AN2865 datasheet(12/17 Pages) STMICROELECTRONICS

12 / 17 page

Dedicated charger detection method

AN2865

12/17

Doc ID 15283 Rev 1

Equation 8

R2 < (VDM_SRC(min) - VDAT_REF(max))/(IDAT_SINK(max))

Equation 9

R2(max) = (0.5 - 0.34)/(100*10-6) = 1600 Ω

This result is consistent with the value in Equation 6 on page 10.

In the case of a dedicated charger, the maximum DP/DM short resistance which is detected

as a charger can be calculated as follows. During phase 1, the value is not significant as

IDAT_SINK is off, therefore the resistor series (R1 + RDCHG_DAT + R2) is connected to a high

voltage on the DP side and is floating on the DM side. The voltage is high on both sides.

During phase one, the DM voltage is equal to the voltage drop over IDAT_SINK. This current

sink is designed to have a maximum voltage drop, while in operation, of 150 mV. This value

is lower than VDAT_REF so it is not critical. In order for the detection to be successful, the DP

line must also be low during phase 2:

Equation 10

VIDAT_SINK(max) + IDAT_SINK(max)*[RDCHG_DAT + (R1 + R2)(max)] < VTHDPL(min)

Equation 11

RDCHG_DAT < [(VTHDPL(min) - VIDAT_SINK(max))/IDAT_SINK(max)] - (R1 + R2)(max)

Considering 1% tolerance for R1 and R2 (470

Ω nominal) we obtain:

Equation 12

RDCHG_DAT(max) = [0.6 - 0.15)/ (100*10-6)] - (470*2*1.01) = 3550.6 Ω

This result is higher than the one found in Equation 3 on page 9 for the current sink method.

The actual result is even higher than this because in the above calculation we have

considered a maximum value for the current flowing in the resistors higher than the real one.

In fact, having two different current sources on the same branch causes the current to be set

to the value of the weak current source (max 30 µA).

This shows that both methods are quite robust and there is sufficient margin in the choice of

the R1 and R2 resistor values.

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