6 Working principle of LM74610-Q1 reverse-voltage range extension

Depletion-mode MOSFETs are by default on when MOSFET V_GS is 0V, unlike enhanced-mode MOSFETs that require V_GS to be greater than the threshold voltage of the MOSFET. In order to turn off the depletion MOSFET, V_GS needs to be <0V (typical ranges are from –1V to –4V). To analyze the effect of the depletion-mode MOSFET in an ideal diode-sense path, let’s look at device operation under these conditions:

• When V_PV– V_PV+: The ideal diode controller is in forward-condition mode, keeping both the power MOSFET Q₁ and depletion FET Q_D on. With these operating conditions, you can calculate the output voltage as V_OUT = V_IN – (I_{D_Q1} R_{DS(on)_Q1}), approximated to V_PV+.
• When V_PV– < V_PV+: The ideal diode controller is in the reverse current blocking condition, with MOSFET Q1 turned off. MOSFET Q_D is in regulation mode as a source follower, maintaining V_CATHODE above V_ANODE, V_CATHODE = V_IN(V_ANODE)+ (V_GSMAX). So the voltage across V_CATHODE to V_ANODE is within the absolute maximum rating V_GSMAX of Q_D (usually <5V), which is far less than the maximum reverse voltage of 45V transient of the LM74610-Q1. The high reverse voltage (V_OUT – V_IN) is sustained by the drain-to-source voltage (V_DS) of Q_D and Q₁.

Selecting the correct depletion MOSFET and power MOSFET depends on these points:

• Choose a V_DS rating of Q₁ and Q_D greater than the maximum peak input voltage.
• Select R_DS(on) such that dissipation across the power-path MOSFET is lowest. The drain current (I_D) of the FET should be higher than the maximum peak current demanded by the output load. Selecting a depletion MOSFET with a drop of 50mV to 100mV across the power MOSFET at the full load current is a good starting point.
• R_DS(on) can be in the hundreds of ohms range (the LM74610-Q1’s floating gate-drive architecture has a large impedance of cathode pin to ground, and the I_CATHODE of the controller is in the microamperes range).

Figure 5 shows test results for a 60V bypass switch solution using the 40V LM74610-Q1 controller.

With properly scaled MOSFETs (Q1 and Q_D), the input voltage range can extend to the V_DS rating of the FETs. This enables high-voltage designs using the same low-voltage controller. Also, extending the input voltage range can also be useful in enterprise, communication, power tool and high-voltage battery-management applications.