The input resistor limits the current that flows into the e-diode when it is forward biased. The threshold current I_FLH is 2.8 mA typ. The recommended operating range for the forward current is 7 mA to 16 mA (e-diode ON). All the electrical specifications are guaranteed in this range. The resistor should be selected such that for typical operating conditions, I_F is 10 mA. Following are the list of factors that will affect the exact value of this current:

1. Supply Voltage V_SUP variation
2. Manufacturer's tolerance for the resistor and variation due to temperature
3. e-diode forward voltage drop variation (at I_F=10 mA, V_F= typ 2.1 V, min 1.8 V, max 2.4 V, with a temperature coefficient <1.35 mV/°C and dynamic impedance < 1 Ω)

Figure 9-2 Configuration 1: Driving the input stage of UCC23511 with a single NMOS and split resistors

Driving the input stage of UCC23511 using a single buffer is shown in Figure 9-3 and using 2 buffers is shown in Figure 9-4

Figure 9-3 Configuration 2: Driving the input stage of UCC23511 with one Buffer and split resistors

Figure 9-4 Configuration 3: Driving the input stage of UCC23511 with 2 buffers and split resistors

Table 9-2 shows the range of values for R_EXT for the 3 different configurations shown in Figure 9-2, Figure 9-3 and Figure 9-4.The assumptions used in deriving the range for R_EXT are as follows:

1. Target forward current I_F is 7mA min, 10mA typ and 16mA max
2. e-diode forward voltage drop is 1.8V to 2.4V
3. V_SUP (Buffer supply voltage) is 5V with ±5% tolerance
4. Manufacturer's tolerance for R_EXT is 1%
5. NMOS resistance is 0.25Ω to 1.0Ω (for configuration 1)
6. R_OH(buffer output impedance in output "High" state) is 13Ω min, 18Ω typ and 22Ω max
7. R_OL(buffer output impedance in "Low" state) is 10Ω min, 14Ω typ and 17Ω max