AN3271 datasheet(24/29 Pages) STMICROELECTRONICS

24 / 29 page

High voltage connection

AN3271

24/29

Doc ID 17933 Rev 2

Equation 6

The closest resistor value from the E6 series is R2 = 33 kΩ.

The monitored threshold is slightly different due to the E6 series resistors:

Equation 7

If using R1 = 150 kΩ and R2 = 33 kΩ, the monitored high voltage threshold is 18.2 V. The

tolerance of resistors R1 and R2 affects the precision of the VSUPPLY-TH threshold.

Let's use dual N-channel / P-channel transistor STS8C5H30L, which has an absolute

maximum gate-source voltage of 16 V. In order to keep some margin, let's use VGS = 15 V

for the calculation. The maximum supply voltage is 30 V (it can't be higher than the

maximum drain-source voltage of the P-channel transistor). The current through resistors

R3 and R4 is 100 µA.

Resistor R4 is:

Equation 8

And resistor R3 is:

Equation 9

The maximum gate threshold voltage of the P-channel transistor is VGS(th)-MAX = 2.5 V,

therefore the P-channel transistor is securely driven for supply voltages down to:

Equation 10

TH –

SUPPLY

–

TH –

–

---------------------------------------------------------

150000

3.2

–

---------------------

32.43k

SUPPLY

–

TH –

150

100

–

3.2

18.2V

SUPPLY

MAX

–

-------------------------------------------------------

–

()

100

–

-----------------------------

150k

-----------

100

–

-----------------------------

150k

SUPPLY

MIN

–

--------------------

2.5

150

-----------------------------------------------------------

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