LTC4099EUDC View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4099EUDC' PDF : 36 Pages View PDF
LTC4099
Operation
the battery to charge. If the input-referred load current
exceeds the input current limit at VBUS, VOUT 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, VOUT 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 VOUT 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 VOUT below the 3.6V instant-on
voltage. To prevent VOUT from falling below this level, an
undervoltage circuit automatically detects that VOUT 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 VOUT. 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 VBUS and
keeps the switching regulator off until VBUS rises above
the rising VUVLO threshold (4.3V). If VBUS falls below the
falling VUVLO threshold (4V), system power at VOUT will
be drawn from the battery via the ideal diodes. The volt-
age at VBUS must also be higher than the voltage at BAT
by VDUVLO, 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
VC 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
16
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 brought
low, and the Bat-Track control of the LTC4099 overdrives
the local VC 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 VOUT,
battery charger efficiency is approximately:
hTOTAL
=
hBUCK
•
VBAT
5V
where hBUCK 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:
hTOTAL
=
hBUCK
•
VBAT
VBAT + 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 VOUT
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 VOUT drops below BAT.
4099fd
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