LTC4120 datasheet(13/24 Pages) AD

13 / 24 page

LTC4126

Rev. A

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OPERATION

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 = 3.3V and 80% when VBAT = 3.0V as calcu-

lated 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 Disconnect/Ship Mode

When no input power is available and the battery voltage

falls to 3.0V (typical), the LTC4126 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 volt-

age rises 80mV (typical) above the BAT pin voltage.

The LTC4126 is also in battery disconnect mode after

initial installation of the battery regardless of its voltage

level. This implements the ship mode functionality.

Pushbutton Control

The LTC4126 is equipped with a pushbutton controller

to turn the DC/DC converter on and off if the EN pin is

not used (held low). A logic high on the EN pin overrides

the pushbutton function and keeps the regulator on. On

the falling edge of the EN signal, the DC/DC shuts off and

1µs later, the pushbutton can control the output as long

as EN remains low. A push on the pushbutton is consid-

ered valid if the PBEN pin is held low for at least 425ms

(typical). Additionally, the PBEN pin needs to return to the

high state for at least 43ms (typical) in between succes-

sive pushes for a push to be considered valid. An invalid

push will not change the state of the converter. A 4MΩ

internal resistor pulls up the PBEN pin to the BAT voltage.

A few different scenarios of valid and invalid pushes are

illustrated in Figure 5.

Figure 5. Various Pushbutton Scenarios

VOUT

TOO SHORT

LONG ENOUGH

425ms

43ms

LONG ENOUGH

PBEN

4126 F05

VOUT

LONG ENOUGH

425ms

43ms

TOO SHORT

(b) HIGH PULSE TOO SHORT, 2ND PUSH IGNORED, DC/DC STAYS ON

PBEN

VOUT

LONG ENOUGH

425ms

43ms

LONG ENOUGH

(a) VALID SUCCESSIVE PUSH, DC/DC TURNS ON AND OFF

PBEN

425ms

Figure 4. Theoretical Maximum Converter

Efficiency vs Battery Voltage

BATTERY VOLTAGE (V)

100

4.2 4.35

3.0

3.9

4126 F04

3.3

3.6

MODE 3

MODE 2

MODE 1

Battery Level Indicator

The LTC4126 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 2. 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 2. 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

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