User's Manual

Silicon Laboratories Finland Oy

Page 24 of 45

Figure 13: USB Connections for Self-Powered Mode

The terminal marked USB_ON can be any free PIO pin. The PIO pin selected must be registered by setting

PSKEY_USB_PIO_VBUS to the corresponding pin number.

Identifier

Value

Function

27Ω nominal

Impedance matching to USB cable

vb1

22kΩ 5%

VBUS ON sense divider

vb2

47kΩ 5%

VBUS ON sense divider

Figure 14: USB Interface Component Values

7.5 Bus-Powered Mode

In bus-powered mode, the application circuit draws its current from the 5V VBUS supply on the USB cable.

WT41u negotiates with the PC during the USB enumeration stage about how much current it is allowed to

consume. On power-up the device must not draw more than 100 mA but after being configured it can draw up

to 500 mA.

For WT41u, the USB power descriptor should be altered to reflect the amount of power required. This is

accomplished by setting PSKEY_USB_MAX_POWER (0x2c6). This is higher than for a Class 2 application

due to the extra current drawn by the Transmit RF PA. By default for WT41u the setting is 300 mA.

When selecting a regulator, be aware that VBUS may go as low as 4.4V. The inrush current (when charging

reservoir and supply decoupling capacitors) is limited by the USB specification. See the USB Specification.

Some applications may require soft start circuitry to limit inrush current if more than 10uF is present between

VBUS and GND. The 5V VBUS line emerging from a PC is often electrically noisy. As well as regulation down

to 3.3V and 1.8V, applications should include careful filtering of the 5V line to attenuate noise that is above the

voltage regulator bandwidth. Excessive noise on WT41u supply pins will result in reduced receiver sensitivity

and a distorted RF transmit signal.