LTC4055-1_15 Datasheet, PDF (18/24 Pages) Linear Technology – USB Power Controller and Li-Ion Charger

LTC4055/LTC4055-1
OPERATION
The nearest 1% value for R_NOM is 115k. This is the value
used to bias the NTC thermistor to get cold and hot trip
points of approximately 0°C and 44°C respectively. To
extend the delta between the cold and hot trip points a
resistor, R1, can be added in series with R_NTC (see Figure 3b).
The values of the resistors are calculated as follows:
R_NOM
=
R_COLD – R_HOT
2.815 – 0.4086
R1 =
⎛
⎝⎜
0.4086 ⎞
2.815 – 0.4086⎠⎟
•
(R_COLD
– R_HOT)
– R_HOT
where R_NOM is the value of the bias resistor, R_HOT and
R_COLD are the values of R_NTC at the desired temperature
trip points. Continuing the example from before with a
desired hot trip point of 50°C:
R_NOM
=
R_COLD – R_HOT
2.815 – 0.4086
=
100k • (3.266 – 0.3602)
2.815 – 0.4086
= 120.8k, 121k is nearest 1%
CHARGER UNDERVOLTAGE LOCKOUT
Internal undervoltage lockout circuits monitor the V_IN and
V_OUT voltages and keep the charger circuits of the part
shut down until V_IN or V_OUT rises above the undervoltage
lockout threshold. The charger UVLO circuit has a built-
in hysteresis of 125mV. Furthermore, to protect against
reverse current in the power MOSFET, the charger UVLO
circuit keeps the charger shutdown if V_BAT exceeds V_OUT.
If the charger UVLO comparator is tripped, the charger
circuits will not come out of shutdown until V_OUT exceeds
V_BAT by 50mV.
SHUTDOWN
The LTC4055/LTC4055-1 can be shut down by forcing the
SHDN pin greater than 1V. In shutdown, the currents on
IN1/IN2, OUT and BAT are decreased to less than 2.5μA
and the internal battery charge timer is reset. All power
paths are put in a Hi-Z state.
R1= 100k • ^{⎡⎣⎢⎛⎝⎜} _{2.8105.4–008.64086}^{⎞⎠⎟} • (3.266 – 0.3602) – 0.3602^{⎤⎦⎥}
= 13.3k, 13.3k is nearest 1%
The final solution is as shown if Figure 3b where R_NOM =
121k, R1 = 13.3k and R_NTC = 100k at 25°C.
CURRENT LIMIT UNDERVOLTAGE LOCKOUT
An internal undervoltage lockout circuit monitors the input
voltage and keeps the current limit circuits of the part in
shutdown mode until V_IN rises above the undervoltage
lockout threshold. The current limit UVLO circuit has a
built-in hysteresis of 125mV. Furthermore, to protect
against reverse current in the power MOSFET, the current
limit UVLO circuit keeps the current limit shutdown if
V_OUT exceeds V_IN. If the current limit UVLO comparator
is tripped, the current limit circuits will not come out of
shutdown until V_OUT falls 50mV below the V_IN voltage.
SUSPEND
The LTC4055/LTC4055-1 can be put in suspend mode by
forcing the SUSP pin greater than 1V. In suspend mode
the ideal diode function from BAT to OUT and the output
charger are kept alive. The rest of the part is shut down
to conserve current and the battery charge timer is reset
if V_OUT becomes less than V_BAT.
V_IN and Wall Adapter Bypass Capacitor
Many types of capacitors can be used for input bypassing.
However, caution must be exercised when using multilayer
ceramic capacitors. Because of the self resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions, such as connecting the charger input to a hot
power source. For more information, refer to Application
Note 88.
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