ISL21060 Datasheet, PDF (18/20 Pages) Intersil Corporation – Precision Reference with Disable

ISL21060
FGA Technology
The ISL21060 voltage reference floating gate references
possess very low drift and supply current. 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 and
include 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 drive. This circuitry
provides excellent accuracy with a trade-off in output noise
level and load regulation due to the MOS device
characteristics. These limitations are addressed with circuit
techniques discussed in other sections.
Micropower Supply Current and Output Enable
The ISL21060 consumes extremely low supply current due
to the proprietary FGA technology. Low noise performance is
achieved using optimized biasing techniques. Supply current
is typically 16µA and noise is 10µV_P-P, benefitting precision,
low noise portable applications, such as handheld meters
and instruments.
The ISL21060 devices have the EN pin, which is used to
Enable/Disable the output of the device. When disabled, the
reference circuitry itself remains biased at a highly accurate
and reliable state. When enabled, the output is driven to the
reference voltage in a relatively short time (about 300µs).
This feature allows multiple references to be connected and
one of them selected. Another application is to disable any
loads that draw significant current, saving power in standby
or shutdown modes.
Board Mounting Considerations
For applications requiring the highest accuracy, board
mounting location should be reviewed. The device uses a
plastic SOIC package, which will subject the die to mild
stresses when the PC board is heated and cooled and
slightly changes shape. Placing the device in areas subject
to slight twisting can cause degradation of the accuracy of
the reference voltage due to these 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. Mounting the device in a cutout also minimizes flex.
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.
If a device is expected to pass through luggage X-ray
machines numerous times, it is advised to mount a 2-layer
(minimum) PC board on the top, and along with a ground
plane underneath will effectively shield it from from 50 to 100
passes through the machine. Since these machines vary in
X-ray dose delivered, it is difficult to produce an accurate
maximum pass recommendation.
Noise Performance and Reduction
The output noise voltage in a 0.1Hz to 10Hz bandwidth is
typically 10µV_P-P. The noise measurement is made with a
bandpass filter made of a 1-pole high-pass filter with a corner
frequency at 0.1Hz and a 2-pole low-pass filter with a corner
frequency at 12.6Hz to create a filter with a 9.9Hz bandwidth.
Noise in the 10kHz to 1MHz bandwidth is approximately
100µV_P-P with no capacitance on the output. This noise
measurement is made with a 2 decade bandpass filter made
of a 1-pole high-pass filter with a corner frequency at 1/10 of
the center frequency and 1-pole low-pass filter with a corner
frequency at 10x the center frequency. Load capacitance up
to 1µF can be added to improve transient response.
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FN6706.4
September 30, 2009