Using the HI7188 with a Single Supply Application Note May 1996 AN9620 Author: John D. Norris Introduction ICL7660S Voltage Converter The HI7188 is an easy-to-use, 16-bit, 8-channel, sigma-delta A/D subsystem ideal for low frequency physical and electrical measurements in scientific, medical, and industrial applications. The device has sufficient on-chip capabilities to support moving the intelligence away from the system controller and towards the sensors for faster and more flexible configurability without the traditional drawbacks of low throughput per channel, higher power and cost per channel, extreme design complexity or excessive software overhead. The ICL7660S Voltage Converter is a monolithic CMOS voltage conversion IC. The device performs supply voltage conversion from positive to negative for an input range of 1.5V to 12V, resulting in complementary output voltages of -1.5V to -12V. Only 2 noncritical external capacitors are needed for the charge pump and charge reservoir functions. The conversion chip contains a series DC power supply regulator, RC oscillator, voltage level translator, and four output power MOS switches. For detailed chip information please refer to the ICL7660S datasheet [1]. The purpose of this application note is to inform the system designer how to use the HI7188 with a positive analog supply if a negative analog supply is not available. The purpose of the negative supply is bias the analog section of the HI7188. There are two approaches to accomplish the single supply task. Each approach will maintain full operation of the HI7188. The first approach would involve using the ICL7660S voltage converter to derive an inexpensive low current negative power supply. The second approach involves offsetting the positive analog supply (AVDD) to 10V, providing +5V to VCM, and simply grounding the negative supply (AVSS). ICL7660S VOLTAGE CONVERTER Functional Description The ICL7660S single supply circuit is shown in Figure 1. This includes the HI7188, an ICL7660 voltage converter, and two 10µF capacitors. The HI7188 datasheet [2] contains information on the HI7188, therefore, this discussion will focus on the voltage conversion circuit. 29 +5V 0.1µF + 10µF - C1 1 NC V+ 8 2 CAP+ NC 7 3 GND LV 6 AVDD + 4.7µF HI7188 26 VCM -5V 4 CAP- VOUT 5 10µF + C 2 9, 30 AVSS 0.1µF FIGURE 1A. VOLTAGE CONVERSION CIRCUIT + FIGURE 1. ICL7660S SINGLE SUPPLY CIRCUIT 1 1-888-INTERSIL or 321-724-7143 | Copyright © Intersil Corporation 1999 Application Note 9620 ICL7660 Theory of Operation [1] ICL7660 Application Discussion The ICL7660S contains all the necessary circuitry to complete a negative voltage converter, with the exception of 2 external capacitors which may be inexpensive 10µF polarized electrolytic types. The mode of operation of the device may be best understood by considering Figure 2, which shows an idealized negative voltage converter. Capacitor C1 is charged to a voltage, V+, for the half cycle when switches S1 and S3 are closed. (Note: Switches S2 and S4 are open during this half cycle.) During the second half cycle of operation, switches S2 and S4 are closed, with S1 and S3 open, thereby shifting capacitor C1 to C2 such that the voltage on C2 is exactly V+, assuming ideal switches and no load on C2. The ICL7660S approaches this ideal situation more closely than existing non-mechanical circuits. The output characteristics of the voltage converter (Figure 1A) can be approximated by an ideal voltage source in series with a resistance as shown in Figure 3. The voltage source has a value of -(V+) with a slight ripple voltage due to the ICL7660S switching between C1 and C2. The output impedance (RO) is a function of the ON resistance of the internal MOS switches (shown in Figure 2), switching frequency, the value of C1 and C2, and the equivalent series resistance (ESR) of C1 and C2. 8 S1 2 RO VOUT V+ + S2 FIGURE 3. ICL7660S EQUIVALENT CIRCUIT VIN C1 3 3 C2 S4 S3 5 VOUT = -VIN 4 7 FIGURE 2. IDEALIZED NEGATIVE VOLTAGE CONVERTER In the ICL7660S, the 4 switches of Figure 2 are MOS power switches; S1 is a P-Channel devices and S2, S3 and S4 are N-Channel devices. The main difficulty with this approach is that in integrating the switches, the substrates of S3 and S4 must always remain reverse biased with respect to their sources, but not so much as to degrade their “ON” resistances. In addition, at circuit start up, and under output short circuit conditions (VOUT = V+), the output voltage must be sensed and the substrate bias adjusted accordingly. Failure to accomplish this would result in high power losses and probable device latchup. This problem is eliminated in the ICL7660S by a logic network which senses the output voltage (VOUT) together with the level translators, and switches the substrates of S3 and S4 to the correct level to maintain necessary reverse bias. The voltage regulator portion of the ICL7660S is an integral part of the anti-latchup circuitry, however its inherent voltage drop can degrade operation at low voltages. Therefore, to improve low voltage operation “LV” pin should be connected to GND, disabling the regulator. For supply voltages greater than 3.5V the LV terminal must be left open to insure latchup proof operation, and prevent device damage. 2 Below is an analysis of this application at 25oC. From the datasheet the MOS switch resistance is typically 23Ω, switching frequency is 10kHz. The capacitors C1 and C2 are 10µF, 16V Kemet Solid Tantalum capacitors type number “T350106K016AS” with ESR of 1Ω at 10kHz. The typical supply current into the HI7188 negative supply is 1mA. The low current requirement of the HI7188 is critical since the conversion chip is ONLY designed for low current applications. As defined in the ICL7660S datasheet, the inverted output voltage drops significantly from -5V to -4.25V when 10mA of current is required. If additional current is needed to drive supplementary devices, multiple ICL7660S units can be placed in parallel [1]. Below are the theoretical calculations for output impedance and ripple voltage. 1 R O ≅ 2 × R SW + ------------------------------------ + 4 × ESR C1 + ESR C2 Ω 0.5f OSC × C 1 1 R O ≅ 2 × 23 + -------------------------------------------------------- + 4 × ESR C1 + ESR C2 3 –6 ( 5 × 10 × 10 × 10 ) R O ≅ 46 + 20 + 5 × 1Ω ≅ 71Ω 1 V RIPPLE ≅ I OUT ----------------------------------------- + 2ESRC 2 2 × f PUMP × C 2 1 V RIPPLE ≅ I OUT ------------------------------------------------- + 2 2 × 10kHz × 10µF V RIPPLE ≅ 7mV Measured Performance The circuit in Figure 1A was added to the evaluation platform to determine performance. The testing consisted of both noise and linearity with the standard -5V supply and a -5V supply derived from the ICL7660S. Noise data was derived from 100 readings while linearity involved single measurements in 0.5V increments. Comparison of the performance data showed no degradation of either noise or linearity while using the ICL7660S voltage converter. Application Note 9620 +10V Supply Operation Measured Performance The application circuit is shown in Figure 4. The positive analog supply (AVDD) is tied to 10V while the negative supply (AVSS) is analog ground. To ensure proper operation, the virtual ground pin (VCM) must be set midway between the positive and negative supplies. In this case +5V. This is critical to ensure the chopper stabilized operational amplifier is biased correctly otherwise performance would be severely degraded. The circuit in Figure 4 was added to the evaluation platform to determine performance. The testing consisted of both noise and linearity with the standard +-5V supplies and the single 10V supply as shown in Figure 4. Noise data was derived from 100 readings while linearity involved single measurements in 0.5V increments. Comparison of the performance data showed no degradation of either noise or linearity while using the single 10V supply circuit. Conclusion 29 +10V AVDD + 0.1µF 4.7µF 26 +5V 9, 30 HI7188 VCM This application note described two methods of using the HI7188 with a single positive analog supply if a negative analog supply was not available. The first method involved the ICL7660S voltage converter to derive an inexpensive low current negative power supply. The second method involved offsetting the positive analog supply (AVDD) to 10V, grounding the negative supply (AVSS) and biasing the virtual ground pin to +5V. References AVSS [1] ICL7660S Datasheet, File Number 3179, Intersil Corporation, Melbourne, Florida, 1994. FIGURE 4. 10V SUPPLY CIRCUIT [2] HI7188 Datasheet, File Number 4016, Intersil Corporation, Melbourne, Florida, 1995. All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 quality systems certification. Intersil semiconductor products are sold by description only. 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