User manual

...the world's most energy friendly microcontrollers
2012-04-24 - Giant Gecko Family - d0053_Rev0.96 244
www.energymicro.com
15.3.2.3 Self-powered Device (with bus-power switch)
A self-powered device (with bus-power switch) may switch power supply to VBUS when connected to
a host. This is typically useful for extending the life of battery-powered devices and enables the use of
coin-cell driven systems with low maximum peak current. The external components required typically
include 2 transistors, 2 diodes and a few resistors. See application note for details. This allows seamless
power supply switching between a battery and the voltage regulator output.
The VREGO Sense High interrupt is used to detect when VBUS becomes present. Software can then
enable the external transistor connected to USB_VREGO, effectively switching the power source. A
regular GPIO pin is used to control this transistor. If necessary, the application may have to reduce the
current consumption before switching to the USB power source. If VBUS voltage is removed, the circuit
switches automatically back to the battery power supply. If necessary software must react quickly to
this event and reduce the current consumption (for example by reducing the clock frequency) to avoid
excessive voltage drop. This configuration is shown in Figure 15.4 (p. 244) .
In this configuration, the VREGO sense circuit must be enabled.
Figure 15.4. Self-powered Device (with bus-power switch)
USB_DP
USB_DM
Standard B
VBUS
D+
D-
GND
USB_VBUS
EFM32
USB_VREGI
USB_VREGO
(ESD Protection)
VDD
1.8V – 3.6V
Dual- Power
Circuit
(enable) GPIO
15.3.2.4 OTG Dual Role Device (5V)
An OTG Dual Role Device (5V) configuration is shown in Figure 15.5 (p. 245) . When 5V is available,
the internal regulator can be used to power the EFM32. An external power switch is needed to control
VBUS power. For overcurrent detection a regular GPIO input pin with interrupt is used. The application
should turn off or limit VBUS power when overcurrent is detected. In OTG mode, the maximum VBUS
decoupling capacitance is 6.5 uF.
In this configuration, the VREGO sense circuit should be left disabled.