LTC4055-1_15 Datasheet, PDF (19/24 Pages) Linear Technology – USB Power Controller and Li-Ion Charger

LTC4055/LTC4055-1
OPERATION
Selecting WALL Input Resistors
The WALL input pin identifies 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_TH(Adapter)
=
V_WALL
•
⎛
⎜¹⁺
⎝
R1^⎞
R2
⎟
⎠
V_HYST (Adapter) =
V_{WALL−HYST}
•
⎛
⎜¹⁺
⎝
R1^⎞
R2
⎟
⎠
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.
R1= R2
•
⎛
⎝⎜⎜
V_TH(Adapter)
V_WALL
−
⎞
1_{⎠⎟⎟}
R1=
10k
•
⎛
⎜
⎝
4.5
1
–
⎞
1_⎟
⎠
=
10k
•
3.5
=
35k
The nearest 1% resistor is 34.8k. Therefore R1 = 34.8k
and the rising trip point should be 4.48V.
V_HYST(Adapter)
≈
30mV
•
⎛
⎜¹⁺
⎝
34.8
10
⎞
⎟
⎠
≈
134mV
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
4055fb
19