Table 4. Design Parameters

8.2.2.1 Digital Isolators

The ISO7220M and ISO7221M isolators are used for this example. Please refer to the ISO722x documentation for device specific information. These devices support data rates of up to 150 Mbps and provide a low propagation delay necessary for bi-directional SPI communication at 12 MHz.

8.2.2.2 Supply Voltage for the LMP92064

A 5-V supply is required for the LMP92064. The LM4040-5 reference is used to generate the required voltage from the load’s –48-V supply. The LM4040-N is a precision micropower shunt voltage reference and is available in several fixed breakdown voltage options. The LM4040-5 provides the required 5-V breakdown voltage.

In a conventional shunt regulator application (Figure 24), the LM4040-5 requires a current limiting resistor connected between the positive supply voltage and the LM4040-5.

Figure 24. Shunt Regulator

The value of the resistor is determined by the positive supply voltage (V_S), the load current (I_L), the reference operating current (I_Q), and the LM4040-5’s reverse breakdown voltage (V_R).

Equation 1.

8.2.2.3 Series Resistor for the Shunt Regulator

The selection of R_S should satisfy two main conditions:

• R_S should be small enough to supply at least the minimum acceptable I_Q to the LM4040-5 even when the supply voltage is at its minimum and the load current is at its maximum value.
• R_S should be large enough so that the current flowing through the LM4040-5 is less than 15 mA when the supply voltage is at its maximum and the load current is at its minimum.

The minimum operating current of the LM4040AIM3-5/NOPB is 80 μA and its maximum operating current is 15 mA.

The typical supply current of the LMP92064 is 13 mA. The measured average current of the circuit, including the isolators, is 38 mA.

Figure 25. Shunt Regulator Voltages

From Figure 25, V_S = 0 V and V_R = –43 V. R_S can be calculated as follows:

Equation 2.

Choosing a smaller resistor, like a 1.1-kΩ resistor, will result in about 39.1 mA to flowing through R_S, providing a margin of 1 mA above the required 38.08 mA.

A variation in the bus voltage (V_BUS) of ±2% would result in less than ±0.87 mA of current variation. Given the additional current margin obtained by the 1.1-kΩ resistor, the shunt regulator would still have more than the required 80 μA of operating current.

The power rating of the series resistor should be selected according to the expected power to be dissipated. In normal operation, the resistor would dissipate 43 V x 39 mA = 1.677 W.

Excess current not used by the LMP92064 and isolators circuits will be burned by in the LM4040-5, and this current should never exceed 15 mA.

8.2.2.4 Voltage Channel Input Resistor Divider

The input buffer amplifier of the LMP92064's voltage channel can tolerate high source impedances, which enables scaling the bus voltage with the use of an external resistor divider. The accuracy of the voltage measurements depends on the accuracy of the components used for the resistor divider as well as the impedance of the divider.

In this example, the voltage channel can sense the voltage across R2 (see Figure 23). The main voltage should be scaled to a range below 2.048V by the resistive divider. If the resistive divider is always connected to the bus voltage, the series resistance of R2 + R3 should be adjusted (while keeping their ratio constant) to limit the current across the resistors within a permissible range for the application.

For simplicity, R2 is set to 1.6kΩ and R3 is set to 46.4 kΩ. The bus voltage of 48 V results in 1.6 V across R2 and the current flowing through R2 and R3 is 1 mA.

8.2.2.5 Sense Resistor Selection

The accuracy of the current measurement depends heavily on the accuracy of the current sense resistor. Its value depends on the application and it is a compromise between signal accuracy, maximum permissible voltage loss and power dissipation in the current sense resistor. High resistance values provide better accuracy at lower currents by minimizing the effects of offset, while low resistance values of minimize voltage loss in the load supply section, but at the expense of low-end accuracy.

In this example, the maximum sense voltage at the input of the current sense channel of the LMP92064 is 75 mV. Given the maximum load current requirement of 1 A, the sense resistor should be selected to be smaller than 75 mΩ.

The resolution at the input of the current sense channel of the device is 20 μV / code. To observe a change in the output code for a 1 mA change in sense current, the sense resistor should be larger than 20 mΩ.