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In PI circuits we often need to drive a power MOSFET that works at 12V with signals from a 5V microcontroller or with TTL levels. The obvious solution is to use a gate driver IC, but they might be expensive and/or hard to obtain in some areas.
Fortunately, the ubiquitous and dirt cheap NE555 (not the CMOS version!) comes handy for this task as well. It has a push/pull output that can drive or sink 250mA, ideal for the large input capacitance of power MOSFETs.
The trick is to add a resistor between the CONT terminal and ground thereby lowering the TRIGGER and THRESHOLD levels below 5V. The NE555 itself is fed with the 12V of the MOSFET's power source.
Calculating the resistor value is very easy by looking at the NE555 internal circuit. Three 5K resistors in series provide the reference TRIG and THRESH voltages, with the CONT terminal tapped at the THRESH node:
The resistor value is
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The level shifter circuit looks like this:
The threshold levels for tuning the MOSFET on and off are different (hystheresis), like this:
In order to avoid the MOSFET turning on accidentally, the output of the driving logic TTL circuitry (or MCU port) should be chosen as "open collector" (or configured as "open drain"), that way resistor R3 keeps the NE555 output at low level by default.
Remember: use the bipolar version (NE555, LM555, SE555). The outputs of the CMOS version (TLC/LMC) cannot deliver the high current required to drive the MOSFET fast enough for PI and the internal resistors are 100K instead of 5K.
Downside of this solution: the resistor bridge alone draws 1.7 mA which is not excessive but more than a proper MOSFET driver IC.
Fortunately, the ubiquitous and dirt cheap NE555 (not the CMOS version!) comes handy for this task as well. It has a push/pull output that can drive or sink 250mA, ideal for the large input capacitance of power MOSFETs.
The trick is to add a resistor between the CONT terminal and ground thereby lowering the TRIGGER and THRESHOLD levels below 5V. The NE555 itself is fed with the 12V of the MOSFET's power source.
Calculating the resistor value is very easy by looking at the NE555 internal circuit. Three 5K resistors in series provide the reference TRIG and THRESH voltages, with the CONT terminal tapped at the THRESH node:
The resistor value is
The level shifter circuit looks like this:
The threshold levels for tuning the MOSFET on and off are different (hystheresis), like this:
In order to avoid the MOSFET turning on accidentally, the output of the driving logic TTL circuitry (or MCU port) should be chosen as "open collector" (or configured as "open drain"), that way resistor R3 keeps the NE555 output at low level by default.
Remember: use the bipolar version (NE555, LM555, SE555). The outputs of the CMOS version (TLC/LMC) cannot deliver the high current required to drive the MOSFET fast enough for PI and the internal resistors are 100K instead of 5K.
Downside of this solution: the resistor bridge alone draws 1.7 mA which is not excessive but more than a proper MOSFET driver IC.
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