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LTC4101

4101f

The SafetySignal impedance is interpreted according to

Table 4.

Table 4. SafetySignal State Ranges

SafetySignal

CHARGE

RESISTANCE

STATUS BITS

DESCRIPTION

Ω to 500Ω

RES_UR,

Underrange

RES_HOT

BATTERY_PRESENT

500

Ω to 3kΩ

RES_HOT

Hot

BATTERY_PRESENT

Ω to 30kΩ

BATTERY_PRESENT

Ideal

30k

Ω to 100kΩ

RES_COLD

Cold

BATTERY_PRESENT

Above 100k

Ω

RES_OR

Overrange

RES_COLD

Note: The underrange detection scheme is a very important feature of the

LTC4101. The RTHA/RSafetySignal 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 modest 100mV of

ground offset between battery and SafetySignal detection circuitry. Such

offsets are anticipated when charging at normal current levels.

The required values for RTHA and RTHB are shown in

Table 5.

Table 5. SafetySignal External Resistor Values

EXTERNAL RESISTOR

VALUE (

Ω)

RTHA

1130

±1%

RTHB

54.9k

±1%

CSS represents the capacitance between the SafetySignal

and GND. CSS may be added to provide additional noise

immunity from transients in the application. CSS cannot

exceed 1nF if the LTC4101 is to properly sense the value

of RSafetySignal.

OPERATIO

When AC is present, the LTC4101 samples the value of the

SafetySignal and updates the ChargerStatus register ap-

proximately every 32ms. The state machine successively

samples the SafetySignal value starting with the RES_OR

≥ RES_COLD threshold, then RES_C0LD ≥ RES_IDEAL

threshold, RES_IDEAL

≥ RES_HOT threshold, and finally

the RES_HOT

≥ RES_UR threshold. Once the SafetySignal

range is determined, the lower value thresholds are not

sampled. The SafetySignal decoder block uses the previ-

ously determined SafetySignal value to provide the appro-

priate adjustment in threshold to add hysteresis. The RTHB

resistor value is used to measure the RES_OR

≥RES_COLD

and RES_COLD

≥ RES_IDEAL thresholds by connecting

the THB pin to VDD and measuring the voltage resultant on

the THA pin. The RTHA resistor value is used to measure

the RES_IDEAL

≥ RES_HOT and RES_HOT ≥ RES_UR

thresholds by connecting the THA pin to VDD and measur-

ing the voltage resultant on the THB pin.

The SafetySignal decoder block uses a voltage divider

network between VDD and GND to determine SafetySignal

range thresholds. Since the THA and THB inputs are

sequentially connected to VDD, this provides VDD noise

immunity during SafetySignal measurement.

When AC power is not available the SafetySignal block

supports the following low power operating features:

1. The SafetySignal is sampled every 250ms or less,

instead of 32ms.

2. A full SafetySignal status is sampled every 30s or less,

instead of every 32ms.

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