LTC4124 datasheet(13/24 Pages) AD

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LTC4126-ADJ

Rev. 0

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OPERATION

another 110ms and the cycle continues. The duration of

110msischosentopreventmodeswitchingatafrequency

which could fall into the audible range. The switch over to

Mode 3 provides more current drive capability at the cost

of efficiency and this is why the converter tries to stay in

Mode 1 or Mode 2 as much as possible.

Converter Efficiency

The LTC4126-ADJ DC/DC converter efficiency varies

throughout the battery voltage range and is very much

dependent on the mode it is operating in. The theoretical

maximumefficiencyinMode1canbeexpressedasfollows:

Efficiency,

ηMode1 =

VOUT

VBAT

⎛

⎝⎜

⎞

⎠⎟

If regulation is maintained at the OUT pin at 1.2V, the

theoretical maximum efficiency is 85.7% when VBAT =

4.2V and 100% when VBAT = 3.6V as calculated from the

above equation.

When the battery voltage is between 3.6V and 3.3V, the

converter can no longer maintain a 1.2V regulation at OUT

at all loads and is operating in Mode 2. However, the upper

limit on the efficiency that the converter can achieve in this

mode is determined by switching losses, ohmic losses,

and quiescent current loss.

When the battery voltage falls to 3.3V, the converter enters

Mode 3 where the theoretical maximum efficiency can be

expressed as follows:

Efficiency,

ηMode3 =

VOUT

VBAT

⎛

⎝⎜

⎞

⎠⎟

In Mode 3, the theoretical maximum efficiency is 72.7%

when VBAT=3.3Vand80%whenVBAT=3.0Vascalculated

from the above equation.

Figure 4 shows graphically the variation of the theoretical

maximum efficiency of the converter over the range of

battery voltages in the three different modes of operation.

Battery Level Indicator

The LTC4126-ADJ is equipped with a battery voltage

monitor which reports various battery voltage levels via

the STAT pins when not charging and the converter is

enabled. See Table 3. Since the STAT pins indicate either

the charger status or the battery levels based on the state

of ACPR, there may be a delay of up to 1µs before the STAT

pins are valid whenever ACPR changes state.

Table 3. Battery Level Indication

ACPR STAT1 STAT2 STATUS

VBAT < 3.2V, Low Battery Alert 3

3.2V < VBAT < 3.3V

3.3V < VBAT < 3.6V

VBAT > 3.6V

Power Available, STAT Pins Indicate Charger Status

Battery Disconnect/Ship Mode

When no input power is available and the battery voltage

falls to 3.0V (typical), the LTC4126-ADJ shuts down most

of its functions to prevent the battery from discharging too

deeply, consuming less than 100nA from the battery. Once

in battery disconnect mode, normal functioning can only

resume when power is applied to the ACIN or VCC pin and

the VCC pin voltage rises 80mV (typical) above the BAT pin

voltage.

The LTC4126-ADJ is also in battery disconnect mode after

initial installation of the battery regardless of its voltage

level. This implements the ship mode functionality.

Figure 4. Theoretical Maximum Converter

Efficiency vs Battery Voltage

BATTERY VOLTAGE (V)

100

4.2

3.0

3.9

4126-ADJ F04

3.3

3.6

MODE 3

MODE 2

MODE 1

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