LTC4075 LINER [Linear Technology], LTC4075 Datasheet - Page 12

• Manufacturer Part Number: LTC4075
• Description: Dual Input USB/AC Adapter Standalone Li-Ion Battery Chargers
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC4075/LTC4075X
Using a Single Charge Current Program Resistor
The LTC4075 can program the wall adapter charge current
and USB charge current independently using two program
resistors, R
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.
12
ADAPTER
ADAPTER
WALL
PORT
USB
WALL
PORT
USB
Figure 2. Full Featured Dual Input Charger Circuit
Figure 3. Dual Input Charger Circuit. The Wall
Adapter Charge Current and USB Charge Current
are Both Programmed to be 500mA
IDC
1µF
1µF
and R
1µF
U
IUSB
1µF
R
2k
1%
IUSB
. Figure 2 shows a charger circuit
R
2k
1%
ISET
U
R
1.24k
1%
IDC
DCIN
USBIN
IUSB
IDC
DCIN
USBIN
IUSB
IDC
LTC4075
GND
LTC4075
W
GND
ITERM
BAT
ITERM
BAT
R
1k
1%
ITERM
800mA (WALL)
R
1k
1%
500mA (USB)
500mA
U
ITERM
+
+
4075 F03
4075 F02
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:
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
affected by the impedance at the charge current program
pin. With no additional capacitance on this pin, the char-
ger is stable with program resistor values as high as 20k
(I
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 LTC4075 automatically reduces the charge
current during high power conditions. The conditions
that cause the LTC4075 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:
CHRG
P
I
CHRG DC
D
= (V
= 50mA); however, additional capacitance on these
−
IN
– V
=
I
CHRG USB
BAT
) • I
−
BAT
=
1000
R
ISET
V
4075Xf