AAT3111 datasheet(11/16 Pages) AAT

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AAT3111

MicroPower™ Regulated Charge Pump

3111.2006.06.1.3

Charge Pump Efficiency

The AAT3111 is a regulated output voltage dou-

bling charge pump. The efficiency (

η) can simply

be defined as a linear voltage regulator with an

effective output voltage that is equal to two times

the input voltage. Efficiency (

η) for an ideal voltage

doubler can typically be expressed as the output

power divided by the input power.

In addition, with an ideal voltage doubling charge

pump, the output current may be expressed as half

the input current. The expression to define the

ideal efficiency (

η) can be rewritten as:

-or-

For a charge pump with an output of 3.3 volts and

a nominal input of 1.8 volts, the theoretical efficien-

cy is 91.6%. Due to internal switching losses and

IC quiescent current consumption, the actual effi-

ciency can be measured at 91%. These figures are

in close agreement for output load conditions from

1mA to 100mA.

Efficiency will decrease as load

current drops below 0.05mA or when the level of

VIN approaches VOUT. Refer to the Typical Char-

acteristics section for measured plots of efficiency

versus input voltage and output load current for the

given charge pump output voltage options.

Short-Circuit and Thermal Protection

In the event of a short-circuit condition, the charge

pump can draw as much as 100mA to 400mA of

current from VIN. This excessive current consump-

tion due to an output short-circuit condition will

cause a rise in the internal IC junction temperature.

The AAT3111 has a thermal protection and shut-

down circuit that continuously monitors the IC junc-

tion temperature. If the thermal protection circuit

senses the die temperature exceeding approxi-

mately 145°C, the thermal shutdown will disable

the charge pump switching cycle operation. The

thermal limit system has 10°C of system hysteresis

before the charge pump can reset. Once the over-

current event is removed from the output and the

junction temperature drops below 135°C, the

charge pump will then become active again. The

thermal protection system will cycle on and off if an

output short-circuit condition persists.

This will

allow the AAT3111 to operate indefinitely in a short-

circuit condition without damage to the device.

Output Ripple and Ripple Reduction

There are several factors that determine the ampli-

tude and frequency of the charge pump output rip-

ple, the values of COUT and CFLY, the load current

IOUT, and the level of VIN. Ripple observed at VOUT

is typically a sawtooth waveform in shape. The rip-

ple frequency will vary depending on the load cur-

rent IOUT and the level of VIN. As VIN increases, the

ability of the charge pump to transfer charge from

the input to the output becomes greater; as it does,

the peak-to-peak output ripple voltage will also

increase.

The size and type of capacitors used for CIN, COUT,

and CFLY have an effect on output ripple. Since

output ripple is associated with the R/C charge time

constant of these two capacitors, the capacitor

value and ESR will contribute to the resulting

charge pump output ripple. This is why low ESR

capacitors are recommended for use in charge

pump applications. Typically, output ripple is not

greater than 35mVP-P when VIN = 2.0V, VOUT =

3.3V, COUT = 10µF, and CFLY = 1µF.

When the AAT3111 is used in light output load appli-

cations where IOUT < 10mA, the flying capacitor CFLY

value can be reduced. The reason for this effect is

when the charge pump is under very light load con-

ditions, the transfer of charge across CFLY is greater

during each phase of the switching cycle. The result

is higher ripple seen at the charge pump output.

This effect will be reduced by decreasing the value

of CFLY. Caution should be observed when decreas-

ing the flying capacitor. If the output load current

rises above the nominal level for the reduced CFLY

value, charge pump efficiency can be compromised.

η(%) = 100

VOUT

2VIN

⎛⎞

⎝⎠

η = POUT = VOUT · IOUT = VOUT

VIN · 2IOUT

2VIN

η =

POUT

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