Table 10. Design Parameters

10.2.2.1 Transceiver Configuration (SN65HVD101)

1. Choose a 24 V nominal dc supply for L+.
2. From the current-limit characteristics in Figure 2 derive the resistor value of R_SET = 4 kΩ for a current limit of I_O(LIM) = 250 mA.
3. Connect V_{CC_SET} to ground for a 3.3V output at V_{CC_OUT}.
4. Connect V_{CC_IN} to V_{CC_OUT} to assure proper output voltage regulation of V_{CC_OUT}.
5. Buffer V_{CC_OUT} with a 3.3μF, 10V ceramic capacitor.
6. Connect the receiver and diagnostic outputs via 10 kΩ pull-up resistors to V_{CC_OUT} to provide defined voltage potentials to the controller inputs during high-impedance states.
7. Connect the driver enable pin, EN, via a 10 kΩ pull-down resistor to ground to maintain the driver disabled during power up.

10.2.2.2 Maximum Ambient Temperature Check

For a 250 mA current limit, the maximum voltage drop across the high-side switch is given with V_RQH = 3 V (taken from Electrical Characteristics: Driver section). This causes an internal power consumption of:

Equation 1.

Multiply this value with the Junction-to-ambient thermal resistance of θ_JA = 33.8 °C/W (taken from Thermal Information) to receive the difference between junction temperature, T_J, and ambient temperature, T_A:

Equation 2.

Add this value to the maximum ambient temperature of T_A = 100 °C to receive the final junction temperature:

Equation 3.

As long as T_J-max is below the recommended maximum value of 150 °C, no overheating will occur.

10.2.2.3 Transient Protection

A commonly applied surge immunity test in digital I/O designs is the application of the 1.2/50 – 8/20 μs combination waveform, specified in IEC61000-4-5, with a source impedance of 500 Ω and a peak test voltage of V_O-pk = 2 kV, which results in a peak surge current of I_S-pk = 4 A.

The test set-up for line-to-line and line-to-ground measurements is shown in Figure 12; the calculation of the surge peak current is shown in Figure 13.

Figure 12. Surge Test Set-up		Figure 13. Peak Current Calculation

10.2.2.4 TVS Evaluation

Because the maximum transceiver supply at L+ is specified with 30 V, the TVS standoff or peak working voltage must be higher than this value. The standoff voltage is the voltage where the TVS does not conduct yet. Transient voltage suppressors with this high standoff have peak pulse powers starting at 200 W. Their peak pulse power however is usually rated based on a 10/1000 μs current pulse, which is commonly applied in telecom application. It is therefore necessary to derate the peak power value from a 10/1000 μs to a 8/20 μs pulse.

For this example the bidirectional, 200 W TVS, SMF33CA, was selected. Its main parameters and their derated values are listed in Table 11. The peak pulse power rating for a 10/1000 μs pulse is shown in Figure 14.

Figure 14. Peak Pulse Power Rating

Figure 15. Pulse Derating Curve

At the pulse duration of 1000 μs the device has a peak pulse power rating of 200 W. To determine the peak power for a 8/20 μs pulse, move up the power rating curve until you hit the 20 μs pulse duration. The peak pulse power rating at this point is about 950 W.

Note these values are valid for 25 °C ambient temperature only. Because the operating ambient temperature in this example is specified with 100 °C however, the peak pulse power must be further derated for the higher ambient temperature using the pulse derating curve in Figure 15. This curve shows that the peak power level at 25 °C drops by 50 % when reaching 100 °C, so from 950 W down to 475 W. This drop is shown in Figure 14 through the arrow pointing down to the second peak power level for a 20 μs pulse duration at 100 °C.

To determine the peak currents for the various peak power ratings, TVS manufacturers advise to assume the maximum clamping voltage as being constant, because this clamping level also presents the device maximum failing voltage if its value is exceeded. The peak current for a given power rating is therefore calculated via:

Equation 4.

So for the 8/20 μs peak power of 475 W at 100 °C, the peak pulse current is I_PP3 = 475 W ÷ 53.3 V = 8.9 A. The new derived values of P_PK and I_PP in combination with the values for breakdown and clamping voltage, V_BR and V_CL, from Table 11, yield a new I-V characteristics of the SMF33CA TVS when exposed to a 8/20 μs pulse.

Figure 16. TVS Characteristic for 8/20 μs Current Pulse

Because the maximum surge current of the CWG in Figure 13 is only 4 A at 2 kV test voltage, the TVS clamping voltage at this level is only 44 V. This voltage is sufficiently below the absolute maximum voltage rating of 50 V for a 100 μs pulse at the L+ and CQ terminals of the SN65HVD101 and SN65HVD102 transceivers, causing no device damage.