AAT3190
Positive/Negative Charge Pump for Voltage Bias
Voltage Reference
Design Procedure and
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
circuit 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.
Component Selection
Output Voltage
The number of charge pump stages required for a
given output varies with the input voltage applied.
The number of stages required can be estimated by:
VOP - VIN
np =
VIN - 2VF
for the positive output and
Enable and Start-up
VON
The AAT3190 is disabled by pulling the EN pin low.
The threshold levels lie between 0.5V and 1.5V.
Even though the quiescent current of the IC during
shutdown is less than 1µA, the positive output volt-
age (VOP) and any load current associated with it
does not disappear without the complete removal of
the input voltage. This is due to the fact that with no
switching of the DRVP pin, the input voltage simply
forward biases the Schottky diodes associated with
the VOP charge pump, providing a path for load cur-
rent to be drawn from the input voltage.
nn =
2VF - VIN
for the negative output.
When solving for np and nn, round up the solution
to the next highest integer to determine the number
of stages required.
VON
The negative output voltage is adjusted by a resis-
tive divider from the output (VON) to the FBN and
REF pin.
Depending on the application, the supplies must be
sequenced properly to avoid damage or latch-up.
The AAT3190 start-up sequence ramps up the VOP
output 200µs after the VON output is present. The
AAT3190-1 ramps up the positive supply before the
negative supply.
The maximum reference voltage current is 50µA;
therefore, the minimum allowable value for R2 of
Figure 1 is 24kΩ. It is best to select the smallest
value possible for R2, as this will keep R1 to a min-
imum. This limits errors due to the FBN input bias
current. The FBN input has a maximum input bias
current of 100nA. Using the full 50µA reference
Over-Temperature Protection
A logic control circuit will shut down both charge
pumps in the case of an over-temperature condition.
current for programming VON
:
VREF
R2
1.2
Under-Voltage Lockout
IPGM
=
=
= 50µA
24.1k
A UVLO circuit disables the AAT3190 when the
input voltage supply is lower than 1.8V nominal.
will limit the error due to the input bias current at
FBN to less than 0.2%:
IFBN 0.1µA
IPGM 50µA
=
= 0.2%
10
3190.2006.01.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
