LTC4069-4.4
12
406944fa
APPLICATIONS INFORMATION
a programmed current higher than 600mA. Since the
LTC4069-4.4 will demand a charge current higher than
the current limit of the input supply, the supply voltage
will drop to the battery voltage plus 600mA times the on-
resistance of the internal PFET. The on-resistance of the
LTC4069-4.4 power device is approximately 450mΩ with a
5V supply. The actual on-resistance will be slightly higher
due to the fact that the input supply will drop to less than
5V. The power dissipated during this phase of charging
is less than 180mW. That is a 82% improvement over the
non-current limited supply power dissipation.
USB and Wall Adapter Power
Although the LTC4069-4.4 allows charging from a USB
port, a wall adapter can also be used to charge Li-Ion
batteries. Figure 4 shows an example of how to combine
wall adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB
port when a wall adapter is present and Schottky diode,
D1, is used to prevent USB power loss through the 1k
pull-down resistor.
Typically a wall adapter can supply signifi cantly more
current than the 500mA-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
are used to increase the charge current to 750mA when
the wall adapter is present.
Stability Considerations
The LTC4069-4.4 contains two control loops: constant-
voltage and constant-current. The constant-voltage loop
is stable without any compensation when a battery is
connected with low impedance leads. Excessive lead
length, however, may add enough series inductance to
require a bypass capacitor of at least 1μF from BAT to
GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
High value capacitors with very low ESR (especially
ceramic) may reduce the constant-voltage loop phase
margin. Ceramic capacitors up to 22μF may be used
in parallel with a battery, but larger ceramics should be
decoupled with 0.2Ω to 1Ω of series resistance.
In constant-current mode, the PROG pin is in the feedback
loop, not the battery. Because of the additional pole created
by the PROG pin capacitance, capacitance on this pin must
be kept to a minimum. With no additional capacitance on
the PROG pin, the charger is stable with program resistor
values as high as 25k. However, additional capacitance
on this node reduces the maximum allowed program
resistor. The pole frequency at the PROG pin should be kept
above 100kHz. Therefore, if the PROG pin is loaded with a
capacitance, C
PROG
, the following equation should be used
to calculate the maximum resistance value for R
PROG
:
R
C
PROG
PROG
≤
π
1
210
5
••
Average, rather than instantaneous, battery current may be
of interest to the user. For example, if a switching power
supply operating in low current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
fi lter can be used on the PROG pin to measure the average
battery current as shown in Figure 5. A 10k resistor has
been added between the PROG pin and the fi lter capacitor
to ensure stability.
V
CC
MP1
MN1
1k
2k
4.02k
I
CHG
D1
Li-Ion
BATTERY
SYSTEM
LOAD
4069 F04
LTC4069-4.4
BAT
USB
POWER
500mA
I
CHG
5V WALL
ADAPTER
750mA
I
CHG
PROG
+
Figure 5. Isolating Capacitive Load on the PROG Pin and Filtering
Figure 4. Combining Wall Adapter and USB Power
4069 F05
C
FILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
R
PROG
LTC4069-4.4
PROG
GND
10k
