The current-sense emulation function creates a scaled replica of the GaN power FET positive drain current at the output of the CS pin. The current-sense emulation gain, G_CSE, is 0.554mA output from the CS pin, I_CS, for every 1A passing into the drain of the low-side GaN power FET, I_D.

The CS pin terminates with a resistor to AGND, R_CS, to create the current-sense voltage input signal to the external power supply controller.

R_CS is determined by solving for the traditional current-sense design resistance, R_CS(trad), and multiplying by the inverse of G_CSE. The traditional current-sense design creates the current-sense voltage, V_CS(trad), by passing the GaN power FET drain current, I_D, through R_CS(trad). The LMG2650 creates the current-sense voltage, V_CS, by passing the CS pin output current, I_CS, through R_CS. The current-sense voltage must be the same for both designs.

The CS pin is clamped internally to a typical 2.5V. The clamp protects vulnerable power-supply controller current-sense input pins from over voltage if, for example, the current sense resistor on the CS pin were to become disconnected.

Figure 7-2 shows the current-sense emulation operation. In both cycles, the CS pin current emulates the GaN power FET drain current while the GaN FET is enabled. The first cycle shows normal operation where the controller turns off the GaN power FET when the controller current-sense input threshold is tripped. The second cycle shows a fault situation where the LMG2650 overcurrent protection turns off the GaN power FET before the controller current-sense input threshold is tripped. In this second cycle, the LMG2650 avoids a hung controller IN pulse by generating a fast-ramping artificial current-sense emulation signal to trip the controller current-sense input threshold. The artificial signal persists until the IN pin goes to logic-low which indicates the controller is back in control of switch operation.