DocID025239 Rev 2

5/14

AN4354

Gate driving circuitry

14

1.1

Advantages of constant current driving of the MOSFET gates

The L648x gate drivers, thanks to the possibility to drive the MOSFET gates using

a programmable constant current, have two main advantages:

1.

The slew rate of the power stage is controlled allowing a precise EMI management.

2.

No gate resistors are required reducing the number of components in the bill of

material.

The output slew rate depends on how fast the MOSFET gate is charged in the Miller plateau

region. In a classic gate driver the charging/discharging current is adjusted adding a resistor

between the gate driver output and the MOSFET gate (see Figure 3). This way the output

slew rate can be limited, but its value depends on the load current (the plateau voltage

changes with the drain current).

Using a true current generator, the L648x gate driver forces a constant gate current during

the entire plateau region regardless of the load current.

Figure 3. Classic gate driver circuit vs. L648x gate driver

1.2

Turn-on sequence

When one of the external MOSFET must be turned-on the gate driving circuitry disables the

respective Miller clamp switch and starts forcing the programmed current into the gate. After

the controlled current time expires, the high-side and the low-side drivers act differently: the

low-side drivers do not limit the gate current; the high-side ones instead limit the gate

current to about 1 mA (see Figure 4).