USB3370 Datasheet, PDF (28/75 Pages) Microchip Technology – Enhanced Single Supply Hi-Speed USB ULPI Transceiver

USB3370
5.7.3.3 VBUS Input Impedance
The OTG Supplement requires an A-Device that supports Session Request Protocol to have a VBUS input impedance
less than 100kΩ and greater the 40kΩ to ground. The USB3370 provides a 75kΩ resistance to ground, R_VB. The R_VB
resistor tolerance is detailed in Table 4-8.
5.7.3.4 VBUS Over Voltage Protection (OVP)
The USB3370 provides an integrated over voltage protection circuit to protect the VBUS pin from excessive voltages
that may be present at the USB connector. The over voltage protection circuit works with an external resistor (R_VBUS)
by drawing current across the resistor to reduce the voltage at the VBUS pin.
When voltage at the VBUS pin exceeds 5.5V, the Over voltage Protection block will sink current to ground until VBUS
is below 5.5V. The current drops the excess voltage across R_VBUS and protects the USB3370 VBUS pin. The required
R_VBUS value is dependent on the operating mode of the USB3370 as shown in Table 5-5.
TABLE 5-5: REQUIRED R_VBUS RESISTOR VALUE
Operating Mode
R_VBUS
Device only
20kΩ ±5%
OTG Host Capable of less than 100mA of
current on VBUS
1kΩ ±5%
Host or OTG Host capable of >100mA
UseExternalVbusIndicator = 1
20kΩ ±5%
The Over voltage Protection circuit is designed to protect the USB3370 from continuous voltages up to 30V on the
R_VBUS resistor.
The R_VBUS resistor must be sized to handle the power dissipated across the resistor. The resistor power can be found
using the equation below:
P_RVBUS
=
-⁽--^V----^p---^r---^o---^t--^e---^c---^t----–----5---.--0---⁾--²-
R_VBUS
Where:
• Vprotect is the VBUS protection required.
• R_VBUS is the resistor value, 1kΩ or 20kΩ.
• P_RVBUS is the required power rating of R_VBUS.
For example, protecting a peripheral or device only application to 15V would require a 20kΩ R_VBUS resistor with a power
rating of 0.05W. To protect an OTG product to 15V would require a 1kΩ R_VBUS resistor with a power rating of 0.1W.
5.7.4
DRIVING EXTERNAL VBUS
The USB3370 monitors VBUS as described in VBUS Monitoring and VBUS Pulsing. For OTG and Host applications,
the system is required to source 5 Volts on VBUS. The USB3370 fully supports VBUS power control using an external
VBUS switch. The USB3370 provides an open drain active low control signal, CPEN_N, that is dedicated to controlling
the VBUS supply when configured as an A-Device.
CPEN_N is driven by setting the DrvVbus or DrvVbusExternal bit of the OTG Control register. To be compatible with
Link designs that support both internal and external VBUS supplies, the DrvVbus and DrvVbusExternal bits in the OTG
Control Register are NOR’d together. This enables the Link to set either bit to access the external VBUS enable
(CPEN_N). This logic is shown in Figure 5-9. DrvVbus and DrvVbusExternal are set to 0 on Power On Reset (POR) as
shown in Section 7.1.1.7.
DS00001915B-page 28
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