LTC4098/LTC4098-1
15
40981fc
For very low battery voltages, the battery charger acts like
a load and, due to limited input power, its current will tend
to pull V
OUT
below the 3.6V instant-on voltage. To prevent
V
OUT
from falling below this level, an undervoltage circuit
automatically detects that V
OUT
is falling and reduces the
battery charge current as needed. This reduction ensures
that load current and voltage are always prioritized while
allowing as much battery charge current as possible. Refer
to Overprogramming the Battery Charger in the Applica-
tions Information section.
The voltage regulation loop compensation is controlled by
the capacitance on V
OUT
. An MLCC capacitor of 10μF is
required for loop stability. Additional capacitance beyond
this value will improve transient response.
An internal undervoltage lockout circuit monitors V
BUS
and
keeps the switching regulator off until V
BUS
rises above
the rising UVLO threshold (4.3V). If V
BUS
falls below the
falling UVLO threshold (4V), system power at V
OUT
will
be drawn from the battery via the ideal diodes. The volt-
age at V
BUS
must also be higher than the voltage at BAT
by approximately 170mV for the switching regulator to
operate.
Bat-Track High Voltage External Switching Regulator
Control
The WALL, ACPR and V
C
pins can be used in conjunction
with an external high voltage step-down switching regulator
such as the LT3653 or LT3480 to minimize heat production
when operating from higher voltage sources, as shown in
Figures 3 and 4. Bat-Track control circuitry regulates the
external switching regulator’s output voltage to the larger
of BAT + 300mV or 3.6V. This maximizes battery charger
efficiency while still allowing instant-on operation when
the battery is deeply discharged.
When using the LT3480, the feedback network should be set
to generate an output voltage between 4.5V and 5.5V. When
high voltage is applied to the external regulator, WALL will
rise toward this programmed output voltage. When WALL
exceeds approximately 4.3V, ACPR is brought low and the
Bat-Track control of the LTC4098/LTC4098-1 overdrives
the local V
C
control of the external high voltage step-down
switching regulator. Therefore, once the Bat-Track control
is enabled, the output voltage is set independent of the
switching regulator feedback network.
Bat-Track control provides a significant efficiency advantage
over the simple use of a 5V switching regulator output to
drive the battery charger. With a 5V output driving V
OUT
,
battery charger efficiency is approximately:
η
TOTAL
=η
BUCK
•
V
BAT
5V
where η
BUCK
is the efficiency of the high voltage switching
regulator and 5V is the output voltage of the switching
regulator. With a typical switching regulator efficiency of
87% and a typical battery voltage of 3.8V, the total battery
charger efficiency is approximately 66%. Assuming a 1A
charge current, nearly 2W of power is dissipated just to
charge the battery!
OPERATION
ACPR
40981 F03
LTC4098/
LTC4098-1
V
OUT
V
OUT
I
SENSE
WALLV
C
LT3653
V
C
HVOK
SYSTEM
LOAD
SW
ACPR
40981 F04
LTC4098/
LTC4098-1
V
OUT
WALLV
C
LT3480
V
C
SYSTEM
LOAD
SW
FB
Figure 3. LT3653 Typical Interface Figure 4. LT3480 Typical Interface
