TD340

8/21

Figure 2 : PWM Analog and Digital Modes

Active (synchronous) rectification for free-wheel current

A motor is an inductive load. When driven in PWM mode, motor current is switched on and off at the

25kHz frequency. When the MOS is switched off, current can not instantaneously drop to zero, a so-called

”free-wheel” current arises in the same direction than the power current. A path for this current must be

provided, otherwise high voltage could arise and destroy the component. The classical way to handle this

situation is to connect a diode in an anti-parallel configuration regarding to the MOS, so that current can

continue to flow through this diode, and finally vanishes by the means of ohmic dissipation, mainly in the

diode due to its 0.8V direct voltage. For high currents, dissipation can be an important issue (eg: 10A x

0.8V makes 8 W!). Furthermore, high speed diodes have to be used, and are expensive.

A more efficient way to handle this problem is to use the high side MOS as a synchronous rectifier. In this

mode, the upper MOS is switched ON when the lower one is switched OFF, and carries the free-wheel

current with much lower ohmic dissipation. Advantages are : one expensive component less (the fast

power diode), and more reliability due to the lower dissipation level.

However, we have to take care not to drive the two MOS simultaneously. To avoid transient problems

when the MOS are switched, a deadtime is inserted between the opening of one MOS, and the closing of

the other one. In the TD340 device, the deadtime is fixed to about 2.5 microseconds. This value is the time

between the commands of the gate drivers, not the deadtime between the actual MOS states because of

the rising and falling times of the gate voltages (due to capacitance), and the MOS characteristics. The

actual value of the deadtime for a typical configuration is about 1.5 microseconds.

Figure 3 shows the synchronous rectification principle

Table 1 summarizes the status of the Mosfets (and the speed and direction of the motor) according to the

Inputs (IN1 and IN2) status in analog and logic modes.

M

µP

TD340

IN1

CF

PWM

PWM

0V

5V

M

µP

TD340

IN1

PWM

PWM

0V

5V

ANALOG INPUT

+ CF (270pF)

DIGITAL INPUT

+ CF GROUNDED

PWM OUTPUT

PWM OUTPUT

CF

Vbatt

Vbatt