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Q&A How to use USB-B to power 5 V microcontroller and peripherals

The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies. One concern is when usi...

posted 9mo ago by Pete W‭  ·  edited 9mo ago by Pete W‭

Answer
#10: Post edited by user avatar Pete W‭ · 2025-12-24T03:04:04Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it I bet it would work. Simple way to make that more robust is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. If it's enough to run the MCU, it could simplify everything.
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it may well work, as the V_OH might approach 3.3V when not loaded. The way to make it robust within the spec, though, is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. If it's enough to run the MCU, and you don't need the 5V operation, then it would simplify everything.
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
#9: Post edited by user avatar Pete W‭ · 2025-12-24T03:00:45Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it I bet it would work. Simple way to make that more robust is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. It may be enough to run the MCU?
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it I bet it would work. Simple way to make that more robust is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. If it's enough to run the MCU, it could simplify everything.
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
#8: Post edited by user avatar Pete W‭ · 2025-12-24T03:00:45Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it I bet it would work. Simple way to make that more robust is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. It may be enough to run the MCU?
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it I bet it would work. Simple way to make that more robust is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. If it's enough to run the MCU, it could simplify everything.
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
#7: Post edited by user avatar Pete W‭ · 2025-12-24T02:58:41Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver V_OH_min (here V_IO-0.7, i.e. V_REG-0.7), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Simple fix is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. It may be enough to run the MCU?
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One concern is when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. The transceiver typically (including yours) makes its own 3.3V from which it powers its serial port circuitry, or at least show it being hooked up like that in the datasheet reference circuit. The concern is then the 3.3V output of the transceiver V_OH_min (here V_IO-0.7 = V_REG-0.7 = 2.4Vmin when Vreg=3.1V), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Though if you tried it I bet it would work. Simple way to make that more robust is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. It may be enough to run the MCU?
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
#6: Post edited by user avatar Pete W‭ · 2025-12-24T02:54:40Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver V_OH_min (here V_IO-0.7, i.e. V_REG-0.7), might be too low for the 5V CMOS levels expected by the MCU Rx pin. The reverse direction typically okay as the USB transceiver's output pins often (including this one) 5V tolerant.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver V_OH_min (here V_IO-0.7, i.e. V_REG-0.7), might be too low for the 5V CMOS levels expected by the MCU Rx pin. Simple fix is a level shifting buffer, like 74LV1T34. The reverse direction (MCU Tx to transceiver Rx) typically okay, as the USB transceiver's output pins are often (including this one) 5V tolerant.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V. You should check its current capability. It may be enough to run the MCU?
  • The potential reason for more regulation, IMO, would be if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
#5: Post edited by user avatar Pete W‭ · 2025-12-24T02:47:59Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver might be pushing it vs the 5V CMOS levels on the MCU Rx pin. (eg V_OH_min on the transceiver too low in that scenario)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver V_OH_min (here V_IO-0.7, i.e. V_REG-0.7), might be too low for the 5V CMOS levels expected by the MCU Rx pin. The reverse direction typically okay as the USB transceiver's output pins often (including this one) 5V tolerant.
#4: Post edited by user avatar Pete W‭ · 2025-12-24T02:38:20Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver might be pushing it vs the 5V CMOS levels on the MCU Rx pin.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver might be pushing it vs the 5V CMOS levels on the MCU Rx pin. (eg V_OH_min on the transceiver too low in that scenario)
#3: Post edited by user avatar Pete W‭ · 2025-12-24T02:37:06Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the model circuit of the datasheet. If so, the 3.3V output of the transceiver might be pushing it vs the 5V CMOS levels on the MCU Rx pin.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the datasheet reference circuit. If so, the 3.3V output of the transceiver might be pushing it vs the 5V CMOS levels on the MCU Rx pin.
#2: Post edited by user avatar Pete W‭ · 2025-12-24T02:36:23Z (9 months ago)
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for an LDO is if you're not satisfied with the USB power quality due to digital noises, and even in that case you can potentially filter it rather than using a regulator, but that depends. The USB bus VDD noise level will be comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though.
  • No startup concerns if you satisfy the bus requirements as per Olin, just make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check is that your USB transceiver chip is outputting the voltage range the MCU expects, they often make their own 3.3V for their outputs, and this might be pushing it if the MCU is looking for 5V CMOS levels on its Rx.
  • The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.
  • The potential reason for more regulation components, IMO, is if you're not satisfied with the USB power quality due to digital noises - which is usually if there is analog circuitry. Even in that case you can potentially filter the VDD rather than using an LDO, but that depends on your exact needs. The USB bus VDD noise level will be noticeable but comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.
  • Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. It may be enough to run the MCU?
  • Regarding startup concerns, it's typically just fine provided you satisfy the bus requirements as per Olin. Probably a good idea to make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.
  • One thing to check when using a dedicated USB transceiver chip with a 5V MCU, is that the transeiver's Rx/Tx is compatible with the voltage range of the MCU. They often make their own 3.3V for their outputs, or at least show it being hooked up like that in the model circuit of the datasheet. If so, the 3.3V output of the transceiver might be pushing it vs the 5V CMOS levels on the MCU Rx pin.
#1: Initial revision by user avatar Pete W‭ · 2025-12-24T02:24:26Z (9 months ago) 
The USB chip will obviously be fine, and so will that MCU at most speeds. In practice the USB VDD coming out of a computer will be pretty well regulated at low frequencies.

The potential reason for an LDO is if you're not satisfied with the USB power quality due to digital noises, and even in that case you can potentially filter it rather than using a regulator, but that depends. The USB bus VDD noise level will be comfortably tolerable for any digital logic, which is the only thing we're seeing in your schematic.

Also the USB transceiver chip often has a feature of a built in regulator to 3.3V - with a modest current capability though. 

No startup concerns if you satisfy the bus requirements as per Olin, just make sure the UVLO on your MCU is configured and activated, which ensures the MCU doesn't get going until the VDD is above its minimum by a meaningful margin.

One thing to check is that your USB transceiver chip is outputting the voltage range the MCU expects, they often make their own 3.3V for their outputs, and this might be pushing it if the MCU is looking for 5V CMOS levels on its Rx.