Datasheet

20
LTC1759
OPERATIO
U
When AC power is not available the thermistor block
supports the following low power operating features:
1. Only the low power THERM_OR detection circuitry is
kept alive to support battery present interrupts.
2. The ChargeStatus() read function forces the thermistor
block to update thermistor status at the beginning of the
read. The low power mode is immediately reentered
upon completion of the read.
The thermistor impedance is interpreted according to
Table 4.
Table 4. Thermistor State Ranges
THERMISTOR CHARGE
RESISTANCE STATUS BITS DESCRIPTION
0Ω to 500Ω THERM_UR, Underrange
THERM_HOT
BATTERY_PRESENT
500Ω to 3kΩ THERM_HOT Hot
BATTERY_PRESENT
3kΩ to 30kΩ (NONE) Ideal
BATTERY_PRESENT
30kΩ to 100kΩ THERM_COLD Cold
BATTERY_PRESENT
Above 100kΩ THERM_OR Overrange
THERM_COLD
The required values for R
WEAK
, R
UR
and R
NR
are shown in
Table 5.
Table 5. Thermistor External Resistor Values
EXTERNAL RESISTOR VALUE ( Ω )
R
WEAK
475k ±1%
R
UR
1k ±1%
R
NR
10k ±1%
Note: The maximum allowed total external capacitance on THERM and
RNR is 75pF, due to settling time requirements.
Figure 5. Simplified V
LIMIT
Circuit Concept (I
LIMIT
Is Similar)
provide a measure of safety with a hardware restriction on
charging current which cannot be overridden by software.
V
LIMIT
12.5k
25k
33k
25k
25k
12.5k
R
LIMIT
V
DD
V
LIM
[3:0]
1759 F05
AC_PRESENT
4
+
–
+
–
+
–
+
–
ENCODER
The V
LIMIT
Decoder Block
The value of an external resistor connected from this pin
to GND determines one of five voltage limits that are
applied to the charger output value. These limits provide
a measure of safety with a hardware restriction on charg-
ing voltage which cannot be overridden by software.
Table 6. I
LIMIT
Trip Points and Ranges
EXTERNAL NOMINAL
RESISTOR CHARGING
(R
ILIMIT
)I
LIMIT
VOLTAGE CURRENT RANGE GRANULARITY
0V
ILIMIT
< 0.09V
DD
0 < I < 1023mA 1mA
10k ±1% 0.17V
VDD
< V
ILIMIT
0 < I < 2046mA 2mA
< 0.34V
VDD
33k ±1% 0.42V
VDD
< V
ILIMIT
0 < I < 4092mA 4mA
< 0.59V
Open (>250k, 0.66V
VDD
< V
ILIMIT
0 < I < 8184mA 8mA
or Short to
V
DD
)
The I
LIMIT
Decoder Block
The value of an external resistor connected from this pin
to GND determines one of four current limits that are used
to limit the maximum charging current value. These limits