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LTC1531 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1531 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC1531 The zero-cross comparator inputs, ZCPOS and ZCNEG, have an input common mode range that allows signal swings near the positive supply rails. The ZCPOS and ZCNEG inputs contain ESD diode protection devices which will clamp input signals that go below GND. The current into the diode should be limited to less than 5mA. The Isolated Thermistor Temperature Controller shows an example phase shift network with attenuation that satis- fies these conditions. The positive input voltage should not exceed the 12V maximum rating or the 5mA input current to the ESD diode clamp. ISOLATION dV/dt The maximum continuous dV/dt across the isolation bar- rier that will still allow the isolated comparator to operate is 50V/ µs. Continuous rates of dV/dt greater than this cause the isolated side to not detect when its power cycle has stopped and a comparison should begin. Figure 4 shows the maximum continuous rate trade-off of fre- quency vs voltage of a sine wave: SR = ( π)(f)(VP-P) where SR = slew rate, VP-P = the peak-to-peak voltage and f = frequency. Noise immunity to intermittent dV/dt rates greater than 50V/ µs can be rejected by the LTC1531. AC Noise Rejection and Things to Avoid Minimizing AC noise pickup at the isolated comparator should follow the following guidelines. 1. Allow the isolated side ground to float. The isolated side should only be common with the isolated circuit. 2. Use hysteresis to decrease sensitivity by using CMPOUT. 3. Use lower impedance circuits if powered by VREG. Avoid large capacitance tied directly to VREG output, since this output does not go Hi-Z (high impedance) during the off time. PC Board Layout The PC board layout should not have copper near the lead frame isolation capacitors. The copper reduces the power coupling and power delivery to the isolated side (see Figure 6). APPLICATIONS INFORMATION TYPICAL APPLICATIONS DESCRIPTION The Isolated Thermistor Temperature Controller (front page) uses a simple AC power rectifier and Zener to provide 5.6V of DC power to the LTC1531. To avoid power dissipation in the 3k, 3W resistor, DC power can be provided with a charge pump circuit similar to the Isolated Switch Control. In this circuit, a thermistor half bridge is used with the 4-input comparator connected to provide the other half of the bridge, V4 = 2.5V, V3 = 0V, giving (V4 + V3)/2 = 1.25V. With the 50k pot set to about 30k, the trip point is 25 °C with a hysteresis of ±0.5°C. Here, VREG turns on and powers the half-bridge while the comparator samples the result. The zero-cross comparator, ZCPOS and ZCNEG, is used to trigger a triac at the 10 °phasepoint. The circuit, R1, R2 and C1, provides the phase shift, θ, as determined by: R2C1 ≅ tan(θ)/2π60Hz and where the attenuation ≅ R2/R1. In this example, R1 = 680k, R2 = 47k and C1 = 0.01 µF, provide a 7V peak input signal with 10 ° of phase lag. Figure 6. PC Board Layout Consideration 1 2 3 4 11 12 13 14 28 27 26 25 18 17 16 15 VCC SHDN ZCNEG ZCPOS VPW CMPOUT VREG ISOGND GND ZCDATA DATA VALID ETCH COPPER FROM THE SHADED AREA V1 V2 V3 V4 SW PACKAGE 28-LEAD PLASTIC SO (ISO) TOP VIEW 1531 F06 |
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