5
FN6239.2
September 28, 2009
FGA Technology
The ISL21032 series of voltage references use the floating
gate technology to create references with very low drift and
supply current. Essentially the charge stored on a floating
gate cell is set precisely in manufacturing. The reference
voltage output itself is a buffered version of the floating gate
voltage. The resulting reference device has excellent
characteristics which are unique in the industry: very low
temperature drift, high initial accuracy, and almost zero
supply current. Also, the reference voltage itself is not limited
by voltage bandgaps or zener settings, so a wide range of
reference voltages can be programmed (standard voltage
settings are provided, but customer-specific voltages are
available).
The process used for these reference devices is a floating
gate CMOS process, and the amplifier circuitry uses CMOS
transistors for amplifier and output transistor circuitry. While
providing excellent accuracy, there are limitations in output
noise level and load regulation due to the MOS device
characteristics. These limitations are addressed with circuit
techniques discussed in other sections.
Board Mounting Considerations
For applications requiring the highest accuracy, board
mounting location should be reviewed. Placing the device in
areas subject to slight twisting can cause degradation of the
accuracy of the reference voltage due to die stresses. It is
normally best to place the device near the edge of a board,
or the shortest side, as the axis of bending is most limited at
that location. Obviously mounting the device on flexprint or
extremely thin PC material will likewise cause loss of
reference accuracy.
Board Assembly Considerations
FGA references provide high accuracy and low temperature
drift but some PC board assembly precautions are
necessary. Normal Output voltage shifts of 100µV to 1mV
can be expected with Pb-free reflow profiles or wave solder
on multi-layer FR4 PC boards. Precautions should be taken
to avoid excessive heat or extended exposure to high reflow
or wave solder temperatures, this may reduce device initial
accuracy.
Post-assembly x-ray inspection may also lead to permanent
changes in device output voltage and should be minimized
or avoided. If x-ray inspection is required, it is advisable to
monitor the reference output voltage to verify excessive shift
has not occurred. If large amounts of shift are observed, it is
best to add an X-ray shield consisting of thin zinc (300µm)
sheeting to allow clear imaging, yet block x-ray energy that
affects the FGA reference.
Special Applications Considerations
In addition to post-assembly examination, there are also
other X-ray sources that may affect the FGA reference long
term accuracy. Airport screening machines contain X-rays
and will have a cumulative effect on the voltage reference
output accuracy. Carry-on luggage screening uses low level
X-rays and is not a major source of output voltage shift,
although if a product is expected to pass through that type of
screening over 100 times it may need to consider shielding
with copper or aluminum. Checked luggage X-rays are
higher intensity and can cause output voltage shift in much
fewer passes, so devices expected to go through those
machines should definitely consider shielding. Note that just
two layers of 1/2 ounce copper planes will reduce the
received dose by over 90%. The leadframe for the device
which is on the bottom also provides similar shielding.
FIGURE 13. Z
OUT
vs f vs C
L
FIGURE 14. V
OUT
NOISE
Typical Performance Curves, ISL21032 Low Voltage Output Reference
V
IN
= 3.0V, I
OUT
= 0mA, T
A
= +25°C Unless Otherwise Specified (Continued)
0
20
40
60
80
100
120
1 10 100 1k 10k 100k 1M
FREQUENCY (Hz)
Z
OUT
(Ω)
NO LOAD
100nF LOAD
1nF LOAD
10nF LOAD
10s/DIV
5µV/DIV
ISL21032
