LTC4124 datasheet(11/24 Pages) AD

11 / 24 page

LTC4126-ADJ

Rev. 0

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OPERATION

Bad Battery Fault

If the battery fails to reach a voltage above VRECHRG by the

end of a full charge cycle of 6 hours, the battery is deemed

faulty and the LED at the CHRG pin indicates this bad

battery fault condition by blinking fast (typically 4.58Hz).

Differential Undervoltage Lockout (DUVLO)

A differential undervoltage lockout circuit monitors the

differential voltage between VCC and BAT and disables

the charger if the VCC voltage falls to within 27mV (typical

ΔVUVLO) of the BAT voltage. This condition is indicated by

a low on the ACPR pin. Charging does not resume until

this difference increases to 80mV at which time the ACPR

pin transitions back high. The DC/DC must be enabled for

proper ACPR indication.

Differential Undervoltage Current Limit (DUVCL)

The LTC4126-ADJ charger also includes differential

undervoltage current limiting (DUVCL) which gradually

reduces the charge current from the full programmed

current towards zero as the VCC-to-BAT differential volt-

age drops from approximately 154mV to 116mV. See the

curve in the Typical Performance Characteristics section.

When the charge current drops below 40% of the full

programmed value, the LED at the CHRG pin blinks fast

(typically4.58Hz)toindicatetheDUVCLfault.Inthereverse

direction, when the charge current rises above 60% of the

full programmed value, the LED at the CHRG pin resumes

slow blinking to indicate normal operation. Due to the

finite hysteresis of the DUVCL comparator, it is possible

under a very narrow region of coupling conditions for the

LTC4126-ADJ to alternate between slow blinking and fast

blinking. This behavior should be construed as operation

at near (but not 100%) full charge current.

TheDUVCLfeatureisparticularlyusefulinsituationswhere

the wireless power available is limited. Without DUVCL,

if the magnetic coupling between the receive coil and the

transmit coil is low, DUVLO could be tripped if the charger

tried to provide the full charge current. DUVLO forces the

chargecurrenttodroptozeroinstantly,allowingthesupply

voltage to rise above the DUVLO threshold and switch on

the charger again. In the absence of DUVCL, this oscilla-

tory behavior would result in intermittent charging. The

DUVCL circuitry prevents this undesirable behavior by

gradually increasing or decreasing the charge current as

input power becomes more or less available.

Temperature Qualified Charging

The LTC4126-ADJ monitors the battery temperature dur-

ing the charging cycle by using a negative temperature

coefficient (NTC) thermistor, placed close and thermally

coupled to the battery pack. If the battery temperature

moves outside a safe charging range, the IC suspends

charging and signals a fault condition via CHRG (blinks

fast at 4.58Hz) and the STAT pins until the temperature

returns to the safe charging range. The safe charging

range is determined by two comparators (Too Hot and Too

Cold) that monitor the voltage at the NTC pin as shown in

the Block Diagram. The rising threshold of the Too Cold

comparator is set to 76.5% of VCC (VCOLD) and the falling

threshold of the Too Hot comparator is set to 34.9% of

VCC (VHOT), each with a hysteresis of 1.5% of VCC around

the trip point to prevent oscillation. If the battery charger

pauses due to a temperature fault, the 6-hour termination

timer also pauses until the thermistor indicates a return

to a safe temperature. Grounding the NTC pin disables

all NTC functionality. Most Li-Ion battery manufacturers

recommend a temperature range of 0°C to 40°C as a safe

charging range.

Charge Status Indication via CHRG, ACPR, and

STAT pins

The status of the battery charger is indicated via the

open-drain CHRG pin as well as by the logic pins STAT1,

STAT2, and ACPR according to Table 2. Indication by the

logic pins is available only when the DC/DC is enabled.

Table 2. Charger Status Indication

CHRG

ACPR STAT1 STAT2 STATUS

Hi-Impedance

Not Charging, No Power,

STAT pins indicate Battery

Level (see Table 3)

Pulled LOW

Done Charging

Blink Slow (1.14Hz)

Charging

Blink Fast (4.58Hz)

Temperature Fault/Bad

Battery

Blink Fast (4.58Hz)

Differential Undervoltage

Current Limit (DUVCL)

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