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LTC1759

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

switches are off and the external RWEAK resistor forms

a voltage divider with RTHERM. 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 RNR and RWEAK resistors form a voltage divider

with RTHERM. 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 RUR and

RWEAK resistors form a voltage divider with RTHERM.

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 RUR/RTHERM divider trip

point of 0.333 • VDD (1V) is well above the 0.047 • VDD

(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

VDD

RNR

THERM

HYSTERESIS

RWEAK

475k

RTHERM

TOTAL

PARASITIC

CAPACITANCE

MUST BE LESS

THAN 75pF

RNR

10k

RNR_SELB

RUR

VDD

THERM_COLD

THERM_UR

THERM_HOT

THERM_OR

1759 F04

RUR_SELB

THERM

LATCH

COLD

HOT

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