ZHCSN87 December 2021 AMC1351
PRODUCTION DATA
This discussion covers the 360-VDC example. The procedure for calculating the resistive divider for the 190-VDC use case is identical.
The 100-μA, cross-current requirement at peak input voltage (360 V) determines that the total impedance of the resistive divider is 3.6 MΩ. The impedance of the resistive divider is dominated by the top resistors (shown exemplary as R1 and R2 in Figure 8-1) and the voltage drop across RSNS can be neglected for a short time. The maximum allowed voltage drop per unit resistor is specified as 75 V; therefore, the total minimum number of unit resistors in the top portion of the resistive divider is 360 V / 75 V = 5. The calculated unit value is 3.6 MΩ / 5 = 720 kΩ and the next closest value from the E96 series is 715 kΩ.
The effective sense resistor value RSNSEFF is the parallel combination of the external resistor RSNS and the input impedance of the AMC1351, RIN. RSNSEFF is sized such that the voltage drop across the impedance at maximum input voltage (360 V) equals the linear full-scale input voltage (VFSR) of the AMC1351 (that is, 5 V). RSNSEFF is calculated as RSNSEFF = VFSR / (VPeak – VFSR) × RTOP, where RTOP is the total value of top resistor string (5 × 715 kΩ = 3575 kΩ). The resulting value for RSNSEFF is 9.96 kΩ. In a final step, RSNS is calculated as RSNS = RIN × RSNSEFF / (RIN – RSNSEFF). With RIN = 1.25 MΩ (typical), RSNS equals 52.47 kΩ and the next closest value from the E96 series is 52.3 kΩ.
Table 8-2 summarizes the design of the resistive divider.
| PARAMETER | 190-VDC LINE VOLTAGE | 360-VDC LINE VOLTAGE |
|---|---|---|
| Unit resistor value (RTOP) | 634 kΩ | 715 kΩ |
| Number of unit resistors in RTOP | 3 | 5 |
| Sense resistor value (RSNS) | 51.1 kΩ | 49.9 kΩ |
| Total resistance value (RTOP + RSNS) | 1953.1 kΩ | 3624.9 kΩ |
| Resulting current through resistive divider (ICROSS) | 97.3 μA | 99.3 μA |
| Resulting full-scale voltage drop across sense resistor RSNS | 4.971 V | 4.956 V |
| Peak power dissipated in RTOP unit resistors | 6 mW | 7.1 mW |
| Total peak power dissipated in resistive divider | 18.5 mW | 35.8 mW |