UCS1001 datasheet(36/59 Pages) MICROCHIP

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USB Port Power Controller with Charger Emulation

Datasheet

Revision 1.2 (05-21-12)

SMSC UCS1001

DATASHEET

6.2.3

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 UCS1001

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 UCS1001 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.

6.2.4

Current Limiting Modes

The UCS1001 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 8.9, "Current Limit Mode Associations"). When operating in the

Detect power state (see Section 4.1.3), the current capacity at VBUS is limited to IBUS_BYP as

described in Section 7.2, "VBUS Bypass Switch".

6.2.4.1

Trip Mode

When using trip current limiting, the UCS1001 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 * IBUS

current) for all current values up to the ILIM.

If the current drawn by a portable device exceeds ILIM, the following occurs:

1. The port power switch will be turned off (trip action).

2. The UCS1001 will enter the Error state and assert the ALERT# pin.

3. The fault handling circuitry will then determine subsequent actions.

Trip current limiting is used when the UCS1001 is in Data Pass-through and Dedicated Charger

Emulation Cycle (except when the BC1.2 DCP or Legacy 2 charger emulation profile is accepted), and

when there’s no handshake.

APPLICATION NOTE: To avoid cycling in trip mode, set ILIM higher than the highest expected portable device

current draw.

Figure 6.1 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 6.1 for

illustrative purposes only; in actual operation only one ILIM can be active at any time.

UCS1001 Datasheet(PDF) 36 Page - Microchip Technology

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