Current Limits Measurement Examples
The following examples use the driver selection reference to evaluate an interface from a Cyclone® series device to the Intel® Cyclone® 10 LP, Cyclone® IV, or Cyclone® III device receiver using the 3.3 V LVTTL I/O standard via unterminated transmission line.
For the interface evaluation in these examples, the current strength of the Cyclone® device is 8 mA.
Input Setup with 2.5 V VCCIO
The supported Intel® device's on-chip PCI diode starts to sink the DC current IDC when driven by a steady state voltage greater than the sum of the supported Intel® device's VCCIO and diode forward voltage in the following figure. The diode is forward-biased when the driver's VCC is 3.3 V or 3.0 V and the IDC must not exceed 10 mA.
Figure 9. Supported Intel® Device Input Setup with 2.5 V VCCIO
The amount of IDC through the diode is determined by the potential voltage difference between the driver and the supported Intel® device's pin, and the current capability of the driver. The diode can limit the transient voltage level to below the specification limit when the driver's VCC is 2.5 V—effectively limiting it to 3.325 V with the assumption that VCCIO is 2.625 V and diode forward voltage is 0.7 V.
Input Setup with 3.0 V VCCIO
The diode might not be forward-biased even when the driver's VCC is 3.3 V as the potential voltage difference between the driver's VCC and the supported Intel® device's VCCIO is less than the diode forward voltage. Therefore, there is no concern on IDC through the diode when driven with an input voltage level of 3.3 V, 3.0 V, or 2.5 V as shown in the following figure.
Figure 10. Supported Intel® Device Input Setup with 3.0 V VCCIO
The forward-bias of the diode occurs only momentarily during overshoot conditions to clamp the overshoot voltage level. In such cases, the diode is effective in limiting the transient voltage level to below the specification limit—effectively limiting it to 3.85 V with the assumption that VCCIO is 3.15 V and diode forward voltage is 0.7 V.
Input Setup with 3.3 V VCCIO
IDC is almost zero at a steady input voltage level as the diode might not be forward-biased as shown in the setup in the following figure. At a higher VCC level of the driver, such as 3.465 V, the diode clamps transient voltage level at 4.165 V with the assumption that diode forward voltage is 0.7 V.
Figure 11. Supported Intel® Device Input Setup with 3.3 V VCCIO
The use of a lower driver current capability reduces the voltage overshoot level. You must ensure that the duration of the overshoot is below these limits:
- For Intel® Cyclone® 10 LP, Cyclone® IV, and Cyclone® III devices, the percentage of high time for an overshoot of 4.15 V can be as high as 18.52% over a 10-year period.
- For Intel® MAX® 10 devices, the percentage of high time for an overshoot of 4.17 V can be as high as 11.7% over a 10-year period.