AAT3110 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – MicroPower™ Regulated Charge Pump

AAT3110
MicroPower™ Regulated Charge Pump
Applications Information
Charge Pump Efficiency
The AAT3110 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.
η = P_OUT / P_IN
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:
η=P_OUT/P_IN=(V_OUT×I_OUT)/(V_IN×2I_OUT)=V_OUT/2V_IN
η(%) = 100(V_OUT / 2V_IN)
For a charge pump with an output of 5 volts and a
nominal input of 3.0 volts, the theoretical efficiency
is 83.3%. Due to internal switching losses and IC
quiescent current consumption, the actual efficien-
cy can be measured at 82.7%. 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 level of V_IN
approaches V_OUT. 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 a much as 100mA to 400mA of cur-
rent from V_IN. This excessive current consumption
due to an output short circuit condition will cause a
rise in the internal IC junction temperature. The
AAT3110 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
charge pump will then become active again. The
3110.2001.9.0.92
thermal protection system will cycle on and off if an
output short circuit condition persists. This will
allow the AAT3110 to operate indefinitely 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 C_OUT and C_FLY, the load current
I_OUT and the level of V_IN. Ripple observed at V_OUT is
typically a sawtooth waveform in shape. The ripple
frequency will vary depending on the load current
I_OUT and the level of V_IN. As V_IN 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.
The size and type of capacitors used for V_IN. C_OUT
and C_FLY 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 50mV_P-P when V_IN = 3.0V, V_OUT =
5.0V, C_OUT = 10µF and C_FLY = 1µF.
When the AAT3110 is used in light output load
applications where I_OUT < 10mA, the flying capaci-
tor C_FLY value can be reduced. The reason for this
effect is when the charge pump is either under very
light load conditions the transfer of charge across
C_FLY 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 C_FLY. Caution should be
observed when decreasing the flying capacitor. If
the output load current rises above the nominal
level for the reduced C_FLY value, charge pump effi-
ciency 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 C_OUT capacitor. The nominal 10µF C_OUT capac-
itor can be increased to 22µF or more. Larger val-
ues for the C_OUT capacitor (22µF and greater) will
by nature have lower ESR and can improve both
high and low frequency components of the charge
13