Adjusting the ISL21007, ISL21009 VOUT Using the ISL95810 ® Technical Brief July 10, 2007 TB473.1 Author: Dipti Desai Introduction The ISL21007, ISL21009 voltage references are fabricated on Intersil’s proprietary Floating Gate Analog (FGA) technology and are very low power, high precision, and low noise. The reference Trim pin enables the user to adjust the output voltage up or down by 2.5%, thereby giving the flexibility to use the part for a wide range of applications. Pinout ISL21007, ISL21009 (8 LD SOIC) TOP VIEW The high terminal of the DCP (RH) is connected to the VOUT pin of the ISL21007, ISL21009, and the Wiper terminal of the DCP part (RW) is connected to the Trim pin of the reference. The Low terminal of the DCP (RL) is connected to ground. The evaluation software available with the ISL95810 evaluation parts allows the user to write to the wiper register (WR) in order to adjust the output voltage of the ISL21007, ISL21009. Ideally, the DCP resistance is approximately 10x smaller than the resistance at the Trim pin. As a result, the DCP resistance will dominate the voltage reference part and it can be externally biased to achieve the desired output voltage adjustment. VVCC CC GND or NC 1 8 DNC VIN 2 7 DNC DNC 3 6 VOUT GND 4 5 TRIM or NC RH RH SDA SDA 2 2 I C I C AND SCL SCL Trim Pin Circuitry WIPER WIPER REGISTER REGISTER RW RW CONTROL CONTROL NON-VOLATILE NON-VOLATILE REGISTER REGISTER WP WP As shown in Figure 1, the Trim input is internally biased by a resistor divider, such that when the Trim pin is floating, the voltage at the pin is VOUT/2. It can be driven externally by using a Digitally Controlled Potentiometer (DCP), such as the ISL95810. As long as the DCP resistance is significantly lower than 625kΩ, the voltage reference part can be trimmed to obtain a VOUT within ±2.5% of the nominal value. RL RL GND GND FIGURE 2. ISL95810 BLOCK DIAGRAM ISL21007, ISL21009 REFERENCE VOUT VOUT VCC RW 1.25MΩ TRIM GND ISL95810 RH VOUT TRIM FEEDBACK ISL21007, ISL21009 VIN 1.25MΩ 1000pF 10, 50kΩ SCL SDA I2C RL GND FIGURE 1. TRIM PIN CIRCUITRY FIGURE 3. CONNECTING THE ISL21007, ISL21009 TO ISL95810 Adjusting VOUT Using ISL95810 The ISL95810 DCP is low noise, and has a low power I2C bus with 256 taps and is implemented with a combination of resistor elements and CMOS switches. The position of the wiper is controlled by the user through the I2C bus interface. The potentiometer has an associated volatile Wiper Register (WR) and a non-volatile Initial Value Register (IVR) that can be directly written to and read by the user. The content of the WR controls the position of the wiper. At power-up, the device recalls the contents of the DCP’s IVR to the WR. A block diagram of the ISL95810 is shown in Figure 21. 1 1. Intersil Corporation ISL95810 data sheet. CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2007. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Technical Brief 473 Figure 4 shows the variation in the output of the ISL21007-25 as the Trim pin is swept from 0 to VOUT using ISL95810. The output voltage of the reference will vary up to nominal VOUT ±2.5%. The step size of the change in output voltage is 488µV due to the fact that the DCP used to adjust VOUT has 256 taps and the allowed Trim is ±2.5% VOUT. When the Trim pin voltage is VOUT/2, the output voltage of the part is the nominal output of the part, in this case 2.50V. Also, when the trim pin is floating at room temperature, the voltage at the trim pin is half the nominal VOUT as a result of the voltage divider at the Trim pin shown in Figure 1. 2.58 2.56 VOUT (V) 2.54 2.52 2.50 Given the parabolic shape of the TC curve, the standard for specifying the TC of a reference is the Box Method2. In the box method, the reference voltage is measured throughout the temperature range from the minimum specified temperature to the maximum specified temperature as shown in Equation 1: 6 ( VREFmax – VREFmin ) ⁄ ( Tmax – Tmin ) TC = ------------------------------------------------------------------------------------------------------------------- • 10 ppm ⁄ ( °C ) VREFnominal (EQ. 1) The two plots in Figure 5 show the change in output voltage over temperature as the Trim pin is adjusted using the DCP. The TC for the case when the voltage reference part is connected to the DCP for VOUT adjustment and when the trim pin is floating are approximately the same in value, 1.7ppm/°C. The trim pin should be left floating when no output voltage adjustment is desired. 2.48 2.46 2.5640 TC = 1.74ppm/°C 2.44 VOUT (V) 2.42 0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0 2.3 2.5 0 VOUT/2 VOUT VTRIM (V) 2.5638 FIGURE 4. VOUT (V) vs TRIM PIN VOLTAGE (V) 2.5636 2.5634 2.5632 2.5630 -40 -25 -10 TABLE 1. VREF (V) AS A FUNCTION OF TRIM PIN VOLTAGE (V) TRIM PIN VOLTAGE (V) VREF (V) 0 VOUT - 2.5% VOUT * 2.5008 VOUT/2 VOUT 2.5006 VOUT VOUT + 2.5% VOUT part Temperature Coefficient (TC) is a measure of the output voltage change with respect to changes in the operating temperature. The TC for ISL21007, ISL21009 is low and varies from 3ppm/°C to 10ppm/°C for B- and D-grades respectively. 20 35 50 65 80 TEMPERATURE (°C) 95 110 125 2.5004 2.5002 2.5000 Effects of Trimming on Voltage Reference Temperature Coefficient -5 TC = 1.71ppm/°C VOUT (V) *VOUT indicates nominal output voltage of the voltage reference TRIM PIN CONNECTED TO DCP 2.4998 TRIM PIN FLOATING -40 -25 -10 -5 20 35 50 65 TEMPERATURE (°C) 80 95 110 125 FIGURE 5. CHANGE IN OUTPUT VOLTAGE OVER TEMPERATURE AS A FUNCTION OF THE TRIM PIN CONNECTION Conclusion The output voltage of the ISL21007 and ISL21009 can be adjusted using the DCP without any significant change in the TC. As seen in Figure 5, even after the adjusting the output voltage, the TC of the voltage reference parts is well within the guaranteed limits of the datasheet specifications. 2. Intersil Corporation AN177 Application Note. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 TB473.1 July 10, 2007