Datasheet

LTC4067
17
4067f
TYPICAL APPLICATIO S
U
Li-Ion Charger from 5V Wall Adapter with Overvoltage
Protection
Figure 5 illustrates an application where the LTC4067 is
used to charge a single-cell Li-Ion battery from a wall
adapter with built-in overvoltage protection.
Overvoltage protection is achieved with the OVI and OVP
pins of the LTC4067 along with an external PFET in se-
ries with the IN pin. This PFET disconnects the LTC4067
from potentially damaging overvoltage conditions that
are caused by attaching the wrong wall adapter. When
the OVI input senses a voltage greater than V
OVTH
, the
OVP output is pulled up to disable the PFET. When OVI is
below this threshold, the OVP output is pulled low, turning
on this PFET. In the event that large in-rush currents are
expected, it is recommended that a decoupling capacitor
be placed from OVI to GND. The body diode of this PFET
must be connected so that it is reverse biased when an
overvoltage condition exists.
A 10nF capacitor is recommended to dynamically pull-up on
the gate of Q1 if a fast edge occurs at the wall input during
a hot-plug. In the event that this capacitor is pre-charged
below the OVI rising threshold when a high voltage spike
occurs, the OVP output cannot guarantee turning off Q1
before the IN pin voltage exceeds the absolute maximum
voltage for this pin. This may occur in the event that the
wall input suddenly steps from 5.5V to a higher voltage.
In this case, a zener diode is also recommended to keep
the IN pin voltage to a safe level.
In the example of Figure 5, the input current limit from
the wall adapter is programmed to 1A with a 1k resistor
from CLPROG to GND (assuming I
LIM0
and I
LIM1
are held
high). And the charge current is programmed to 500mA
via the 2k resistor from the PROG pin to GND.
An optional second external PFET connected between
OUT and BAT serves as a high-performance ideal diode;
to connect the load to the battery with an extremely low
impedance. This ideal diode is controlled by the GATE
output pin whenever the wall adapter is not present or
the load demands more current than is available from
the wall adapter input (Connect the source of the PFET to
OUT and the drain to BAT).
Instantaneous monitoring of both input current and
charge current is achieved by measuring the voltages at
the CLPROG and PROG pins respectively.
Low-power shutdown is engaged by any of the following.
Disabling an external NFET tied in series with the PROG
pin resistor, by fl oating the PROG pin with a single-pole
switch, by tying R
PROG
to an open-drain output, by pulling
Figure 5. Li-Ion Charger/Controller with Overvoltage Protection
LTC4067
OUT
GATE
1-CELL
Li-Ion
10μF
IN
INPUT
CURRENT
MONITOR
LOAD
ENABLEQ3
Q2
R
PROG
2k
R
CLPROG
1k
IN
CHRG
BATNTC
I
LIM0
PROG
I
LIM1
CLPROG
GND
WALL
ADAPTER
1μF 10μF
D1
OPTIONAL
Q1
Q1, Q2: IFR7404
CHARGE CURRENT
MONITOR
4067 F05
OVP
OVI
OPTIONAL
SOURCE
DRAIN
10nF