AAT3111
MicroPower™ Regulated Charge Pump
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.
Applications Information
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.
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 ripple
frequency will vary depending on the load current
IOUT and the level of VIN. As VIN increases the abili-
ty 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.
η = POUT / PIN
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 (h) can be rewritten as:
η = POUT / PIN = (VOUT × IOUT) / (VIN × 2IOUT) =
VOUT / 2VIN
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
η(ꢀ) = 100(VOUT / 2VIN)
For a charge pump with an output of 3.3 volts and
a nominal input of 1.8 volts, the theoretical efficien-
cy is 91.6ꢀ. Due to internal switching losses and
IC quiescent current consumption, the actual effi-
ciency can be measured at 91ꢀ. 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.
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
applications where IOUT < 10mA, the flying capaci-
tor CFLY value can be reduced. The reason for this
effect is when the charge pump is under very light
load conditions, 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
Short Circuit and Thermal Protection
In the event of a short circuit condition, the charge
pump can draw a much as 100mA to 400mA of cur-
rent from VIN. This excessive current consumption
due to an output short circuit condition will cause a
rise in the internal IC junction temperature. The
AAT3111 has a thermal protection and shutdown
circuit that continuously monitors the IC junction
temperature. If the thermal protection circuit sens-
es the die temperature exceeding approximately
145°C, the thermal shutdown will disable the
charge pump switching cycle operation. The ther-
mal 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
the value of CFLY
.
Caution should be observed
when decreasing the flying capacitor. If the output
load current rises above the nominal level for the
reduced CFLY value, charge pump efficiency can be
compromised.
There are several methods that can be employed to
reduce output ripple depending upon the require-
ments of a given application. The most simple and
straightforward technique is to increase the value of
the COUT capacitor. The nominal 10µF COUT capac-
itor can be increased to 22µF or more. Larger val-
ues for the COUT capacitor (22µF and greater) will by
nature have lower ESR and can improve both high
3111.2002.3.0.91
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