AAT3183 Datasheet, PDF (9/16 Pages) Advanced Analogic Technologies – 300mA Inductorless Step-Down Converter

AAT3183
300mA Inductorless Step-Down Converter
Charge Pump Operation
The AAT3183 step-down charge pump is imple-
mented using a fixed 1/2x (gain) converter topolo-
gy. This configuration allows efficient energy trans-
fer with a single ceramic flying capacitor. The
arrangement of the internal switches requires that
the voltage on the flying capacitor is greater than
the output voltage plus the input voltage headroom
to account for a parasitic voltage drop.
Energy is transferred to the flying capacitor and out-
put during alternate ‘charge’ and ‘discharge’ inter-
vals. The amount of energy transferred from the
input voltage source to flying capacitor is proportion-
al to the differential voltage across the flying capaci-
tor (V_DIFF = V_IN - V_OUT) which occurs during the
‘charge’ interval multiplied by the switching frequen-
cy. The step-down charge pump transfers energy to
the output during both the ‘charge’ and ‘discharge’
intervals. Figure 1 illustrates the energy transfer
mechanism during ‘charge’ and ‘discharge’ intervals.
PFM control compensates for changes in the input
voltage and output current by modulating the fre-
quency of switching intervals to maintain the
desired output voltage. The output voltage is
sensed through an internal resistor divider and
compared against a reference voltage by an error
amplifier. As the output voltage decreases, the
voltage at the input to the error amplifier decreas-
es. The error signal increases the effective switch-
ing frequency; providing increased current to the
output current thus maintaining the desired output
voltage. At light loads, the effective switching fre-
quency is greatly reduced which maintains output
regulation while minimizing switching losses.
Operating efficiency (η) is defined as the output
power divided by the input power.
η
=
P_OUT
P_IN
=
(V_OUT
(V_IN
·
·
I_OUT)
I_IN)
With a constant output current and 1/2x (gain)
operation, the input current is constant regardless
of input voltage. The input current is equal to 50%
[1/2x (gain)] of the output current.
I_IN = ½I_OUT
η=
(V_OUT · I_OUT)
(V_IN · ½I_OUT)
=
2 · V_OUT
V_IN
ENERGY
TRANSFER
VIN
C_FLY(CHARGE)
V_DIFF
VOUT
C_OUT ^IOUT
RLOAD
VIN
ENERGY
TRANSFER
CFLY(DISCHARGE)
VOUT
C_OUT ^IOUT
RLOAD
GND
Figure 1a: Step-Down Charge
Pump “CHARGE” Interval.
GND
Figure 1b: Step-Down Charge
Pump “DISCHARGE” Interval.
3183.2007.07.1.2
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