Microchip Technology DM183037 Data Sheet

DS30575A-page 546

 2012 Microchip Technology Inc.

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.

27.6.1

BUS POWER ONLY

In Bus Power Only mode, all power for the application
is drawn from the USB (

). 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 V

BUS

 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 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.

27.6.2

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 

 for an example. 

Note that an attach indication is added to indicate when
the USB has been connected and the host is actively
powering V

BUS

.

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. 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 V

BUS

 pin of the USB cable.

27.6.3

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 

 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
V

BUS

 is driven high. See 

 and 

 for

descriptions of those requirements. Additionally, dual
power devices must never source current onto the 5V
V

BUS

 pin of the USB cable.

V

DD

V

USB

3V3

V

SS

V

BUS

~5V

3.3V

Low I

Q

 Regulator

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. 

V

DD

V

USB

3V3

V

SS

V

SELF

~3.3V

Attach Sense

100 k



100 k



V

BUS

~5V

5.5V Tolerant
I/O Pin

V

DD

V

USB

3V3

I/O Pin

V

SS

Attach Sense

V

BUS

V

SELF

100 k



~3.3V

~5V

100 k



3.3V

Low I

Q

Regulator