AN2865 datasheet(9/17 Pages) STMICROELECTRONICS

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AN2865

Current sink detection method

Doc ID 15283 Rev 1

9/17

The DM check during phase 1 is needed to ensure that PS2 to USB adapters, pulling the

DM line high, are not recognized as chargers. If a PS2 port is recognized as a charger, the

high current drawn during the charging process could damage old PC motherboards which

is why the detection is stopped if the DM line is high during this phase.

On the other hand, if DM is low during phase 1, phase 2 is entered. If a dedicated charging

port is connected to the DP/DM lines, the voltage source connected to the DP line pulls the

DM line over the VDAT_REF threshold because of the short-circuit between DP and DM inside

the charging port. The same happens when a charging host port is connected. After

detecting the VDAT_SRC voltage on the DP line, the charging host port connects a voltage

source to the DM line which exceeds the VDAT_REF threshold.

Both ports allow the detection to be successful.

On the other hand, if a standard host port is connected to the DP-DM lines, the DM voltage

during phase 2 is always low because of the pull-down resistor connected to it. This causes

the detection to fail.

The maximum value of RDCHG_DAT recognized as a charger can be easily calculated:

VDAT_SRC minus the voltage drop on the series R1+R2+ RDCHG_DAT must not go below

VDAT_REF. The worst-case conditions are obtained using the minimum value of VDAT_SRC

and (R1+R2) and the maximum value for VDAT_REF and IDAT_SINK:

Equation 1

VDAT_SRC(min) - IDAT_SINK(max)(R1+R2+RDCHG_DAT) > VDAT_REF(max)

Therefore,

Equation 2

RDCHG_DAT < [(VDAT_SRC(min) - VDAT_REF(max))/IDAT_SINK(max)]- (R1+R2)(max)

Considering 1% tolerance for R1 and R2 (470

Ω nominal) and min/max values taken from

the USB battery charging specification and the STUSBCD01B datasheet, we obtain:

Equation 3

RDCHG_DAT < [(0.615-0.34)/(100*10-6)]-(470*2*1.01) = 1800.6 Ω

Figure 5.

STUSBCD01B current sink method applied to a dedicated charger

DAT_REF

DAT_SINK

DAT_SRC

Phase 1

STUSBCD01

DAT_REF

DAT_SINK

DAT_SRC

Phase 2

STUSBCD01

LOW

HIGH

Dedicated

Charger

I = 0

Dedicated

Charger

I =

DAT_SINK

DAT_REF

DAT_SINK

DAT_SRC

Phase 1

STUSBCD01

DAT_REF

DAT_SINK

DAT_SRC

Phase 2

STUSBCD01

LOW

HIGH

Dedicated

Charger

I = 0

Dedicated

Charger

I =

DAT_SINK

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