schematics Buck switching regulator based on ATtiny84a -- please critique!

  
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The microcontroller won`t be able to drive the gate of Q1 very hard (usually GPIO pins can only source a few milliamps) so your turn-on and turn-off will be very slow. This will limit how well your high-side switch will behave. You don`t have a gate-to-source resistor on Q1, so you`re solely dependent on the GPIO keeping the MOSFET on or off. If t
schematics Buck switching regulator based on ATtiny84a -- please critique! - schematic

he GPIO pin goes high-impedance, the MOSFET may turn itself on if the gate picks up a charge from the environment. which is crazy high power since D is going to be high (input is close to output). Also, the 225mA or so that will flow will also be burned in Q1, which isn`t healthy since it`s a relatively small device. Your purely resistive feedback network is a bad idea. You really need some compensation and/or filtering. Your comparator will be hyper-fast and could react to switching noise, pickup, ripple, etc. - since you don`t seem to be using an error amplifier with compensation to control the gain and phase, you`re going to need some cap across R5 (and some luck). You don`t have input reverse-polarity protection and an input fuse in your power train. Big no-no, especially when the source is battery-based (big short-circuit sourcing capability). That being said, this isn`t a simple project by any stretch. Your schematic is largely incomplete and lacks basic safety protection that any power supply (especially ones that run at high power levels like yours) will need. Even after the good answer by Madmanguruman, there are additional things that should be noted. The main difficulty with this design will be the high current being processed. I`ll pay attention mainly to the power processing components, power modulator, and filtering. Power FET is P channel. IRF4905: Rdson=0. 02@25C, 0. 034@150C; Ciss=3500pF. Conduction loss...



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