FUSB307B
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15
VBUS Monitoring and Measurement
The FUSB307B can monitor the presence of VBUS and
will report it on PWRSTAT.VBUS_VAL and interrupt
ALERT.I_PORT_PWR.
VBUS_VAL is set according to VBUS thresholds in
vVBUSthr.
The FUSB307B also supports a more precise voltage
measurement via an on−board ADC. The voltage on VBUS
is measured at a rate of tVBUSsample and it is reported on
VBUS_VOLTAGE_L/H register. The precision of the
measurement is +/2% with a resolution of 25 mV LSB.
In addition to providing the μProcessor an accurate
measurement of VBUS, the measurement in
VBUS_VOLTAGE will be used when monitoring various
user defined thresholds:
• Voltage alarms in registers VALARMLCFG and
VALARMHCFGL
• VBUS Disconnect Threshold in registers
VBUS_SNK_DISCL and VBUS_SNK_DISCH
• VBUS Stop Discharge Threshold in registers
VBUS_STOP_DISCL and VBUS_STOP_DISCH
• The FUSB307B implements Low and High VBUS
Voltage Alarms that can be programmable via
VALARMLCFG and VALARMHCFG respectively. If
the High or the Low thresholds are crossed, the
FUSB307B will signal an interrupt on
ALERTL.I_VBUS_ALRM_HI or
ALERTH.I_VBUS_ALRM_LO respectively.
These alarms can be disabled by writing
PWRCTRL.DIS_VALARM to one
ALERTL.I_PORT_PWR is asserted if the bit−wise AND
of PWRSTAT and PWRSTAMSK results in any bits that
have the value 1.
VBUS Discharge
Manual Discharge
There are two types of manual discharge circuits
implemented: A bleed discharge for low current and a force
discharge. The bleed discharge can be manually enabled by
writing a one to register bit
PWRCTRL.EN_BLEED_DISCH. When enabled, the bleed
discharge provides a low current load on VBUS of 7 kΩ
(max.) via RBLEED. The force discharge is used to quickly
discharge VBUS to vSafe0V by applying a dynamic load to
VBUS via RFULL_DISCH. The force discharge can be
manually enabled by writing a one to register bit
PWRCTRL.FORCE_DISCH. When RFULL_DISCH is
applied, the maximum slew rate allowed for discharging
VBUS does not exceed vSrcSlewNeg 30 mV/μs as it is
specified in the USB−PD spec.
Automatic discharge bit PWRCTRL. AUTO_DISCH
must be disabled before enabling force discharge.
Automatic Source Discharge after a Disconnect
Automatic discharge can be enabled by setting
PWRCTRL. AUTO_DISCH register bit. When in Source
mode the FUSB307B will fully discharge VBUS to vSafe5V
(max.) within tSafe5V and to vSafe0V within tSafe0V when
a Disconnect occurs. The FUSB307B is in Source mode
when the SRC output is asserted.
The FUSB307B in Source mode will detect a Disconnect
if the CCSTAT.CCx_STAT field for the monitored CC pin
indicates SRC.Open and enable the FULL Discharge
pull−down device. The monitored CC pin is specified by
TCPC_CTRL.ORIENT.
Figure 14. VBUS Auto Discharge as Source
Cable Disconnect
(CCSTAT change)
tSafe5V
tSafe0V
vSafe0V
vSafe5V
time
VBUS
Apply R
