connected
to a suspended USB port, an LDO from V
BUS
to V
OUT
provides either low power or high power USB suspend
current to the application.
Bat-Track Input Current Limited Step Down Switching
Regulator
The power delivered from V
BUS
to V
OUT
is controlled
by a 2.25MHz constant-frequency step-down switching
regulator. To meet the USB maximum load specification,
the switching regulator contains a measurement and
control system that ensures that the average input cur-
rent remains below the level programmed at CLPROG.
V
OUT
drives the combination of the external load and the
battery charger.
If the combined load does not cause the switching power
supply to reach the programmed input current limit, V
OUT
will track approximately 0.3V above the battery voltage.
By keeping the voltage across the battery charger at this
low level, power lost to the battery charger is minimized.
Figure 1 shows the power path components.
If the combined external load plus battery charge current is
large enough to cause the switching power supply to reach
the programmed input current limit, the battery charger
will reduce its charge current by precisely the amount
necessary to enable the external load to be satisfied. Even
if the battery charge current is programmed to exceed the
allowable USB current, the USB specification for average
input current will not be violated; the battery charger will
reduce its current as needed. Furthermore, if the load cur-
rent at V
OUT
exceeds the programmed power from V
BUS
,
load current will be drawn from the battery via the ideal
diodes even when the battery charger is enabled.
The current at CLPROG is a precise fraction of the V
BUS
current. When a programming resistor and an averaging
capacitor are connected from CLPROG to GND, the voltage
on CLPROG represents the average input current of the
switching regulator. As the input current approaches the
programmed limit, CLPROG reaches 1.188V and power
delivered by the switching regulator is held constant.
Several ratios of current are available which can be set
to correspond to USB low and high power modes with a
single programming resistor.
The input current limit is programmed by various com-
binations of the D0, D1 and D2 pins as shown in Table 1.
OPERATION
