DESIGN FEATURES LTC1531 Isolated Comparator by Wayne Shumaker Basic Operation The LTC1531 is an isolated, self-powered comparator that receives power and communicates through internal isolation capacitors. The internal isolation capacitors provide 3000VRMS of isolation between the comparator and its output. This allows the part to be used in applications that require high voltage isolated sensing without the need to provide an isolated power source. The isolated side provides a 2.5V pulsed reference output that can deliver 5mA for 100µ s using the power stored on the isolated external capacitor. A 4-input, dual-differential comparator samples at the end of the reference pulse and transmits the result back to the nonisolated side. The nonisolated, powered side latches the result of the comparator and provides a zero-cross comparator output for triggering a triac. Typical applications include isolated temperature sensing and control, isolated voltage monitoring and other sensing applications riding on top of high common mode voltages, such as the AC power line. The block diagram in Figure 1 shows the basic components of the LTC1531. The nonisolated powered side toggles between pumping AC voltage through the capacitive barrier to the isolated side, where it is rectified and stored on an external capacitor tied to VPW, and listening for a comparison result. When the isolated-side VPW voltage reaches 3.3V, the comparison circuitry is enabled. On the next listen cycle, the 2.5V VREG output pulses on for 100µ s, at the end of which a comparison is done, with the result transmitted back to the nonisolated side. If a valid result is received, the DATA output is updated and the VALID output pulses on for 1ms. When the latched DATA output is high, the zero-cross comparator output is enabled for firing a triac whenever the zero-cross comparator inputs pass through 0V. Figure 2 represents a typical VPW start-up sequence, showing VREG output pulses after VPW reaches 3.3V. Thereafter, whenever V PW reaches 3.3V the comparator samples during the next listen period in the power/ listen cycle. Figure 2 shows typical sampling with light loading on VREG. Sampling is not uniform but depends on the combination of VPW = 3.3V and the 800Hz power/listen cycle. The comparator samples at a typical rate of 200Hz–300Hz. The actual sampling rate depends on the internal and external loading on the 2.5V VREG output and the charging rate to the VRIPPLE VCC = 5V CVPW = 1µF IVREG = 5mA 3.3 tSAMPLE 2.5 VPW VPW (V) Introduction VREF 100 0 200 300 TIME (ms) NOTES: VRIPPLE DEPENDS ON CVPW AND IVPW + IVREG tSAMPLE DEPENDS ON IVPW + IVREG 1531 F01 Figure 2. Typical V PW power-up and VREG samples ISOLATION BARRIER POWERED SIDE ISOLATED SIDE VPW 11 VCC V1 18 3.3V DET VCC 1 VOLTAGE PUMP TRANSMIT AND DRIVER VCC VALID 25 + LATCH + DATA 26 Q D VCC V2 17 Σ V3 16 COMPARE – TIMING Σ V4 15 TIMING DECODE 2.5V REG – R POWER-ON RESET VREG 13 ZCDATA 27 CMPOUT 12 GND 28 ZERO-CROSS COMPARATOR 4 3 ZCPOS ZCNEG 2 14 SHDN ISOGND 1531 BD Figure 1. LTC1531 block diagram 24 Linear Technology Magazine • November 1998 DESIGN FEATURES 3.3 VREG (V) 2.5 0 0 10 20 30 TIME (ms) 40 NOTE: NONPERIODIC SAMPLES DUE TO DEPENDENCE ON VPW > 3.3V AND THE POWER-LISTEN CYCLE SAMPLING 1531 F02 Figure 3. Typical V REG and VPW with IVREG = 100µA external capacitor on VPW. This charging rate, through the internal isolation capacitors to VPW, can be modeled as a 100k source resistance and a 5.5V source with V CC = 5V. Figure 4 shows typical sampling periods for different load currents and supply voltages. The sample rate does not depend on the external storage capacitor, whose value should be chosen to minimize ripple on VPW for different VREG loads. VPW can also be used to power continuous, low current circuits, such as the LT1495 op amp or the LTC1540 comparator, provided that such circuits do not prevent VPW from reaching 3.3V. Isolated Comparator The LTC1531 isolated switched capacitor comparator has four inputs that sum the voltages together to perform the following comparison: AC 120V HEATER 25Ω TECCOR Q4008L4 OR EQUIVALENT NEUTRAL VCC = 4.5V 20 15 VCC = 5.5V 10 VCC = 5V 5 0 0 1 2.5k 5W 3 4 1531 F03 thermistor and a resistor that is driven by the 2.5V VREG output. As the thermistor resistance rises with temperature, the voltage across the thermistor increases. When it exceeds the voltage across R4, the comparator output becomes zero and the triac control to the heater is turned off. Hysteresis can be added in the temperature control by using CMPOUT and R5. A 10° phase-shifted AC line signal is supplied through R1, R2 and C1 to the zero-cross comparator for firing the triac. In the overtemperature detect application in Figure 6, an isolated thermocouple is cold junction compensated with the micropower LT1389 reference and the Yellow Springs thermistor. The micropower LT1495 op amp provides gain to give an overall 0°C–200°C temperature range, adjustable by changing the 10M feedback resistor. The isolated comparator is connected to compare at 1.25V or ISOLATION BARRIER C1 0.01µF 2 IVREG (mA) Figure 4. Typical average tSAMPLE vs IVREG The LTC1531 can be used to isolate sensors such as in the isolated thermistor temperature controller in Figure 5. In this circuit, a comparison is made between the voltages across a R2 47k R1/(R1 + R2) = ATTENUATION R2 • C1 = Tan(θ)/(2π60Hz) θ = DESIRED PHASE LAG COMPARISON V1 – V3 > V4 – V2 R = RO • exp (B/T – B/TO) B = 3807 TO = 298°K + 390Ω 150Ω 2N2222 OR 2N3904 25 Applications R1 680k IN4004 30 (V1 + V2)/2 > (V3 + V4)/2 By rearranging the equation, for example, a dual differential comparison can be performed: (V1 – V4) > (V3 – V2) or (V1 – V3) > (V4 – V2) The comparator inputs have a railto-rail input range. They sample once at the end of the 100µ s VREG pulse. Their summing nature allows midVREG referencing, for example, by connecting V3 to VREG and V4 to ISOGND, which sums together to provide 1.25V for the negative comparator input. In the isolated temperature control application (Figure 5), the comparator is used to compare the voltage across the thermistor to the voltage across R4, with (V1 – V3) > (V4 – V2). The isolated comparator has an isolated output, CMPOUT, which can be used for hysteresis. This output is Hi-Z except when VREG is on; then the output is either 2.5V or 0V, depending on the result of the previous comparison. This output, in combination with the comparator, can be used to create a delta-sigma modulator for transmitting isolated voltage signals across the isolation barrier, as in the isolated voltage sense application (Figure 6). tSAMPLE (ms) VCC = 5V, CVPW = 1µF IVREG = 100µA VCC SHDN ZC + ZC – LED 1k 1µF VPW 2.5V ZCDATA VREG V1 THERM 30k YSI 44008 V2 DATA 5.6V + Q D – + 100µF VALID GND V3 V4 CMPOUT LTC1531 ISOGND R5 HYSTERESIS 1M R4 50k 1531 TA01 Figure 5. Isolated thermistor temperature controller Linear Technology Magazine • November 1998 25 DESIGN FEATURES the center of the temperature range. In this case, VTRIP goes high when the temperature exceeds 100°C. The LTC1531 can use the high impedance nature of CMPOUT as a duty-cycle modulator, as in the isolated voltage sense application in Figure 7. The duty-cycle output of the comparator is smoothed with the + ISOLATION BARRIER VCC Conclusion LT1490 rail-to-rail op amp to reproduce the voltage at VIN . The output time constant, R2 • C2, should approximately equal the input time constant, 35 • R1 • C1. The factor of 35 results from CMPOUT being on for only 100µ s at an average sample rate of 300Hz. The LTC1531 is a versatile part for sensing signals that require large isolation voltages. The ability of the LTC1531 to supply power through the isolation barrier simplifies applications; it can be combined with other micropower circuits in a variety of isolated signal conditioning and sensing applications. 1M LT1389 2.2µF 1.74M 10M 2.5V ZCDATA VREG V1 Q D 1.13k 10.7k + VTRIP + 33k LT1495 V2 DATA THERM 30k YSI 44008 10.2k VPW – ZC + ZC – SHDN – VCC V3 – K + V4 VALID GAIN SET FOR 0°C TO 200°C CMPOUT LTC1531 ISOGND – GND 1531 TA08 UNUSED OP AMP LT1495 COLD JUNCTION COMPENSATES 0°C TO 60°C OUTPUT, VTRIP = 1 AT ≥100°C RESPONSE TIME = 10 sec RESOLUTION = 4mV ≥ 0.5°C + Figure 6. Overtemperature detect ISOLATION BARRIER VCC R2 10M RESOLUTION = 4mV SETTLING TIME CONSTANT = 10 sec + VCC C2, 1µF SHDN ZC + ZC – 2.5V ZCDATA VCC – VREG VIN 0V TO 2.5V FULL-SCALE INPUT V1 V2 DATA + Q D – V3 V4 VCC LT1490 + VOUT 0V – VCC FULL-SCALE OUTPUT R3 10M 2.2µF VPW VALID CMPOUT 10k GND LTC1531 10k ISOGND R1 1M C1 0.22µF 1531 TA05 Figure 7. Isolated voltage detect for the latest information on LTC products, visit www.linear-tech.com 26 Linear Technology Magazine • November 1998