What's the purpose of these pull-up resistors?

Other Hardware General Electronics
1

Hello everybody, I'm taking a look at this schematic which represents a controller for a 16x8 RGB LED matrix. I can't understand why there are pull-up resistors at the output of the 74HC138 since this component is not an open collector type. Firstly I thought that they invert the 138 output signal (I'm not sure about that tho); if this was their purpose, wouldn't be better using a 74HC238 which is exactly the same of his "brother" but has an 'active HIGH' tecnology? Thanks for replies

2

Perhaps they are more for the IRF7314 gate inputs than the 74HC138 outputs. Maybe they keep the FETs turned off during power-up.

3

Thanks for your response, it makes sense to me. Same applies for the pull-down probably resistors upon the IRF7314s drains, do you agree?

4

Matt:
Thanks for your response, it makes sense to me. Same applies for the pull-down probably resistors upon the IRF7314s drains, do you agree?

Yes.

5

Ok, what do you think about series resistors at the outputs of the 74HC138? I'd use them (something like 220-330 Ohm) in order to limit the output current when the gate capacitor is discharged. I honestly think that they're needed to avoid damages at the IC.

6

NO... But they are Very useful in "De-Spiking" the Vcc as they 'smooth' out the off to on transitions by slowing very slightly the rise time of the gate pulse so that the instantaneous or edge current requirements are reduced, they are also useful to prevent a nasty oscillation that can occur in some types of rapid switching. The oscillation is caused by certain layouts and the GM or transconductance of the Mosfet. Gate resistors should be in the range of 47R to 470R for fast repetitive pulses and for slow or occasional switching they can be pretty much a whatever. In the Main the resistors serve two valid points one is a reduction in the required power supply bypassing necessary for fast repetitive switching and second is safety... If and when a Fet fails or the return path to ground fails or is lost having a current limiter between the Mosfet and your Arduino is a good idea, Ya Think?

Doc

7

I got your point, thanks for the explanations!