ZXBM1016

Issue 1 - February 2007

7

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© Zetex Semiconductors plc 2007

SPD - Speed control input

The voltage applied to the SPD pin provides control over the fan motor speed by varying the Pulse

Width Modulated (PWM) drive ratio at the Ph1Lo and Ph2Lo outputs. The input voltage range for

the control signal is 3V to 1V, representing 0% to 100% drive respectively.

If variable speed control is not required this pin can be left with an external potential divider to

set a fixed speed or tied through a 10k

resistor to ground to provide full speed i.e. 100% PWM

drive.

The advantage of a fixed potential divider is that the benefit of the current limit and tail-end

current control can be achieved. Neither function will operate with the pin tied directly to ground.

If full speed with current limit and tail-end current control is required then connect the SPD pin

ground through a 10k

resistor with a 0.1 F capacitor in parallel. This capacitor is used for the

tail-end current control shaping.

If required this pin can also be used as an enable pin. The application of a voltage >3.0V will force

the PWM drive fully off, in effect disabling the drive.

A voltage can be set on this pin using a potential divider between the ThRef and Gnd. This voltage

higher voltage on the SPD pin represents a lower speed it therefore restricts the lower speed

range of the motor. If this feature is not required the pin is left tied to ThRef so no minimum speed

will be set.

If the fan is being controlled from an external voltage source and the minimum speed feature is

required then a 10k

resistor should be placed in series with the external control voltage and the

SPD pin.

whilst running and when in the lock condition.

When a motor is running under normal conditions this pin provides a triangular waveform related

to the speed of the motor. This waveform is used by the tail-end current control circuit to

determine the end of the commutation cycle.

If the fan stops rotating for any reason then this pin takes on the conventional operation of a

Locked Rotor monitor. In this condition and after the motor has stopped, a predetermined time

the fan re-starts.

start-up function in conjunction with the current monitor circuit described towards the end of this

section.

pin. Any selection of the value will need consideration when in the run and lock modes. Typically

a 0.47 F will provide for most applications.