AAT3110 Datasheet, PDF (14/20 Pages) Advanced Analogic Technologies – MicroPower™ Regulated Charge Pump

AAT3110
MicroPower™ Regulated Charge Pump
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
-or-
η(%)
=
100
⎛
⎝
V_OUT
2V_IN
⎞
⎠
For a charge pump with an output of 5.0V and a
nominal input of 3.0V, 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 the level of
V_IN approaches V_OUT. Refer to the Typical Char-
acteristics section of this datasheet for measured
plots of efficiency versus input voltage and output
load current for the given charge pump output volt-
age 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
14
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 become active again. The ther-
mal protection system will cycle on and off if an out-
put short-circuit condition persists. This will allow
the AAT3110 to operate indefinitely under short-cir-
cuit conditions without damaging 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 rip-
ple frequency will vary depending on the load current
I_OUT and the level of V_IN. As V_IN increases, the abil-
ity 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 C_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 capaci-
tor 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 30mV_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 appli-
cations where I_OUT < 10mA, the flying capacitor C_FLY
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 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 decreas-
ing the flying capacitor. If the output load current
rises above the nominal level for the reduced C_FLY
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
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