ISL21090

8

FN6993.0

June 8, 2011

Device Operation

Bandgap Precision References

The ISL21090 uses a bandgap architecture and special trimming

circuitry to produce a temperature compensated, precision voltage

reference with high input voltage capability and moderate output

current drive.

Applications Information

Board Mounting Considerations

For applications requiring the highest accuracy, board mounting

location should be reviewed. The device uses a plastic SOIC

package, which subjects the die to mild stresses when the

printed circuit (PC) board is heated and cooled, which slightly

changes the shape. Because of these die stresses, placing the

device in areas subject to slight twisting can cause degradation

of reference voltage accuracy. It is normally best to place the

device near the edge of a board, or on the shortest side, because

the axis of bending is most limited in 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

Some PC board assembly precautions are necessary. Normal

output voltage shifts of 100µV to 500µV 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.

Noise Performance and Reduction

The output noise voltage in a 0.1Hz to 10Hz bandwidth is typically

bandpass filter. The filter is 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 (3dB) at 9.9Hz, to create a filter with a 9.9Hz

bandwidth. Noise in the 10Hz to 1kHz bandwidth is approximately

This noise measurement is made with a 2 decade bandpass filter.

The filter is made of a 1-pole high-pass filter with a corner

frequency at 10Hz of the center frequency, and 1-pole low-pass

filter with a corner frequency at 1kHz. Load capacitance up to

10µF can be added but will result in only marginal improvements

in output noise and transient response.

Turn-On Time

Normal turn-on time is typically 150µs, as shown in Figure 12.

The circuit designer must take this into account when looking at

power-up delays or sequencing.

Temperature Coefficient

The limits stated for temperature coefficient (Tempco) are governed

by the method of measurement. The overwhelming standard for

specifying the temperature drift of a reference is to measure the

reference voltage at two temperatures, take the total variation,

ppm/°C. This is the “Box” method for specifying temperature

coefficient.

Output Voltage Adjustment

The output voltage can be adjusted above and below the

factory-calibrated value via the trim terminal. The trim terminal is

the negative feedback divider point of the output op amp. The

positive input of the amplifier is about 1.216V, and in feedback,

so will be the trim voltage. The trim terminal has a 5000Ω

resistor to ground internally, and in the case of the 2.5V output

version, there is a feedback resistor of approximately 5000Ω

The suggested method to adjust the output is to connect a very

high value external resistor directly to the trim terminal and

connect the other end to the wiper of a potentiometer that has a

much lower total resistance and whose outer terminals connect

output adjust range will be ±6.3mV. It is important to minimize

the capacitance on the trim terminal to preserve output amplifier

stability. It is also best to connect the series resistor directly to

the trim terminal, to minimize that capacitance and also to

minimize noise injection. Small trim adjustments will not disturb

the factory-set temperature coefficient of the reference, but

trimming near the extreme values can.