LTC4100 datasheet(15/24 Pages) LINER

15 / 24 page

LTC4100

sn4100 4100is

OPERATIO

ues, when there is sufficient DCIN voltage to charge the

battery.

10. Writing a zero value to ChargingVoltage() function.

11. Writing a zero value to ChargingCurrent() function.

The SafetySignal Decoder Block

This block measures the resistance of the SafetySignal

and features high noise immunity at critical trip points. The

low power standby mode supports only battery presence

SMB charger reporting requirements when AC is not

present. The SafetySignal decoder is shown in Figure 4.

The value of RTHA is 1.13k and RTHB is 54.9k.

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 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

The SafetySignal impedance is interpreted according to

Table 4.

RTHA

1.13k

RSafetySignal

RTHB

54.9k

VDD

4100 F04

THA_SELB

THB_SELB

RES_OR

RES_COLD

RES_H0T

RES_UR

LATCH

SafetySignal

CONTROL

MUX

REF

HI_REF

LO_REF

TH_HI

TH_LO

THA

THB

CSS

VLIM

12.5k

25k

33k

25k

12.5k

RVLIM

VDD

VLIM [3:0]

4100 F05

AC_PRESENT

ENCODER

Figure 5. Simplified VLIM Circuit Concept (ILIM is Similar)

Figure 4. SafetySignal Decoder Block

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