L99MH98 datasheet(30/147 Pages) STMICROELECTRONICS

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Table 24. Free-wheeling mode

HB_WHEELx, x=1...8

Setting

Passive free-wheeling

Active free-wheeling on HS of the half-bridge x

Active free-wheeling on LS of the half-bridge x

Passive free-wheeling

In the active free-wheeling case, the MOSFET is actively switched off by the pre-driver. In active free-wheeling the

current is fixed. It is possible to configure two different currents according to the STRONG_ON_WHEELx, x =

1...8, bit: 4 mA if the bit is set to 0, 30 mA if the bit is set to 1.

If the microcontroller, in a not correct way, should program a PWM and an active free-wheeling in a MOSFET at

the same time, L99MH98 assigns the PWM to the selected MOSFET.

4.2

Indirect current measurement for external MOSFET

A new and innovative current sense method, different from the classical voltage drop on the shunt resistors, has

been designed and here reported. The proposed method has the great advantage of not using external resistors,

with the consequence of a lower cost of the application and no voltage drop on the sensing resistors.

The new current sense method is made up of 5 different phases:

1. CSOs settings according to the used external MOSFETs;

2. Rds(on) and temperature calibration at application level;

3. Rds(on) prediction curve stored in the external μC;

4. Live temperature measurement for MOSFET Rds(on) prediction;

5. Real time Vds measurement via CSO pin and external MOSFET current calculation (done at μC level).

Figure 15. Overview of indirect current measurement

4.2.1

Rds(on) calibration at application level

The first phase of current sensing is that of a correct measurement of the Rds(on) of each MOSFET of the 4

external H-bridges. This is because the Rds(on) of the MOSFETs have a large variability that can go up to ±25%

of the typical value. Thus, to have a precise measurement of the current, a calibration must be performed during

the manufacturing process. The Rds(on) calibration of each MOSFET must be done using the following strategy

at room temperature:

1. Power on the MOSFET under measurement;

2. A fixed current, 1 A for instance, is sent in the drain pin. The Vds is measured by L99MH98;

3. The microcontroller calculates the Rds(on) = Vds/1 A. This value is stored in the microcontroller memory. This

value represents the Rds(on) of the MOSFET at room temperature;

4. The curve of the variation of the Rds(on) as function of the temperature is stored in the microcontroller

memory.

L99MH98

Gates driver

DS14611 - Rev 3

page 30/147

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