FUSB3307

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13

Figure 10. VBUS Discharge by FUSB3307 via DISC Pin

VBUS is discharged through a resistor (R1) via the DISC

pin of the FUSB3307 as shown in the highlighted section in

Figure 10.

The external resistor R1 value is dependent on the total

bulk capacitance (C8) of this power source so that VBUS is

discharged within the time limits dictated by USB PD. A

typical value for R1 is 39

W, 1 W and in addition, there is

internal resistance that causes a expected discharge current

electrical tables above). If the load current to the Sink is

debounce time) such that the internal discharge is not

needed, then the FUSB3307 will automatically disable

internal discharge.

Upon power up, the FUSB3307 will discharge VBUS in

case there is any voltage on VBUS since the only way a Sink

can be attached per Type C specification is if VBUS is

discharged to ground (below VSafe0V) upon attach. The

discharge resistance limits are governed by the Type C

specification when not sourcing power on VBUS

no external load/discharge resistor is connected to VBUS

other than R1 to the FUSB3307 discharge DISC pin.

A TVS diode connected from VBUS to ground and shown

in the figures ([SZ]ESD7241) allow operating voltages up

to 24 V covering the entire VBUS range of 3.3 V to 21 V for

a USB PD PPS contract. This can be replaced by a TVS that

covers the VBUS range for the use case of this design if

needed.

USB Type C specification requires that the supply voltage

is not sourced on VBUS until an attach per Type C

specification has been determined. Dual back−to−back

FET’s for the load switch are not needed for reverse voltage

protection since it is unlikely that VBUS is charged from an

external source. For interoperability with legacy connectors,

there is a case where a Type A to Type C cable is first plugged

into a Type A port of a power source which then supplies 5 V

on VBUS of the cable. Then the Type C connector is plugged

into this design which is not plugged into the AC outlet nor

gets it power from the DC input depending on the design.

The 5 V from the cable will forward conduct through the Q1

FET and charge the bulk capacitor C8. This doesn’t cause an

issue, since the FUSB3307 will power up, check the CC1

and CC2 lines and realize it is not a legitimate Sink device

plugged in and stay detached. Upon unplugging the A to C

tables above) will discharge VBUS to ground if the input

voltage is still unavailable. Even if the input power is

supplied to this design during this incorrect connection,

VCC will regulate to 5 V which will prevent the previously

forward bias body diode of Q1 FET from conducting and the

FUSB3307 will wait for a legitimate Sink to be attached

before turning on FET Q1. The maximum bulk capacitance

is specified in the Type C specification to handle this fault

case as shown in Table 5 from the USB Type C specification

so as to allow for just one FET use for optimum efficiency.

Table 5. TYPE−C SPECIFICATION FOR VBUS BULK CAPACITANCE

Symbol

Notes

Min

Max

Units

VBUS

Capacitance

Capacitance for source−only ports between VBUS and GND pins on

receptacle when VBUS is not being sourced.

3000

mF

Capacitance for DRP ports between VBUS and GND pins on recep-

tacle when VBUS is not being sourced.

10

mF