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26

FT51A Advanced MCU with 8051 Compatible Core IC Datasheet

Version 1.5

Doc ument N o.: FT _000877

C learance N o.: FT DI#420

5 USB Power Configurations

The following sections illustrate a possible USB power configuration for the FT51A. The illustrations have

omitted pin numbers for ease of understanding since the pins differ between the various package options.

5.1 USB Bus Powered Configuration

FT51A

1

2

3

4

5

SHIELD

Ferrite

Bead

GND

GND

VCC

GND

VCC5V

VCC3V3

USBDM

USBDP

VCC

100nF

100nF

10nF

4.7uF

+

27R

27R

GND

47pF

47pF

Figure 5-1 Bus Powered Configuration

Figure 5-1 illustrates the FT51A in a typical USB bus powered design configuration. A USB bus powered

device gets its power from the USB bus. Basic rules for USB bus powered devices are as follows –

i)

On plug-in to USB, the device should draw no more current than 50mA.

ii)

In USB Suspend mode the device should draw no more than 500uA.

iii)

A device that consumes more than 100mA cannot be plugged into a USB bus powered hub.

iv)

No device can draw more than 500mA from the USB bus.

The power descriptors in the internal MTP memory of the FT51A should be programmed to match the

current drawn by the device.

A ferrite bead is connected in series with the USB power supply to reduce EMI noise from the FT51A and

associated circuitry being radiated down the USB cable to the USB host. The value of the Ferrite Bead

depends on the total current drawn by the application. A suitable range of Ferrite Beads is available from

Laird Technologies (http://www.lairdtech.com) for example Laird Technologies Part # MI0805K601R-10.