AAT3111 Datasheet, PDF (11/16 Pages) Advanced Analogic Technologies – MicroPower™ Regulated Charge Pump

AAT3111
MicroPower™ Regulated Charge Pump
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.
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 = ^POUT = ^{VOUT · IOUT} = ^VOUT
P_IN V_IN · 2I_OUT 2V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT
2V_IN
⎞
⎠
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
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 as much as 100mA to 400mA of
current from V_IN. 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 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 cur-
rent I_OUT and the level of V_IN. As V_IN 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 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 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 35mV_P-P when V_IN = 2.0V, V_OUT =
3.3V, C_OUT = 10µF, and C_FLY = 1µF.
When the AAT3111 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.
3111.2006.06.1.3
11