LTC4098/LTC4098-1
16
40981fc
With Bat-Track, battery charger efficiency is approximately:
η
TOTAL
=η
BUCK
•
BAT
BAT + 0.3V
With the same assumptions as above, the total battery
charger efficiency is approximately 81%. This example
works out to less than 1W of power dissipation, or almost
60% less heat.
See the Typical Applications section for complete circuits
using the LT3653 and LT3480 with Bat-Track control.
Overvoltage Protection
The LTC4098/LTC4098-1 can protect itself from the inad-
vertent application of excessive voltage to V
BUS
or WALL
with just two external components: an N-channel FET and
a 6.04k resistor. The maximum safe overvoltage magnitude
will be determined by the choice of the external NMOS and
its associated drain breakdown voltage.
The overvoltage protection module consists of two pins.
The first, OVSENS, is used to measure the externally ap-
plied voltage through an external resistor. The second,
OVGATE, is an output used to drive the gate pin of an
external FET. The voltage at OVSENS will be lower than
the OVP input voltage by (I
OVSENS
• 6.04k) due to the
OVP circuit’s quiescent current. The OVP input will be
200mV to 400mV higher than OVSENS under normal
operating conditions. When OVSENS is below 6V, an in-
ternal charge pump will drive OVGATE to approximately
1.88 • OVSENS. This will enhance the N-channel FET and
provide a low impedance connection to V
BUS
or WALL
which will, in turn, power the LTC4098/LTC4098-1. If
OVSENS should rise above 6V (6.35V OVP input) due to
a fault or use of an incorrect wall adapter, OVGATE will
be pulled to GND, disabling the external FET to protect
downstream circuitry. When the voltage drops below 6V
again, the external FET will be reenabled.
In an overvoltage condition, the OVSENS pin will be
clamped at 6V. The external 6.04k resistor must be sized
appropriately to dissipate the resultant power. For example,
a 1/10W 6.04k resistor can have at most √P
MAX
• 6.04kΩ
= 24V applied across its terminals. With the 6V at OVSENS,
the maximum overvoltage magnitude that this resistor can
withstand is 30V. A 1/4W 6.04k resistor raises this value
to 44V. WALL’s absolute maximum current rating of 10mA
imposes an upper protection limit of 66V.
The charge pump output on OVGATE has limited output
drive capability. Care must be taken to avoid leakage on
this pin, as it may adversely affect operation.
See the Applications Information section for examples of
multiple input protection, reverse input protection, and a
table of recommended components.
Ideal Diode from BAT to V
OUT
The LTC4098/LTC4098-1 have an internal ideal diode as
well as a controller for an external ideal diode. Both the
internal and the external ideal diodes are always on and
will respond quickly whenever V
OUT
drops below BAT.
If the load current increases beyond the power allowed
from the switching regulator, additional power will be pulled
from the battery via the ideal diodes. Furthermore, if power
to V
BUS
(USB or wall power) is removed, then all of the
application power will be provided by the battery via the
ideal diodes. The ideal diodes will be fast enough to keep
V
OUT
from drooping with only the storage capacitance
required for the switching regulator. The internal ideal
diode consists of a precision amplifier that activates a
large on-chip MOSFET transistor whenever the voltage at
V
OUT
is approximately 15mV (V
FWD
) below the voltage at
BAT. Within the amplifier’s linear range, the small-signal
resistance of the ideal diode will be quite low, keeping
the forward drop near 15mV. At higher current levels, the
OPERATION
