Datasheet

PIC18(L)F2X/45K50
DS30684A-page 372  2012 Microchip Technology Inc.
24.6 USB Power Modes
Many USB applications will likely have several different
sets of power requirements and configuration. The
most common power modes encountered are Bus
Power Only, Self-Power Only and Dual Power with
Self-Power Dominance. The most common cases are
presented here. Also provided is a means of estimating
the current consumption of the USB transceiver.
24.6.1 BUS POWER ONLY
In Bus Power Only mode, all power for the application
is drawn from the USB (Figure 24-9). This is effectively
the simplest power method for the device.
In order to meet the inrush current requirements of the
USB 2.0 specifications, the total effective capacitance
appearing across V
BUS and ground must be no more
than 10 µF. If not, some kind of inrush liming is
required. For more details, see section 7.2.4 of the
USB 2.0 specification.
According to the USB 2.0 specification, all USB devices
must also support a Low-Power Suspend mode. In the
USB Suspend mode, devices must consume no more
than 2.5 mA from the 5V V
BUS line of the USB cable.
The host signals the USB device to enter the Suspend
mode by stopping all USB traffic to that device for more
than 3 ms. This condition will cause the IDLEIF bit in
the UIR register to become set.
During the USB Suspend mode, the D+ or D- pull-up
resistor must remain active, which will consume some
of the allowed suspend current: 2.5 mA budget.
FIGURE 24-9: BUS POWER ONLY
24.6.2 SELF-POWER ONLY
In Self-Power Only mode, the USB application provides
its own power, with very little power being pulled from
the USB. Figure 24-10 shows an example.
In order to meet compliance specifications, the USB
module (and the D+ or D- pull-up resistor) should not
be enabled until the host actively drives V
BUS high.
The application should never source any current onto
the 5V V
BUS pin of the USB cable.
FIGURE 24-10: SELF-POWER ONLY
VDD
VUSB
VSS
VBUS
VDD
VUSB
VSS
VSELF