L9908 datasheet(65/152 Pages) STMICROELECTRONICS

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Where Qgd is the Ext. FET gate-drain charge and Idrv_source/sink_peak are the predrivers Pull-up/Pull-down

current. Driver peak current should not be confused with drivers Average Current which is the average current the

driver is delivering over full FET switching period i.e.

IDRV_AVERAGE=Qgfsw

(8)

Where Qg is the Ext. FET total gate charge and fsw is the predriver switching frequency.

Output peak current limitation

Since pre-drivers output current is not programmable the peak level can be reduced by means of an external

series resistances in order to achieve the following goals:

•

dump ringing due to parasitic inductances/capacitances;

•

dump ringing due to high voltage/current switching dv/dt, di/dt, and Ext. FET’s body-diode reverse recovery

charge;

•

optimize the switching losses;

•

reduce electromagnetic interference (EMI).

Figure 38. Pre-driver peak output current limitation circuit

The maximum PU/PD peak output currents can be estimated as follows:

IOUT_PU_PK=MINDRV_pu_iout, VHIGH

Rlim_on+RPU+Rg

(9)

IOUT_PD_PK=MINDRV_pu_iout, VHIGH−Vdfw

Rlim_on∥Rlim_off+RPD+Rg

(10)

Where: Vhigh is the driver`s output stage supply, Vdfw is the external diode forward voltage, Rg is the Ext. FET

series gate resistance, and Rlim_on/off are the external limitation resistances.

Example: if Rg = 1.6 Ω, Vdfw = 0.75 V, VHIGH = 10 V to have maximum peak current below 0.8 A the external

resistance shall be:

IOUT_PU_PK= 10V

Rlim_on+1.15Ω+1.6Ω=0.8A Rlim_on+2.75Ω ≈12.5Ω

Rlim_on≈9.75Ω

(11)

IOUT_PD_PK= 10V−0.75V

Rlim_on Rlim_off+1.15Ω+1.6Ω=0.8A Rlim_on Rlim_off

+2.75Ω ≈11.56Ω 9.75Ω Rlim_off≈ 8.81Ω Rlim_on≈4.63Ω

(12)

L9908

Functional description

DS13546 - Rev 5

page 65/152

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