AN2785 datasheet(17/51 Pages) STMICROELECTRONICS

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AN2785

Application examples

Doc ID 14785 Rev 1

17/51

In the example of Figure 10 the logic inversion of the hall signal is provided by a comparator

which acts as a logic inverter. The comparator is directly available in one of the two L6393

drivers. The remaining comparator, belonging to the other L6393 IC, can be used as in the

example to provide a PWM current control by comparing the sense voltage coming from the

shunt resistor to a certain threshold voltage Vth, and by connecting the open-drain output of

the comparator with both the two BRAKE inputs of the two L6393s. The analog input signal

Vth is limited by the resistive divider and the small signal diode to a fixed minimum level.

With this kind of current mode control, the reference signal Vth regulates directly the torque

applied to the fan motor and then indirectly the speed. The type of current control obtained

with this architecture is a peak current control with constant OFF time, dimensioned by the

RC network on the open-drain output of the sense comparator. Note that the active pull-

down of the BRAKE signal operated by the PWM comparator decreases the load current

during the OFF time in “slow decay” mode, because the load is shorted by the bridge. If “fast

decay” is desired, the PWM signal can be applied to the SD pins of the two drivers.

Figure 11

shows the waveform of the load current (flowing in the shunt resistor) and the Vth

torque reference signal.

Figure 11.

Current modulation (flowing in the shunt resistor) changing the Vth

signal

The rotation detection circuit works as a high-pass filter and can detect if the fan rotor is

moving or not. In the event of a stall, the RD output goes to ground and provides information

on the fan’s stall condition.

Note that several different combinations of the circuitry available in the L6393 can be used.

For example, if the inverted hall signal is already available (or is implemented using a small

BJT or any other circuitry), the first comparator belonging to one of the two L6393s can be

used for any other purpose.

AN2785 Datasheet(PDF) 17 Page - STMicroelectronics