Datasheet
LTC4099
16
4099fd
the battery to charge. If the input-referred load current
exceeds the input current limit at V
BUS
, V
OUT
will range
between the no-load voltage and slightly below the battery
voltage as indicated by the shaded region of Figure 2. If
there is no battery present when this happens, V
OUT
may
collapse to ground. In such cases the input-referred load
current should be maintained below the programmed input
current level in order to keep the V
OUT
and BAT voltages
within specified limits.
For very low battery voltages, the battery charger acts
like a load and, due to the input current limit circuit, 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. See Overprogramming the Battery
Charger in the Applications 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 V
UVLO
threshold (4.3V). If V
BUS
falls below the
falling V
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 V
DUVLO
, or approximately 200mV, for the switching
regulator to operate.
Bat-Track Auxiliary High Voltage Switching Regulator
Control
As shown in the Block Diagram, the WALL, ACPR and
V
C
pins can be used in conjunction with an external high
voltage Linear Technology step-down switching regula-
tor,
such as the LT3480 or LT3653, to minimize heat
production when operating from higher voltage sources.
Bat-Track control circuitry regulates the external switching
regulator’s output voltage to the larger of BAT + 300mV
or 3.6V in much the same way as the internal switching
regulator. This maximizes battery charger efficiency while
still allowing instant-on operation when the battery is
deeply discharged.
The feedback network of the high voltage regulator should
be set to program 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 b
rought
low, and the Bat-Track control of the LTC4099 overdrives
the local V
C
control of the external high voltage step-down
switching regulator. Once the Bat-Track control is enabled,
the output voltage is independent of the switching regula
-
tor
feedback network.
Bat-Track control provides a significant efficiency advantage
over the use of a simple 5V switching regulator output to
drive the battery charger. With a 5V output driving V
OUT
,
battery charger efficiency is approximately:
h
TOTAL
= h
BUCK
•
V
BAT
5V
where h
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!
With Bat-Track, battery charger efficiency is approximately:
h
TOTAL
= h
BUCK
•
V
BAT
V
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 LT3480 and LT3653 with Bat-Track control.
Ideal Diode from BAT to V
OUT
The LTC4099 has an internal ideal diode as well as a con-
troller 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.
operaTion