Datasheet

19
LTC1759
OPERATIO
U
The Thermistor Decoder Block
This block measures the resistance of the battery’s ther-
mistor and features high noise immunity at critical trip
points. The low power standby mode supports all SMB
charger reporting requirements when AC is not present.
The thermistor decoder is shown in Figure 4.
Thermistor sensing is accomplished by a state machine
that reconfigures the switches of Figure 4 using RNR_SELB
and RUR_SELB. The three allowable modes are as follows:
1. Overrange Detection. The RUR_SELB and RNR_SELB
switches are off and the external R
WEAK
resistor forms
a voltage divider with R
THERM
. The resulting voltage is
monitored at THERM, compared to an internal refer-
ence and sampled at the output of the OR comparator.
This detection mode is always active allowing low
power operation when AC power is not available.
2. Cold/Ideal/Hot Range Detection. The RNR_SELB
switch is on and the RUR_SELB switch is off. The
external R
NR
and R
WEAK
resistors form a voltage divider
with R
THERM
. The resulting voltage is monitored at
THERM, compared to an internal reference and sampled
at the output of the cold and hot comparators. This
detection mode is only activated if OR tested low.
3. Underrange Detection. The RNR_SELB switch is off
and the RUR_SELB switch is on. The external R
UR
and
R
WEAK
resistors form a voltage divider with R
THERM
.
The resulting voltage is monitored at RNR, compared to
an internal reference and sampled at the output of the
UR comparators. This detection mode is only activated
if HOT tested high.
NOTE: The underrange detection scheme is a very impor-
tant feature of the LTC1759. The R
UR
/R
THERM
divider trip
point of 0.333 • V
DD
(1V) is well above the 0.047 • V
DD
(140mV) threshold of a system using a 10k pull-up. A
system using a 10k pull-up would not be able to resolve the
important underrange to hot transition point with a mod-
est 100mV of ground offset between battery and ther-
mistor detection circuitry. Such offsets are anticipated
when charging at normal current levels.
Figure 4. Thermistor Decoder Block
V
DD
V
DD
R
NR
THERM
HYSTERESIS
R
WEAK
475k
1%
R
THERM
TOTAL
PARASITIC
CAPACITANCE
MUST BE LESS
THAN 75pF
R
NR
10k
1%
RNR_SELB
R
UR
1k
1%
V
DD
V
DD
THERM_COLD
THERM_UR
THERM_HOT
THERM_OR
1759 F04
RUR_SELB
+
+
+
+
THERM
LATCH
COLD
HOT
OR
UR