LTC4100_09 datasheet(18/30 Pages) LINER

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LTC4100

4100fb

OPERATION

SafetySignal sensing is accomplished by a state machine

that reconﬁgures the switches of Figure 4 using THA_SELB

and THB_SELB, a selectable reference generator, and two

comparators. This circuit has two modes of operation

based upon whether AC is present.

When AC is present, the LTC4100 samples the value of

the SafetySignal and updates the ChargerStatus register

approximately 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 SafetySig-

nal range is determined, the lower value thresholds are

not sampled. The SafetySignal decoder block uses the

previously determined SafetySignal value to provide the

appropriate 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

measuring the voltage resultant on the THB pin.

The SafetySignal decoder block uses a voltage divider

network between VDD and GND to determine SafetySig-

nal 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.

The SafetySignal impedance is interpreted according to

Table 4.

Table 4. SafetySignal State Ranges

SafetySignal

RESISTANCE

CHARGE STATUS BITS

DESCRIPTION

0Ω to 500Ω

RES_UR

RES_HOT

BATTERY_PRESENT

Underrange

500Ω to 3kΩ

RES_HOT

BATTERY_PRESENT

Hot

3kΩ to 30kΩ

BATTERY_PRESENT

Ideal

30kΩ to 100kΩ

RES_COLD

BATTERY_PRESENT

Cold

Above 100kΩ

RES_OR RES_COLD

Overrange

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

LTC4100. 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 LTC4100 is to properly sense the value

of RSafetySignal.

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