ltc4078 Linear Technology Corporation, ltc4078 Datasheet - Page 12

• Manufacturer Part Number: ltc4078
• Description: Dual Input Li-ion Battery Charger With Overvoltage Protection
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC4078/LTC4078X
Using a Single Charge Current Program Resistor
The LTC4078/LTC4078X can program the wall adapter
charge current and USB charge current independently using
two program resistors, R
charger circuit that sets the wall adapter charge current
to 800mA and the USB charge current to 500mA.
In applications where the programmed wall adapter
charge current and USB charge current are the same, a
single program resistor can be used to set both charge
currents. Figure 3 shows a charger circuit that uses one
charge current program resistor.
In this circuit, the programmed charge current from both the
wall adapter supply is the same value as the programmed
charge current from the USB supply:
12
Figure 2. Dual Input Charger with Independent Charge Currents
ADAPTER
ADAPTER
I
WALL
PORT
CHRG DC
WALL
PORT
USB
USB
Figure 3. Dual Input Charger Circuit. The Wall Adapter
Charge Current and USB Charge Current Are Both
Programmed to Be 500mA
1μF
1μF
C1
C1
= I
C2, 1μF
CHRG USB
C2, 1μF
R1
2k
1%
R1
2k
1%
R2
1.24k
1%
DCIN
USBIN
IUSB
IDC
DCIN
USBIN
IUSB
IDC
IDC
=
LTC4078
LTC4078
1000V
GND
and R
R
GND
BATDET
BATDET
ISET
ITERM
ITERM
BAT
BAT
IUSB
R3
2k
1%
R3
2k
1%
. Figure 2 shows a
800mA (WALL)
500mA (USB)
R4
3.9k
R4
3.9k
500mA
+
+
4.2V
Li-Ion
BATTERY
PACK
4.2V
Li-Ion
BATTERY
PACK
4078X F02
4078X F03
Stability Considerations
The constant-voltage mode feedback loop is stable without
any compensation provided a battery is connected to the
charger output. However, a 1μF capacitor with a 1Ω series
resistor is recommended at the BAT pin to keep the ripple
voltage low when the battery is disconnected.
When the charger is in constant-current mode, the charge
current program pin (IDC or IUSB) is in the feedback loop,
not the battery. The constant-current mode stability is af-
fected by the impedance at the charge current program pin.
With no additional capacitance on this pin, the charger is
stable with program resistor values as high as 20k (I
= 50mA); however, additional capacitance on these nodes
reduces the maximum allowed program resistor.
Power Dissipation
When designing the battery charger circuit, it is not
necessary to design for worst-case power dissipation
scenarios because the LTC4078/LTC4078X automatically
reduce the charge current during high power conditions.
The conditions that cause the LTC4078/LTC4078X to
reduce charge current through thermal feedback can be
approximated by considering the power dissipated in the
IC. Most of the power dissipation is generated from the
internal charger MOSFET. Thus, the power dissipation is
calculated to be:
P
age (either DCIN or USBIN), V
and I
temperature at which the thermal feedback begins to
protect the IC is:
Example: An LTC4078/LTC4078X operating from a 5V wall
adapter (on the DCIN input) is programmed to supply
800mA full-scale current to a discharged Li-Ion battery
with a voltage of 3.3V.
D
P
T
T
is the dissipated power, V
A
A
D
BAT
= 120°C – P
= 120°C – (V
= (V
is the charge current. The approximate ambient
IN
– V
BAT
D
IN
) • I
• θ
– V
JA
BAT
BAT
) • I
IN
BAT
BAT
is the input supply volt-
is the battery voltage
• θ
JA
CHRG
4078xfb