AAT2820
Triple-Output Charge Pump Regulator
Positive Charge Pump Regulator
Enable
During the first half-cycle, the N-channel MOSFET
turns on and charges the flying capacitor C12
(Figure 2). During the second half-cycle, the N-
channel MOSFET turns off and the P-channel
MOSFET turns on, level shifting C12 the input volt-
age. This connects C12 in parallel with the reser-
voir capacitor C24. If the voltage across C24 plus a
diode drop is less than the level shifted by the fly-
ing capacitor (C12 + VIN), charge is transferred
from C12 to C24 until the diode turns off. If the
positive charge pump output is connected to
ground, output may not recover until power is recy-
cled. Loads greater than the maximum load condi-
tion may cause permanent damage to the device;
please review the Maximum Line Load Curves and
the Electrical Characteristics table.
In the normal operating state, the AAT2820 typical-
ly consumes 5mA of quiescent operating current.
By pulling the enable pin (EN) low, the AAT2820
disables all three outputs. Once the device is shut
down, the supply current drops to less than 1µA to
maximize battery life.
The AAT2820 gives the application an option to
independently turn on/off the positive and negative
charge pump outputs. These two outputs can be
disabled by pulling the EN/PN pin low. The thresh-
old levels lie between 0.5V and 1.5V. Depending
on the application, the supplies must be sequenced
properly to avoid damage or latch-up.
Soft-Start and Start-up Sequence
The AAT2820 has an internal soft-start circuit to
guarantee a smooth transition to 5V for the main out-
put when the device is enabled (typical 200µs). This
device has two versions for the start-up sequence.
The AAT2820 ramps up the positive charge pump
after the negative charge pump is present; the
AAT2820-1 ramps up the positive charge pump
before the negative charge pump.
Voltage Reference
The voltage reference is a simple band gap with an
output voltage equal to VBE + K*VT. The band gap
reference amplifier has an additional compensation
capacitor from the negative input to the output. This
capacitor serves to slow down the circuit during
startup and soft starts the voltage reference and the
regulator output from overshoot. The reference cir-
cuit amplifier also increases the overall PSRR of the
device. An 80kΩ resistor serves to isolate and buffer
the amplifier from a small internal filter capacitor and
an optional large external filter capacitor.
Over-Temperature Protection
To protect the AAT2820, as well as the system appli-
cation, this device has a thermal protection circuit
that will shut down all the charge pumps if the die
temperature rises above the preset internal thermal
limit. This protects the device if the ambient temper-
ature exceeds the operating limit for the device.
IN
IN
1/2 A3
C11
VIN
OSC
DRVN
CTL
1/2 A4
C12
OSC
CTL
DRVP
BAT54SDW
BAT54SDW
VON = -(R1/R2) x VREF
VON
R1
FBN
R4
VOP
FBP
⎛
R4⎞
R3⎠
VOP
=
1 +
× VREF
R2
⎝
VREF
1.2V
C23
R3
C24
VREF
1.2V
C5
GND
GND
Figure 1: Negative Charge Pump Block Diagram. Figure 2: Postive Charge Pump Block Diagram.
10
2820.2006.04.1.4
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