LTC4055 Linear Technology, LTC4055 Datasheet - Page 19

• Manufacturer Part Number: LTC4055
• Description: USB Power Controller and Li-Ion Linear Charger
• Manufacturer: Linear Technology

OPERATIO
Selecting WALL Input Resistors
The WALL input pin identifies the presence of a wall
adaptor. This information is used to disconnect the inputs
IN1/IN2 from the OUT pin in order to prevent back conduc-
tion to whatever may be connected to the inputs. It also
forces the ACPR pin low when the voltage at the WALL pin
exceeds the input threshold. The WALL pin has a 1V rising
threshold and approximately 30mV of hysteresis.
It needs to be noted that this function is disabled when the
only power applied to the part is from the battery. There-
fore the 1V threshold only applies when the voltage on
either IN1/IN2 or OUT is 100mV greater than the voltage
on BAT and the voltage on IN1/IN2 or OUT is greater than
the V
The wall adapter detection threshold is set by the following
equation:
where V
old, V
R1 is the resistor from the wall adapter input to WALL and
R2 is the resistor from WALL to GND.
Consider an example where the V
somewhere around 4.5V. Resistance on the WALL pin
should be kept relatively low (~10k) in order to prevent false
tripping of the wall comparator due to leakages associated
with the switching element used to connect the adapter to
OUT. Pick R2 to be 10k and solve for R1.
R
R
V
V
HYST
TH
1
1 10
UVLO
WALL
(
Adapter
R
TH
(
2
Adapter
(Adapter) is the wall adaptor detection thresh-
(3.8V typ) threshold.
k
is the WALL pin rising threshold (typically 1V),
•
•
V
4 5
TH
)
1
.
U
V
(
)
Adapter
WALL
–
V
WALL
1
V
WALL HYST
10
•
)
k
1
• .
1
3 5 35
R
R
TH
2
1
•
(Adapter) is to be set
1
k
R
R
2
1
The nearest 1% resistor is 34.8k. Therefore R1 = 34.8k and
the rising trip point should be 4.48V.
The hysteresis is going to be approximately 124mV for
this example.
Power Dissipation
The conditions that cause the LTC4055 to reduce charge
current due to the thermal protection feedback can be
approximated by considering the power dissipated in the
part. For high charge currents and a wall adapter applied
to V
Where P
voltage, V
charge current. It is not necessary to perform any worst-
case power dissipation scenarios because the LTC4055
will automatically reduce the charge current to maintain
the die temperature at approximately 105 C. However, the
approximate ambient temperature at which the thermal
feedback begins to protect the IC is:
Example: An LTC4055 operating from a wall adapter with
5V at V
ambient temperature above, which the LTC4055 will begin
to reduce the 0.8A charge current, is approximately:
P
T
T
T
T
V
OUT
A
A
A
A
D
HYST
= 105 C – P
= 105 C – (V
= 105 C – (5V – 3V) • 0.8A • 37 C/W
= 105 C – 1.6W • 37 C/W = 105 C – 59 C = 46 C
= (V
, the LTC4055 power dissipation is approximately:
OUT
D
(
BAT
Adapter
OUT
is the power dissipated, V
providing 0.8A to a 3V Li-Ion battery. The
is the battery voltage and I
– V
BAT
D
)
OUT
•
) • I
30
JA
– V
mV
BAT
BAT
•
) • I
1
BAT
34 8
10
•
.
OUT
BAT
JA
LTC4055
is the supply
134
is the battery
mV
19
4055p