AAT3183_08 Datasheet, PDF (9/15 Pages) Advanced Analogic Technologies – 300mA Inductorless Step-Down Converter

ChargePump^TM
Integrated soft-start limits inrush current, maintains
monotonic turn-on characteristics and eliminates output
voltage overshoot. The device includes short-circuit pro-
tection and a self-recovering over-temperature (thermal)
protection.
Charge Pump Operation
The AAT3183 step-down charge pump is implemented
using a fixed 1/2x (gain) converter topology. This con-
figuration allows efficient energy transfer 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 output
during alternate ‘charge’ and ‘discharge’ intervals. The
amount of energy transferred from the input voltage
source to flying capacitor is proportional to the differential
voltage across the flying capacitor (V_DIFF = V_IN - V_OUT)
which occurs during the ‘charge’ interval multiplied by the
switching frequency. The step-down charge pump trans-
fers energy to the output during both the ‘charge’ and
‘discharge’ intervals. Figure 1 illustrates the energy trans-
fer mechanism during ‘charge’ and ‘discharge’ intervals.
PFM control compensates for changes in the input voltage
and output current by modulating the frequency of
switching intervals to maintain the desired output volt-
age. 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,
PRODUCT DATASHEET
AAT3183
300mA Inductorless Step-Down Converter
the voltage at the input to the error amplifier decreases.
The error signal increases the effective switching fre-
quency; providing increased current to the output current
thus maintaining the desired output voltage. At light
loads, the effective switching frequency 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) opera-
tion, the input current is constant regardless of input
voltage. The input current is equal to 50% [1/2x (gain)]
of the output current.
A conventional LDO regulator maintains input current
which is equal to the output current. Operation efficiency
(η) of an LDO regulator is as follows:
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
GND
Figure 1a: Step-Down Charge
Pump “CHARGE” Interval.
ENERGY
TRANSFER
VOUT
CFLY(DISCHARGE)
C_OUT
IOUT
RLOAD
GND
Figure 1b: Step-Down Charge
Pump “DISCHARGE” Interval.
3183.2008.02.1.3
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