ltc4069edc-trpbf Linear Technology Corporation, ltc4069edc-trpbf Datasheet - Page 12

• Manufacturer Part Number: ltc4069edc-trpbf
• Description: Standalone 750ma Li-ion Battery Charger In 2 X 2 Dfn With Ntc Thermistor Input
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC4069
will demand a charge current higher than the current limit
of the voltage 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
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
240mW. That is a 76% improvement over the non-current
limited supply power dissipation.
USB and Wall Adapter Power
Although the LTC4069 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 significantly 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 contains two control loops: constant-volt-
age and constant-current. The constant-voltage loop is
stable without any compensation when a battery is con-
nected with low impedance leads. Excessive lead length,
however, may add enough series inductance to require a
12
ADAPTER
5V WALL
POWER
750mA
500mA
Figure 4. Combining Wall Adapter and USB Power
I
I
USB
CHG
CHG
MP1
U
1k
D1
U
MN1
V
CC
LTC4069
4.02k
PROG
BAT
W
I
CHG
2k
+
4069 F04
Li-Ion
BATTERY
U
SYSTEM
LOAD
Figure 5. Isolating Capacitive Load on the PROG Pin and Filtering
bypass capacitor of at least 1µF from BAT to GND. Further-
more, 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 ce-
ramic) may reduce the constant-voltage loop phase mar-
gin. 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, addi-
tional 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
equation should be used to calculate the maximum resis-
tance value for R
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
filter can be used on the PROG pin to measure the average
R
PROG
≤
LTC4069
2
GND
π
PROG
•
PROG
10
5
1
•
:
C
R
PROG
PROG
10k
4069 F05
C
FILTER
PROG
CHARGE
CURRENT
MONITOR
CIRCUITRY
, the following
4069fa