Datasheet

LTC4055/LTC4055-1
18
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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
RR
RRRR
NOM
COLD HOT
COLD HOT HOT
=
=
⎛
⎝
⎜
⎞
⎠
⎟
()
–
.–.
.
.–.
•––
2 815 0 4086
1
0 4086
2 815 0 4086
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
RR
k
Rk
NOM
COLD HOT
==
()
=
=
⎛
⎝
⎜
⎞
⎠
⎟
()
⎡
⎣
⎢
⎤
⎦
⎥
=
–
.–.
•. –.
.–.
•
.
.–.
• . –. –.
2 815 0 4086
100 3 266 0 3602
2 815 0 4086
1 100
0 4086
2 815 0 4086
3 266 0 3602 0 3602
120.8k, 121k is nearest 1%
13.3k, 13.3k is nearest 1%
The fi nal 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.
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.
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.