PIC18F47J53 Datasheet, PDF (400/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

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23.6 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.
23.6.1 BUS POWER ONLY
In Bus Power Only mode, all power for the application
is drawn from the USB (Figure 23-9). This is effectively
the simplest power method for the device.
In order to meet the inrush current requirements of the
USB 2.0 Specification, the total effective capacitance
appearing across VBUS and ground must be no more
than 10 µF. If not, some kind of inrush timing 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 VBUS 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 23-9:
BUS POWER ONLY
Low IQ Regulator
VBUS
~5V
3.3V
VDD
VUSB
VSS
23.6.2 SELF-POWER ONLY
In Self-Power Only mode, the USB application provides
its own power, with very little power being pulled from
the USB. See Figure 23-10 for an example.
Note that an attach indication is added to indicate when
the USB has been connected and the host is actively
powering VBUS.
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 VBUS high.
One of the 5.5V tolerant I/O pins may be used for this
purpose.
The application should never source any current onto
the 5V VBUS pin of the USB cable.
FIGURE 23-10:
VBUS
~5V
VSELF
~3.3V
SELF-POWER ONLY
Attach Sense
100 k
5.5V Tolerant
I/O Pin
VDD
100 k
VUSB
VSS
DIAGRAMS:OWER ONLY
23.6.3 DUAL POWER WITH SELF-POWER
DOMINANCE
Some applications may require a dual power option.
This allows the application to use internal power
primarily, but switch to power from the USB when no
internal power is available. See Figure 23-11 for a
simple Dual Power with Self-Power Dominance mode
example, which automatically switches between
Self-Power Only and USB Bus Power Only modes.
Dual power devices must also meet all of the special
requirements for inrush current and Suspend mode
current, and must not enable the USB module until
VBUS is driven high. See Section 23.6.1 “Bus Power
Only” and Section 23.6.2 “Self-Power Only” for
descriptions of those requirements. Additionally, dual
power devices must never source current onto the 5V
VBUS pin of the USB cable.
FIGURE 23-11: DUAL POWER EXAMPLE
Low IQ
Regulator
3.3V
VBUS
~5V
100 k Attach Sense
I/O Pin
VDD
100 k
VSELF
~3.3V
VUSB
VSS
Note:
Users should keep in mind the limits for
devices drawing power from the USB.
According to USB Specification 2.0, this
cannot exceed 100 mA per low-power
device or 500 mA per high-power device.
DS39964B-page 400
Preliminary
 2010 Microchip Technology Inc.