Pull Down Resistor Placement

April 23, 2018

An example of a pull down resistor in a digital circuit can be seen in the figure. The second circuit puts a large limit on gate charging current and the voltage divider on the gate will affect on resistance increased dissipation on the mosfet.

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I mean it often does that but its real intent is to do exactly as the name says.

Pull down resistor placement. Placement of termination resistor pull up and pull down are different from series and parallel terminations. If thats an issue. Pull down to prevent the motor from turning on if the driver isnt actively driving the gate.

This pull down resistor configuration is particularly useful for digital circuits like latches counters and flip flops that require a positive one shot trigger when a switch is momentarily closed to cause a state change. Pull down resistors work in the same manner as pull up resistors except that they pull the pin to a logical low value. A pull down resistor works in the same way as the previous pull up resistor except this time the logic gates input is tied to ground logic level 0 low or it may go high by the operation of a mechanical switch.

Pull up and pull down resistors are used to make the signal high and low wrt the resistor value. When the switch is closed the input is damn sure tied to ground. Terminations methodology is for signal integrity or delay corrections.

The purpose of a pull x resistor is not about preventing a short. This depends on whether the ic drives the output high or low when it wants to assert it. Resistor to limit gate charging current.

It is to pull then input up or down. The 10k or 20k or 50kwhatever pull down would be used if leakage from the mcu or emi sufficient enough to charge the gate capacitance is a problem at any point during the operation cycle of the final design. As more realistic example consider the pin tied to an powerful output buffer with a low output impedance of 5.

They are connected between ground and the appropriate pin on a device. Choosing an appropriate pull uppull downresistor for open drain outputs ben hopf. It is typically used in combination with components such as switches and transistors which physically interrupt the connection of subsequent components to ground or to v cc.

Otoh if you are more wisely using a pull down resistor of 10k ohms to ground then you really wont need a series resistor on the jtag line at all because the low output impedance of the jtag programmer can easily drive the line high. The first thing to recognize when dealing with an open drain output is whether a pull upor a pull down resistor is needed. In electronic logic circuits a pull up resistor or pull down resistor is a resistor used to ensure a known state for a signal.

But when the switch is openthe input is floating.

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