LTC4055/LTC4055-1
19
4055fb
OPERATION
Selecting WALL Input Resistors
The WALL input pin identifi es the presence of a wall adapter.
This information is used to disconnect the inputs IN1/IN2
from the OUT pin in order to prevent back conduction 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.
Therefore the 1V threshold only applies when the voltage
on either IN1/IN2 or OUT is 100mV greater than the volt-
age on BAT and the voltage on IN1/IN2 or OUT is greater
than the V
UVLO
(3.8V typ) threshold.
The wall adapter detection threshold is set by the follow-
ing equation:
V Adapter V
R
R
V Adapte
TH WALL
HYST
() •
(
=+
⎛
⎝
⎜
⎞
⎠
⎟
1
1
2
rrV
R
R
W ALL HYST
)•=+
⎛
⎝
⎜
⎞
⎠
⎟
−
1
1
2
where V
TH
(Adapter) is the wall adapter detection threshold,
V
WALL
is the WALL pin rising threshold (typically 1V), 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
TH
(Adapter) is to be set
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.
RR
V Adapter
V
Rk
TH
WALL
12 1
110
45
= −
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
•
()
•
.
11
1103535–•.
⎛
⎝
⎜
⎞
⎠
⎟
==kk
The nearest 1% resistor is 34.8k. Therefore R1 = 34.8k
and the rising trip point should be 4.48V.
V Adapter mV mV
HYST
() •
.
≈ +
⎛
⎝
⎜
⎞
⎠
⎟
≈30 1
34 8
10
134
The hysteresis is going to be approximately 134mV for
this example.
Power Dissipation
The conditions that cause the LTC4055/LTC4055-1 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
OUT
, the LTC4055/LTC4055-1 power
dissipation is approximately:
P
D
= (V
OUT
– V
BAT
) • I
BAT
where P
D
is the power dissipated, V
OUT
is the supply
voltage, V
BAT
is the battery voltage and I
BAT
is the battery
charge current. It is not necessary to perform any worst-
case power dissipation scenarios because the LTC4055/
LTC4055-1 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:
T
A
= 105°C – P
D
• θ
JA
T
A
= 105°C – (V
OUT
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4055/LTC4055-1 operating from a wall
adapter with 5V at V
OUT
providing 0.8A to a 3V Li-Ion
battery. The ambient temperature above, which the
LTC4055/LTC4055-1 will begin to reduce the 0.8A charge
current, is approximately:
T
A
= 105°C – (5V – 3V) • 0.8A • 37°C/W
T
A
= 105°C – 1.6W • 37°C/W = 105°C – 59°C = 46°C
