FT4222HQ-C-R datasheet(32/49 Pages) FTDI

32 / 49 page

FT4222H USB2.0 TO QUADSPI/I2C BRIDGE IC

1.3

Document No.: FT_001011 Clearance No.: FTDI#405

7.2

Self Powered Configuration with 5V Source Input

Figure 7.2 Self Powered Configuration with 5V Source Input

Figure 7.2 illustrates the FT4222H in a typical USB2.0 self-powered configuration. A USB self-powered

device gets its power from its own power supply, 5V, and does not draw current from the USB bus. The

basic rules for USB self-powered devices are as follows –

A self-powered device should not force current down the USB bus when the USB host or hub

controller is powered down.

ii)

A self-powered device can use as much current as it needs during normal operation and USB

suspend as it has its own power supply.

iii)

A self-powered device can be used with any USB host, a bus powered USB hub or a self-

powered USB hub.

The power descriptor in the internal OTP memory of the FT4222H should be programmed to a value of

zero (self-powered).

In order to comply with the first requirement above, the USB bus power (USB connector pin 1) is used to

control the VBUS_DET pin of the FT4222H device. When the USB host or hub is powered up an internal

1.5kΩ resistor on DP is pulled up to +3.3V, thus identifying the device to the USB host or hub. When the

USB host or hub is powered off, the VBUS_DET pin will be low and the FT4222H is held in a suspend

state. In this state the internal 1.5kΩ resistor is not pulled up to any power supply (hub or host is

powered down), so no current flows down DP via the 1.5kΩ pull-up resistor. Failure to do this may cause

some USB host or hub controllers to power up erratically.

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