LTC4077
13
4077fa
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, IUSB or IUSBL) is in the feed-
back 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 20k; 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 LTC4077 automatically reduces the charge
current during high power conditions. The conditions
that cause the LTC4077 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
IN
– V
BAT
) • I
BAT
P
D
is the power dissipated, V
IN
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
= 105°C – P
D
• θ
JA
T
A
= 105°C – (V
IN
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4077 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.
Assuming θ
JA
is 40°C/W (see Thermal Considerations),
the ambient temperature at which the LTC4077 will begin
to reduce the charge current is approximately:
T
A
= 105°C – (5V – 3.3V) • (800mA) • 40°C/W
T
A
= 105°C – 1.36W • 40°C/W = 105°C – 54.4°C
T
A
= 50.6°C
The LTC4077 can be used above 50.6°C ambient, but
the charge current will be reduced from 800mA. The ap-
proximate current at a given ambient temperature can be
approximated by:
I
C T
V V
BAT
A
IN BAT JA
=
°105 ±
( ±
•
)
q
Using the previous example with an ambient tem-
perature of 60°C, the charge current will be reduced to
approximately:
I
C C
V V C W
C
C A
I m
A
BAT
BAT
=
° °
°
=
°
°
=
105 60
5 3 3 4
0
45
68
662
±
( ± . ) • /
/
It is important to remember that LTC4077 applications do
not need to be designed for worst-case thermal conditions,
since the IC will automatically reduce power dissipation
when the junction temperature reaches approximately
105°C.
APPLICATIO S I FOR ATIO
W UU
U
