LTC4120 datasheet(11/24 Pages) AD

11 / 24 page

LTC4126

Rev. A

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OPERATION

the VCC-to-BAT differential voltage drops from approxi-

mately 154mV to 116mV. See the curve in the Typical

Performance Characteristics section. When the charge

current reaches approximately 3.1mA, the LED at the

CHRG pin blinks fast (typically 4.58Hz) to indicate the

DUVCL fault. In the reverse direction, when the charge cur-

rent reaches approximately 4.5mA, 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 condi-

tions for the LTC4126 to alternate between slow blinking

and fast blinking. This behavior should be construed as

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

The DUVCL feature is particularly useful in situations

where 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 charge current to drop to zero instantly, allowing

the supply voltage to rise above the DUVLO threshold and

switch on the charger again. In the absence of DUVCL, this

oscillatory 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 monitors the battery temperature during 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 out-

side 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 tempera-

ture 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 tem-

perature 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 1. Indication by the logic

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

Table 1. Charger Status Indication

CHRG

ACPR STAT1 STAT2 STATUS

Hi-Impedance

Not Charging, No Power,

STAT pins indicate Battery

Level (see Table 2)

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)

The open-drain CHRG pin has an internal 300µA (typical)

pull-down. An LED can be connected between this pin and

VCC to indicate the charging status and any fault condition

as indicated in the table above. The ACPR, STAT1, and

STAT2 pins are digital CMOS logic outputs that can be

interpreted by a microprocessor. The low level of these

three pins is referenced to GND and the high level is ref-

erenced to the OUT pin voltage (typically 1.2V). Hence

the status indication via these three pins is only available

if the DC/DC converter is turned on via the EN pin or the

push button. Status indication via the CHRG pin is always

available during charging.

DC/DC CONVERTER

To supply the system load from the battery to the OUT pin,

the LTC4126 contains a proprietary low-noise multi-mode

charge pump DC/DC converter which can be switched

on by applying a minimum voltage of 1.1V to the EN

pin or by pushing the pushbutton. The converter can be

active simultaneously with the charger. The switching fre-

quency of the charge pump is set to either 50kHz or 75kHz

depending on the mode of operation. This frequency is

chosen to keep any switching noise out of the audio band.

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