ltc1759cg-trpbf Linear Technology Corporation, ltc1759cg-trpbf Datasheet - Page 19

• Manufacturer Part Number: ltc1759cg-trpbf
• Description: Smart Battery Charger
• Manufacturer: Linear Technology Corporation

OPERATIO
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
2. Cold/Ideal/Hot Range Detection. The RNR_SELB
switches are off and the external R
a voltage divider with R
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.
switch is on and the RUR_SELB switch is off. The
external R
MUST BE LESS
CAPACITANCE
PARASITIC
THAN 75pF
TOTAL
NR
and R
R
475k
1%
WEAK
U
R
10k
1%
WEAK
NR
R
THERM
R
1%
1k
UR
THERM
resistors form a voltage divider
THERM
. The resulting voltage is
V
R
DD
NR
WEAK
HYSTERESIS
RNR_SELB
resistor forms
Figure 4. Thermistor Decoder Block
V
DD
V
DD
3. Underrange Detection. The RNR_SELB switch is off
NOTE: The underrange detection scheme is a very impor-
tant feature of the LTC1759. The R
point of 0.333 • V
(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.
V
DD
with R
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.
and the RUR_SELB switch is on. The external R
R
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.
RUR_SELB
WEAK
+
–
+
–
THERM
UR
OR
resistors form a voltage divider with R
+
+
COLD
–
–
HOT
. The resulting voltage is monitored at
DD
(1V) is well above the 0.047 • V
THERM
LATCH
THERM_COLD
THERM_UR
THERM_HOT
THERM_OR
UR
1759 F04
/R
THERM
LTC1759
divider trip
19
THERM
UR
and
DD
.