MM516
User's Manual
24
Revision 1.4 7-8-2011
Hardware Description Manual
5.2.5 Bus-Powered Mode
In bus-powered mode, the application circuit draws its current from the 5V VBUS supply on the USB cable.
BlueCore 5 chip negotiates with the PC during the USB enumeration stage about how much current it is allowed
to consume.
For Class 2 Bluetooth applications, CSR recommends that the regulator used to derive 3.3V from VBUS is rated at
100mA average current and should be able to handle peaks of 120mA without foldback or limiting. In bus-powered
mode, BlueCore 5 chip requests 100mA during enumeration.
For Class 1 Bluetooth applications, 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.
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 10F 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 the 1.8V supply to the analog supply pins of BlueCore 5 chip will result in
reduced receive sensitivity and a distorted RF transmit signal.
Figure 8
USB Connections for Bus-Powered Mode
