FUSB3307 datasheet(14/21 Pages) ONSEMI

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FUSB3307

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Capacitance to ground on the connector side VBUS

connection (source of Q1) can be added if needed but it

hasn’t been shown in the application diagrams above. If

added, it is recommended it doesn’t exceed 1

mF for

recovery from the source−source case mentioned above.

Voltage and Current Sensing Operation

As mentioned above (see Power Up and Assumptions),

the resistor ratio from VCC to ground formed by resistors R8

and R9 (typically 1:10 ratio where R8=120k and R9=13.3k)

and sensed via FUSB3307 VFB pin will sense the voltage

for VCC in order to set a new voltage. For Figure 5 offline

design, this will be done via the FUSB3307 CATH pin, the

opto−coupler, resistor R10 and the primary side PWM

controller operation. R11 provides a bias current to the

CATH pin feedback circuit within the FUSB3307 and is

optional. For Figure 6 buck−boost design, this will be done

via the FUSB3307 CATH pin controlling the buck−boost

PWM via its COMP pin. The FUSB3307 will automatically

control the CATH pin based on the desired voltage as

determine by the USB PD contract and the existing VCC

voltage sensed by VFB. If FUSB3307’s PD communication

is not responded to by the Sink upon initial attach, the

FUSB3307 will continue with 5 V VBUS Type C operation

until the Sink detaches. The external compensation network

formed by C6/R7/C7 and R6/C5 need to be selected to

achieve stable operation over the range of VBUS voltage

and current transitions as shown in Figure 11.

Figure 11. Compensation Network for Constant

Voltage / Constant Current (CV/CC) Feedback

For the offline design, C10 may be needed as well. For the

DC/DC design, there may be a need for additional

compensation networks from COMP pin to ground or from

COMP to the NCV81599 supply.

The current is sensed via a small resistor R4 (5 m

typically) connected between the USB−C connector ground

and the main ground plane of the power source (secondary

side ground for offline design) as shown in Figure 12.

Figure 12. Current Sense Network

A low pass filter formed by R5/C4 provides a stable signal

for IS+ and IS− pins of the FUSB3307 to sense this current

for over−current protection for fixed voltage PD contracts,

constant current operation for PPS contracts and cable

compensation if selected from the trim table (Table 6). It is

expected that the USB−C connector ground is connected

only to the current sense network resistors R4 and R5 and the

connector TVS ground connections and not to the main

ground plane of the NCV81599 for the DC/DC design or

secondary side power ground for the offline design

(FUSB3307 ground connection). However, the FUSB3307

consumes very little current and so it should have a

negligible impact on this current sensing if the FUSB3307

ground connection is on the USC−C connector ground if it

is more convenient in the Printed Circuit Board (PCB)

layout.

When in a PPS contract, if a PPS_Status message is

requested, the FUSB3307 will measure the current with an

internal 10−bit Analog to Digital Converter (ADC) based on

the above description and report it back to the Sink on the

PPS_Status message. The voltage is also reported back but

it is measured off VCC with the ADC not VFB pin since the

VFB pin is only used for voltage feedback. Thus if the

voltage feedback resistor divider connected to VFB is

modified to be slightly different from the 1:10 ratio

expected, the voltage sensing for this PPS_Status message

will not be affected.

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