AN3271 datasheet(23/29 Pages) STMICROELECTRONICS

23 / 29 page

AN3271

High voltage connection

Doc ID 17933 Rev 2

23/29

Equation 2

where

R2 is the second resistor of the voltage divider.

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

R3 / R4 ratio has to guarantee a secure open and close of the P-channel transistor. It is

possible to use for example dual N-channel / P-channel transistor STS8C5H30L in a tiny

SO8 package, which can switch voltages up to 30 V and currents up to 5 A.

The absolute maximum gate-source voltage VGS has to be considered, therefore:

Equation 3

where

R4 is the resistor between the P-channel transistor and N-channel transistor,

VSUPPLY–MAX is the possible maximum of VSUPPLY voltage,

VGS is the absolute maximum gate-source voltage or lower,

I34 is the current through the R3 and R4 resistors.

Equation 4

where,

R3 is the resistor between the gate and source of the P-channel transistor.

Example

Let's use STM6600ES24DM6F, which has active high EN output and undervoltage

threshold VTH– = 3.2 V.

We would like to monitor the supply voltage VSUPPLY for 18 V, the current through the

resistor divider R1/R2 is I12 = 100 µA and therefore:

Equation 5

The closest resistor value from the E6 series is R1 = 150 kΩ.

TH –

SUPPLY

–

TH –

–

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

SUPPLY

MAX

–

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

-----------

SUPPLY

–

TH –

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

3.2

–

100

–

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

148k

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