LTC4124 datasheet(12/24 Pages) AD

12 / 24 page

LTC4126-ADJ

Rev. 0

For more information www.analog.com

OPERATION

Figure 2. DC/DC Converter Thevenin Equivalent Circuit

in Mode 2: 3-to-1 Step-Down

Figure 3. VOUT vs Battery Voltage at IOUT = 0

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

referenced 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

pushbutton. 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-ADJ 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 pressing the pushbutton. The converter can

be active simultaneously with the charger. The switching

frequency of the charge pump is set to either 50kHz or

75kHzdependingonthemodeofoperation.Thisfrequency

ischosentokeepanyswitchingnoiseoutoftheaudioband.

Modes of Operation

The charge pump DC/DC converter has 3 modes of opera-

tion depending on the battery voltage. For VBAT > 3.6V, the

charge pump operates in 3:1 step-down mode (Mode 1)

and provides a regulated 1.2V output. In Mode 1, the

maximum output current that the DC/DC converter can

provide is limited by internal current limit circuitry to ap-

proximately 65mA.

DC/DC CONVERTER

ROL

IOUT

VOUT

4126-ADJ F02

+–

–

VBAT

HEARING

AID ASIC

BATTERY VOLTAGE (V)

1.4

1.2

0.8

0.4

1.0

0.6

0.2

3.0

3.9

4126-ADJ F03

3.3

4.2

3.6

MODE 3

MODE 2

MODE 1

This is referred to as Mode 2. The Thevenin equivalent

circuit of the converter in Mode 2 is shown in Figure 2,

where ROL is the effective open-loop output resistance of

the converter. ROL is typically 4.6Ω at room temperature

for VBAT = 3.5V and fSW = 50kHz. It varies with the battery

voltage, the switching frequency of the converter, and the

temperature of the die. Figure 2 can be used to determine

the output voltage (VOUT) for a specific load current (IOUT)

using the following equation:

VOUT =

VBAT

–IOUT •ROL

When the battery voltage falls below 3.3V, the charge

pump switches to 2:1 step-down mode (Mode 3) and

again provides a regulated 1.2V output. In Mode 3, the

maximum output current that the DC/DC converter can

provide decreases with battery voltage but does not fall

below approximately 35mA. See the curve in the Typical

Performance Characteristics. The variation of the output

voltage versus the battery voltage for the various modes

of operation is shown in Figure 3.

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

charge pump still operates in 3:1 step-down mode, but it

can no longer maintain 1.2V regulation and provides one-

third of the battery voltage at its output (only at no load).

Handling Large Loads

WhileoperatinginMode1orMode2(3:1step-downmode),

if a large load at the output causes the output voltage to

drop below 1.1V, the converter automatically switches

over to Mode 3 (2:1 step-down mode) and attempts to

regulate the output at 1.2V. The converter stays in Mode 3

for 110ms (typical) and then returns to the previous mode.

If the large load condition persists and VOUT drops below

1.1V again, the converter switches back into Mode 3 for

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