AN2785 datasheet(18/51 Pages) STMICROELECTRONICS

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Application examples

AN2785

18/51

Doc ID 14785 Rev 1

6.2.2

Voltage mode - example 1

A different approach could be used to drive a single-phase BLDC fan: the voltage mode

control. In the following example, an application circuit using the voltage mode approach is

provided. This solution does not control the load current as in the previous circuit, but

directly controls the duty cycle of the voltage applied to the load connected to the full-bridge.

Additionally, the motor current is limited for protection purposes. As in the previous example,

the diagonal of the full-bridge is selected by feeding the single-phase Hall Effect sensor

signal directly to the PHASE input of one L6393 and the inverted signal to the PHASE input

of the other L6393. Unlike the circuit shown in Figure 10, in this particular example the

PHASE inversion is provided by a simple bipolar transistor since the two L6393 available

comparators are already used for other purposes. Note that the PHASE inversion, as other

analog/digital functions, can be obtained using various combinations of the features of the

L6393 drivers or, where needed, some external components. The PWM voltage control is

implemented using one L6393 comparator which compares the voltage coming from a

triangle generator with an external voltage reference. As in the previous circuit, the external

reference threshold is also limited to a minimum value by the network composed by a

resistive divider plus a small signal diode (see Figure 12). The result of the PWM

comparator is fed into the BRAKE inputs of the two L6393s. As mentioned previously, the

pull-down of the two BRAKE inputs provides a “slow decay” mode for the current control,

shorting the motor load through the two low sides. If “fast decay” is required, the PWM signal

can be applied to the SD pins of the two drivers. The remaining comparator can be used to

implement a simple over-current protection obtained by reading the shunt resistor voltage

between the low sides and ground. The open-drain output of the comparator can be

connected directly to the two SD pins of the L6393s so as to set to high impedance the full-

bridge stage after over-current detection. The RC network on this open drain output sets a

disable time after the fault detection. If over-current protection is not needed, one of the

integrated comparators can be used for other purposes (such as triangle waveform

generation, for example).

AN2785 Datasheet(PDF) 18 Page - STMicroelectronics