PIC18F47J53 Datasheet, PDF (402/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

PIC18F47J53 FAMILY
EQUATION 23-2: CALCULATING USB TRANSCEIVER CURRENT^†
For this example, the following assumptions are made about the application:
• 3.3V will be applied to VUSB and VDD, with the core voltage regulator enabled.
• This is a full-speed application that uses one interrupt IN endpoint that can send one packet of 64 bytes every
1 ms, with no restrictions on the values of the bytes being sent. The application may or may not have additional
traffic on OUT endpoints.
• A regular USB “B” or “mini-B” connector will be used on the application circuit board.
In this case, PZERO = 100% = 1, because there should be no restriction on the value of the data moving through the
IN endpoint. All 64 kbps of data could potentially be bytes of value, 00h. Since ‘0’ bits cause toggling of the output state
of the transceiver, they cause the USB transceiver to consume extra current charging/discharging the cable. In this
case, 100% of the data bits sent can be of value ‘0’. This should be considered the “max” value, as normal data will
consist of a fair mix of ones and zeros.
This application uses 64 kbps for IN traffic out of the total bus bandwidth of 1.5 Mbps (12 Mbps), therefore:
64 kbps
Pin = _{1.5 Mbps} = 4.3% = 0.043
Since a regular “B” or “mini-B” connector is used in this application, the end user may plug in any type of cable up to
the maximum allowed 5m length. Therefore, we use the worst-case length:
LCABLE = 5 meters
Assume IPULLUP = 2.2 mA. The actual value of IPULLUP will likely be closer to 218 A, but allowance for the worst-case.
USB bandwidth is shared between all the devices which are plugged into the root port (via hubs). If the application is
plugged into a USB 1.1 hub that has other devices plugged into it, your device may see host to device traffic on the
bus, even if it is not addressed to your device. Since any traffic, regardless of source, can increase the IPULLUP current
above the base 218 A, it is safest to allow for the worst-case of 2.2 mA.
Therefore:
IXCVR = ^{(40 mA • 3.3V • 1 • 0.043 • 5m)} + 2.2 mA = 3.9 mA
(3.3V • 5m)
† The calculated value should be considered an approximation and additional guardband or
application-specific product testing is recommended. The transceiver current is “in addition to” the
rest of the current consumed by the PIC18F47J53 family device that is needed to run the core,
drive the other I/O lines, power the various modules, etc.
DS39964B-page 402
Preliminary
 2010 Microchip Technology Inc.