6.4 USB Power Supply Configurations

A USB host or hub supplies nominal 5V power on the VBUS wire for use by USB devices that are directly connected. In addition, any USB device may have its own power supply. As stated in the USB 2.0 specification, USB devices that rely solely on power from the USB cable are called bus-powered devices. In contrast, those having a separate power source are called self-powered devices.

One important design consideration is the maximum allowed slew rate on the device’s VDD pin(s). Exceeding this value may cause the device to lock up or even damage the device. Because the slew rate of VBUS can be more than 0.5 V/μs at the moment the USB is connected, it may be necessary to include components in the design to reduce the slew rate on the device’s VDD pin(s). These components will be referred to generically as Slew Rate Limiting Components or SRLC. One example of an SRLC is the MIC94165 high-side load switch with reverse blocking and soft start.

The USB peripheral’s internal transceiver requires a nominal 3.3V power supply voltage on the VUSB pin for successful data transfer operations on USB, provided by an external source or generated by an internal VDD-to-VUSB (3.3V) voltage regulator called USB VREG. For operating the USB VREG, a minimum supply voltage VDD ≥ VDDUMIN is required. The USB VREG has these configuration bits:
  • The USBVREG bit in the VUSB Control (VUSBCTRL) register of the SYSCFG peripheral enables/disables the VUSB regulation.
  • The USBSINK bit in the System Configuration 1 (SYSCFG1) fuse controls whether the USB VREG can sink current. It is recommended to leave this feature enabled.

Refer to the Electrical Characteristics section for standard operating conditions.

The flexibility of external or internally generated VUSB allows the following seven power configurations to be supported (with mnemonics in parentheses):
  • 5V Bus-powered (5b)
  • 5V Self-powered (5s)
  • 3.3V Self-powered (3s)
  • Dual-powered 5V Bus-dominance and 5V Self-powered (5b5s)
  • Dual-powered 5V Bus-dominance and 3.3V Self-powered (5b3s)
  • Dual-powered 5V Bus-dominance and Self-powered by variable 2V to 3V (5b2s)
  • Dual-powered 5V Self-dominance and 5V Bus-powered (5s5b)

5b (5V Bus-powered)

In power configuration 5b, the VBUS supply from the USB receptacle is connected, either directly or via SRLC, to the device's VDD pin(s). When the USB is not connected, the device is not powered. When USB is connected, the nominal 5V VBUS supply powers the device, and the internal voltage regulator must also be enabled, generating the required 3.3V on the VUSB pin.
Figure 6-1. Power Configuration 5b

5s (5V Self-powered)

In power configuration 5s, a local 5V supply is connected, either directly or via SRLC, to the device's VDD pin(s). Also, enable the internal voltage regulator while the USB is connected, generating the required 3.3V on the VUSB pin. The internal voltage regulator may be disabled when the USB is not connected.
Figure 6-2. Power Configuration 5s

3s (3.3V Self-powered)

In power configuration 3s, the VDD and VUSB pins are connected, and a local supply provides 3.3V power to them, either directly or via SRLC. The internal voltage regulator is unused, and both the USBVREG bit in the SYSCFG.VUSBCTRL register and the USBSYNC bit in the SYSCFG1 fuse can be written to `0’.
Figure 6-3. Power Configuration 3s

5b5s (Dual-powered 5V Bus-dominance and 5V Self-powered)

In power configuration 5b5s, the USB VBUS supply is always used to power the device when the USB is connected. VBUS is connected, directly or via SRLC, to the device's VDD pin(s). The internal voltage regulator must also be enabled, thereby generating the required 3.3V on the VUSB pin. When the USB is not connected, typically through reverse-blocking circuitry (RBC) such as a Schottky diode or a transistor configuration, a local 5V supply provides power to the VDD pin(s). The internal voltage regulator may be disabled when the USB is not connected by writing the USBVREG bit in the SYSCFG.VUSBCTRL register to `0’.
Figure 6-4. Power Configuration 5b5s

5b3s (Dual-powered 5V Bus-dominance and 3.3V Self-powered)

In power configuration 5b3s, the USB VBUS supply is always used to power the device when the USB is connected. VBUS is connected, directly or via SRLC, to the device's VDD pin(s). The internal voltage regulator must also be enabled, thereby generating the required 3.3V on the VUSB pin. A local 3.3V supply provides power to the VDD pin(s) through RBC when the USB is disconnected. The internal voltage regulator will be inoperable when the USB is not connected because VDD is below VDDUMIN.
Figure 6-5. Power Configuration 5b3s

5b2s (Dual-powered 5V Bus-dominance and Self-powered by Variable 2V to 3V)

In power configuration 5b2s, the USB VBUS supply is always used to power the device when the USB is connected. VBUS is connected, directly or via SRLC, to the device's VDD pin(s). The internal voltage regulator must also be enabled, generating the required 3.3V on the VUSB pin. When the USB is not connected, a local supply that can vary from 2V to 3V, such as a battery, provides power to the device’s VDD pin(s), typically through RBC. The internal voltage regulator will be inoperable when the USB is not connected because VDD is below VDDUMIN.
Figure 6-6. Power Configuration 5b2s

5s5b (Dual-powered 5V Self-dominance and 5V Bus-powered)

In power configuration 5s5b, the device is powered by a local 5V supply whenever available. The local 5V supply is connected, directly or via SRLC, to the device's VDD pin(s). If the local supply fails or is unavailable, the device will be powered only when the USB is connected because VBUS provides power to the device’s VDD pin(s), typically through RBC. The internal voltage regulator must be enabled when the USB is connected, generating the required 3.3V on the VUSB pin.
Figure 6-7. Power Configuration 5s5b