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CPC5002G Datasheet(PDF) 8 Page - IXYS Corporation |
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CPC5002G Datasheet(HTML) 8 Page - IXYS Corporation |
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8 / 14 page  INTEGRATED CIRCUITS DIVISION 8 www.ixysic.com R01 CPC5002 3 Functional Description 3.1 Introduction The CPC5002 provides two independent galvanically isolated high speed open-drain output optical isolators in a single 8-pin package. It exhibits excellent isolation (3750Vrms) and speed (10Mbps typical), and operates over a wide range of supply voltages (2.7V to 5.5V). Because the active circuits have been fabricated in a CMOS process, the device requires much less supply current (1.4mA typical with VDD = 3.3V) and can run at much lower LED currents (1.4mA minimum) than similar devices fabricated with bipolar processes. 3.2 Functional Description An open-drain output of the CPC5002 will activate and sink current when the light generated by the LED and passed across the barrier to the photodetector is sufficient. The minimum level of input current necessary to initiate this behavior is referred to as the LED Input Threshold Current (ITH) and is a function of the optical current transfer ratio of the device. To provide consistent performance over the LED Input Threshold Current range, the recommended typical LED drive current (IF) over temperature and all operating conditions, is 1.5mA. This recommendation is provided to offer a balance in the propagation delays on both the falling and rising edges of the signal pulse being buffered across the barrier. The absolute value of the mismatch in the delay of these two edges is Pulse Width Distortion. In the specifications these delays are identified as tPHL and tPLH while the distortion is PWD. In general, choosing a higher LED drive current will decrease tPHL, the propagation time for the output to go from high to low. This is mostly due to the LED generating more light more quickly as it turns on. However, if IF is more than 2 x ITH then increasing the LED drive current further will cause tPLH, the propagation time for the output to go from low to high, to increase. Excess levels of IF makes the difference between tPLH and tPHL (also known as pulse width distortion) greater. Pulse width distortion is often of interest when the signal being isolated is a clock. Keeping the LED drive current near 1.5mA and using the minimum RPU and CL at the output reduces the worst case pulse width distortion and is thus recommended for best waveform fidelity. When using 1.5mA of LED drive current and when the CPC5002 is driving a fast output bus (one with minimum RPU and CL), the average tPHL will usually be slightly longer than the average tPLH. In this case, reduction of average pulse width distortion can be accomplished by using a small feed forward capacitor. The capacitor boosts the instantaneous current applied to the LED at turn-on (reducing tPHL) while leaving the applied DC input current at 1.5mA (tPLH unchanged). Examples of the feed forward capacitor (CFWD) are shown in "Figure 1. Inverting Configuration” on page 9 and "Figure 2. Non-Inverting Configuration” on page 9. Increasing the value of the feed forward capacitor causes tPHL to decrease. For a 499 pullup into a 20pF load capacitance (CL), a 10pF capacitor across the series resistor will minimize pulse width distortion of an average unit. When parallel digital signals are to be isolated, propagation delay skew (tPSK) becomes important. It is defined as the absolute value of the difference between the maximum and minimum propagation delays (i.e. the worse of t PLH or tPHL) for any group of optical isolator channels operating under the same conditions. For the CPC5002, the delay tPLH has a wider variation with differing optical current transfer ratios than the delay tPHL. Additionally, tPLH will exhibit variation due to RPU and CL differences between channels. If one channel is to be used as a clock and another for data, it is recommended to use the CPC5002 output falling edge to latch the data as this edge will exhibit less channel-to-channel or part-to-part timing variation and thus will reduce worst case timing skew. In general the current transfer ratio matching between the two channels in a single CPC5002 is better than the ratio matching between multiple parts. Thus the channel to channel skew for two signals isolated through the same CPC5002 will be statistically better than skew measured between signals isolated through multiple parts. |
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