FUSB3317

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11

Figure 8. Voltage and Current Sensing Circuits

+

−

CATH

IFB

VFB

VDD

VDD

IS+

IS−

Buck or Buck−Boost

PWM Control

VBAT

COMP

PWM

V1

Constant Current Control

Constant Current Limit (CL or CC) or control is enabled

during USB PD explicit contracts with a PPS APDO. When

CL (or CC) mode is enabled, the supply will decrease the

output voltage as the load increases in order to maintain a

fixed output current. Output current is sensed via a

current

the IS+ and IS− pins. The sensed signal is internally

amplified, and this amplified voltage is connected to the

non

*inverting input of the internal operational amplifier.

Similar to the constant voltage amplifier circuit, it also plays

a role as a shunt regulator to regulate the constant output

current. In order to compensate output current regulation,

one capacitor and one resistor are connected between the

IFB and CATH pins as shown in Figure 8. The constant

output current can be calculated using Equation 2. 5 m

Ω is

typically used for the sense resistor.

40

(eq. 2)

Since the voltage across the IS+ and IS− pins is small, the

sensing resistor should be positioned as close as possible to

the pins. An RC filter can be added to the pins to reduce the

noise seen on the circuit.

Output Over Current Protection

Over Current Protection (OCP) is enabled during USB PD

explicit contracts with PDOs (i.e. not PPS APDOs). When

OCP mode is enabled, the supply will regulate the output

voltage until the load current exceeds the OCP threshold, at

which point it will cause a fault condition and disable the

output voltage and disconnects from the attached Sink.

Similar to Constant Current Control, the FUSB3317 detects

difference being the output of the CC amplifier disconnected

from the CATH signal. When the load current exceeds the

and the FUSB3317 enters Auto Restart Mode after OCP is

not detected.

Discharge Functionality

Discharge circuits are implemented on the DISC pin to

discharge the output capacitors quickly during voltage

and/or current transitions and to fully discharge VBUS when

required. The discharge circuits in the FUSB3317 are sized

to meet the timing requirements in the USB PD

specification. Since the output load can discharge the load

sufficiently during heavy loads, the discharge circuits are

The operation of the discharge circuits is shown in Table 2

and Table 3.

Table 2. DISCHARGE DURING VOLTAGE

TRANSITIONS

Step Size

New VBUS

DC_EN

v0.5 V

>13 V

7 ms

Enabled

<13 V

19 ms

Enabled

>0.5 V

>13 V

55 ms

Enabled

<13 V

221 ms

Enabled

Table 3. DISCHARGE UPON DETACH, PROTECTION

MODE OPERATION OR HARD RESET

Initial VBUS

Final VBUS

DC_EN

3.3 V ~ 21 V

vSafe0V

(<0.8 V)

221 ms

Enabled

Protection Mode and Auto Restart Operation

The FUSB3317 provides Output Over Voltage Protection

(OVP), Under Voltage Protection (UVP), Output Over

Current Protection (OCP), External Over Temperature

Protection via NTC (E_OTP), Internal Over Temperature

Protection (I_OTP), D+/D− line Over Voltage Protection

(D_OVP) and CC line Over Voltage Protection (CC_OVP).

When a protection mode is triggered, the DC/DC is disabled

and the discharge circuits are enabled to protect the Sink

are disabled to indicate to the Sink device that the Source is

not ready to provide power. The functionality described is

shown in Figure 9. Once the fault conditions are removed,

the FUSB3317 will re−enable the DISC discharge circuit

and begin the auto

auto

*restart timer expires, the CC pull*up currents will be

enabled to allow a Sink device to attach.