AN2785 datasheet(37/51 Pages) STMICROELECTRONICS

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AN2785

Below-ground voltage on the OUT pin

Doc ID 14785 Rev 1

37/51

The overall below-ground voltage on the OUT pin can be calculated as follows.

Equation 5

Equation 6

Where:

●

VFPK is the free-wheeling diode transient peak forward voltage; it depends mainly on

the device technology and on the dIF / dt of the current in the diode. Typical values may

range from some volts to more than 10 V. Figure 28 shows the VFP vs. dIF/dt of the

STTH1L06 diode.

●

dIF / dt is the current slope in the low side IGBT/MOSFET and may have a value from

some tens to some hundreds of A/µs. Its value depends mainly on the power switch

characteristics and in part on the driving current.

●

LPARASITIC represents the sum of all parasitic inductances on the current path and

mainly depends on the PCB layout. In general, during the design of the power

application it is important to pay attention to the layout of the power bridges in order to

limit this parameter. Typical values of a good layout are in the order of some tens of nH.

Note that it is also useful to use RSENSE resistors with low parasitic inductance for the

same reason.

●

RSENSE * ILOAD product is the value of the VSENSE voltage and is typically less than 1 V,

also for thermal dissipation issues on the same resistor.

●

VF is the forward voltage of the free-wheeling diode and is usually less than 2 V.

Figure 28.

Transient peak forward voltage vs. dIF/dt of STTH1L06 diode

OUTmin

static

≥

SENSE

LOAD

⋅

()

–

contributi

static

min_

OUT

contributi

dynamic

PARASITIC

FPK

spike

BGV

⋅

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