UCS81003 datasheet(33/108 Pages) MICROCHIP

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DS20005334A-page 33

UCS81003

7.0

USB PORT POWER SWITCH

assure

compliance

various

charging

specifications, the UCS81003 contains a USB port

power switch that supports two current-limiting modes:

Trip and Constant Current (variable slope). The current

limit (ILIM) is pin selectable (and may be updated via the

and a separate short-circuit current limit.

The port power switch is on in the Active state (except

when VBUS is discharging).

7.1

Current Limiting

7.1.1

CURRENT LIMIT SETTING

The UCS81003 hardware set current limit (ILIM), can be

one of eight values (see Table 11-1). This resistor value

is read once upon UCS81003 power-up. The current

limit can be changed via the SMBus/I2C after power-

up; however, the programmed current limit cannot

exceed the hardware set current limit.

At power-up, the communication mode (Stand-Alone or

SMBus/I2C) and hardware current limit (ILIM) are

determined via the pull-down resistor (or pull-up

resistor, if connected to VDD) on the COMM_SEL/ILIM

pin, as shown in Table 11-1.

7.1.2

SHORT CIRCUIT OUTPUT

CURRENT LIMITING

Short circuit current limiting occurs when the output

current is above the selectable current limit (ILIMx). This

event will be detected and the current will immediately

be limited (within tSHORT_LIM time). If the condition

remains, the port power switch will flag an Error

condition and enter the Error state (see Section 5.1.5

“Error State Operation”).

7.1.3

SOFT START

When the PWR_EN control changes states to enable

the port power switch, or an Attach Detection event

occurs in the Detect power state and the PWR_EN

control is already enabled, the UCS81003 invokes a

soft start routine for the duration of the VBUS rise time

(tR_BUS). This soft start routine will limit current flow

from VS into VBUS while it is active. This circuitry will

prevent current spikes due to a step in the portable

device current draw.

In the case when a portable device is attached while the

PWR_EN pin is already enabled, if the bus current

exceeds ILIM, the UCS81003 current limiter will respond

within a specified time (tSHORT_LIM) and will operate

normally at this point. The CBUS capacitor will deliver

the extra current, if any, as required by the load change.

7.1.4

CURRENT-LIMITING MODES

The UCS81003 current limiting has two modes: Trip

and Constant Current (variable slope). Either mode

functions at all times when the port power switch is

closed. The current limiting mode used depends on the

Active state mode (see Section 9.9 “Current Limit

Mode Associations”). When operating in the Detect

power

state

(see

Section 5.1.3

“Detect

State

Operation”), the current capacity at VBUS is limited to

IBUS_BYP as described in Section 8.2 “VBUS Bypass

Switch”.

7.1.4.1

Trip Mode

When using Trip Current Limiting, the UCS81003 USB

port power switch functions as a low-resistance switch

and rapidly turns off if the current limit is exceeded.

While operating using Trip Current Limiting, the VBUS

output voltage will be held relatively constant (equal to

the VS voltage minus the RON xIBUS current) for all

current values up to the ILIM.

If the current drawn by a portable device exceeds ILIM,

the following occurs:

The port power switch will be turned off (Trip

action).

The UCS81003 will enter the Error state and

assert the ALERT# pin.

The fault handling circuitry will then determine

subsequent actions.

Trip Current Limiting is used by default when the

UCS81003 is in Data Pass-Through and Dedicated

Charger Emulation Cycle (except when the BC1.2 DCP

charger emulation profile is accepted), and when

there’s no handshake. This method is also used when

charger emulation is active.

7.1.4.2

Constant Current Limiting

(Variable Slope)

Constant Current Limiting is used when a portable

device handshakes using the BC1.2 DCP charger

emulation profile and the current drawn is greater than

ILIM (and ILIM < 1.68A). It is also used in BC1.2 CDP

mode and during the DCE Cycle when a charger emu-

lation profile is being applied and the emulation timeout

is active.

In CC mode, the port power switch allows the attached

portable device to reduce VBUS output voltage to less

than the input VS voltage while maintaining current

delivery. The V/I slope depends on the user set ILIM

value. This slope is held constant for a given ILIM value.

Note:

To avoid cycling in Trip mode, set ILIM

higher than the highest expected portable

device current draw.

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