UCS1002-3 datasheet(31/114 Pages) MICROCHIP

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DS20006852A-page 31

UCS1002-3

7.0

USB PORT POWER SWITCH

assure

compliance

various

charging

specifications, the UCS1002-3 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 register set). The switch also includes soft start

circuitry 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 UCS1002-3 hardware set current limit (ILIM), can

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

value is read once upon UCS1002-3 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 is detected and the current immediately becomes

limited (within tSHORT_LIM time). If the condition

remains, the port power switch flags an Error condition

and enters the Error state (see Section 5.1.5 “Error

State Operation”).

7.1.3

SOFT START

When the PWR_EN control changes its state to enable

the port power switch, or an Attach Detection event

occurs in the Detect power state and the PWR_EN

control is enabled, the UCS1002-3 invokes a soft start

routine for the duration of the VBUS rise time (tR_BUS).

This soft start routine limits current flow from VS into

VBUS while it is active. This circuitry prevents 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 enabled, if the bus current exceeds

ILIM, the UCS1002-3 current limiter responds within a

specified time (tSHORT_LIM) and operates normally at

this point. The CBUS capacitor delivers the extra

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

7.1.4

CURRENT-LIMITING MODES

The UCS1002-3 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 UCS1002-3 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 is held relatively constant (equal to the

VS voltage minus the RON x IBUS 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 is turned OFF (Trip

action).

The UCS1002-3 enters the Error state and

assert the ALERT# pin.

The

fault

handling

circuitry

determines

subsequent actions.

Trip Current Limiting is used by default when the

UCS1002-3 is in Data Pass-Through and Dedicated

Charger Emulation Cycle (except when the BC1.2 DCP

charger emulation profile is accepted), and when there

is no handshake.

Figure 7-1 shows operation of current limits in trip

mode with the shaded area representing the USB 2.0

specified VBUS range. Dashed lines indicate the port

power switch output will go to zero (e.g., trip) when ILIM

is exceeded. Note that operation at all possible values

of ILIM are shown in Figure 7-1for illustrative purposes

only; in actual operation only one ILIM can be active at

any time.

Note:

To avoid cycling in Trip mode, set ILIM

higher than the highest expected portable

device current draw.

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