Hi All,
My customer wants to know forward regulation mode.
What kind of usage state is assumed?
Also, what kind of behavior is the gate voltage in Forward regulation mode?
It is a yellow part of the figure below.
I made the top side 6 V.
Please point out if it is wrong.
Best Regards,
Koji Hayashi
Hello Koji,
In forward regulation mode, anode to cathode voltage, gate of the external MOSFET is controlled by LM74700 to achieve 20mV of anode to cathode voltage.
Let us assume the startup behaviour. During startup, as the LM74700 small time to turn on, body diode of the external MOSFET will conduct and cathode will be one diode drop lower than anode. ANODE - CATHODE is about 600mV is > 50mV, Gate of the MOSFET will be turned on to achieve 20mV ANODE-CATHODE voltage. Now Value of the gate voltage will be dependent on the Load current:
For MOSFET "DMT6007LFG", at 2A load current, to meet the 20mV ANODE-CATHODE regulation, GATE-ANODE is regulated close to 3.5V above, so that RdsON is 10mohm.
That is 10mohm * 2A = 20mV. Please refer to the below picture.
Now, the load current increases to 4A, then GATE-ANODE is regulated close to 6V so that RdsON is 5mohm (because 5mohm * 4A = 20mV).
Now if the load current reduces to 1A, the GATE-ANODE is regulated close to 3.25V so that RdsON is 20mOhm.
In normal operation, GATE-ANODE is expected to operate in the range of Vth (of MOSFET) to 12V. In the above case it will be 3.25V to 8V (or 10V).
We recommend to select the FET such that 20mV/LoadCurrent < RdsON < 100mV/LoadCurrent.
Regards,
Kari.
