LTC4097
15
4097f
APPLICATIONS INFORMATION
LTC4097
DCIN
USBIN
IUSB
IDC
BAT
HPWR
ITERM
R
ISET
2k
1%
R
ITERM
2k
1%
WALL
ADAPTER
USB
PORT
1µF1µF
+
100mA
(USB, HPWR = LOW)
500mA
4097 F02
GND
4.2V
1-CELL
Li-Ion
BATTERY
LTC4097
DCIN
USBIN
IUSB
IDC
BAT
VNTC
HPWR
NTC
CHRG
ITERM
R
IDC
1.24k
1%
WALL
ADAPTER
USB
PORT
1µF1µF
+
800mA (WALL)
100mA/500mA (USB)
4097 F03
GND
R
IUSB
2k
1%
R
NTC
100k
R
NTCBIAS
100k
1k
4.2V
1-CELL
Li-Ion
BATTERY
R
ITERM
2k
1%
Figure 2. Dual Input Charger Circuit. The
Wall Adapter Charge Current and USB Charge
Current are Both Programmed to be 500mA
Figure 3. Full Featured Dual Input Charger Circuit
Using a Single Charge Current Program Resistor
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 2 shows a charger circuit that uses one charge cur-
rent program resistor. In this circuit, one resistor programs
the same charge current for each input supply.
II
V
R
CHRG DC CHRG USB
SET
() ( )
==
1000
The LTC4097 can also program the wall adapter charge
current and USB charge current independently using two
program resistors, R
IDC
and R
IUSB
. Figure 3 shows a
charger circuit that sets the wall adapter charge current
to 800mA and the USB charge current to 500mA.
Stability Considerations
The constant-voltage mode feedback loop is stable without
any compensation provided a battery is connected to the
charger output. However, a 4.7µF 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 affected 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 20KΩ (I
CHRG
=
50mA); however, additional capacitance on these nodes
reduces the maximum allowed program resistor.
Power Dissipation
When designing the battery charger circuit, it is not neces-
sary to design for worst-case power dissipation scenarios
because the LTC4097 automatically reduces the charge
current during high power conditions. The conditions
that cause the LTC4097 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 MOSFET pass device. Thus,
the power dissipation is calculated to be:
P
D
= (V
CC
– V
BAT
) • I
BAT
P
D
is the power dissipated, V
CC
is the input supply volt-
age (either DCIN or USBIN), V
BAT
is the battery voltage
and I
BAT
is the charge current. The approximate ambient
temperature at which the thermal feedback begins to
protect the IC is:
T
A
= 115°C – P
D
• θ
JA
T
A
= 115°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4097 operating from a 5V USB adapter
(on the USBIN input) is programmed to supply 500mA
full-scale current to a discharged Li-Ion battery with a
voltage of 3.3V. Assuming θ
JA
is 60°C/W (see Thermal
Considerations), the ambient temperature at which the
LTC4097 will begin to reduce the charge current is ap-
proximately:
T
A
= 115°C – (5V – 3.3V) • (500mA) • 60°C/W
T
A
= 115°C – 0.85W • 60°C/W = 115°C – 51°C
T
A
= 64°C
