VCC - Applied Voltage

This is the device internal circuitry supply voltage. For

5V to 12V fans this can be supplied directly from the Fan

Motor supply. For fans likely to run in excess of the 18V

maximum rating for the device this will be supplied

from an external regulator such as a Zener diode.

SetTh - Set Threshold Voltage

The ZXBM1015 contains a current monitor circuit used

to sense the current flowing in the motor winding and

this pin is used to control how the circuit responds to

that current.

The device works in a threshold feedback mode using a

potential divider to the Set Threshold pin. This

potential divider is used to set a voltage that will be

compared with the voltage generated by the current in

a Sense resistor attached in the Low-Side ground

return of the external H-Bridge driver. When the current

in the Sense resistor, and thus the voltage, rises above

the SetTh pin threshold the controller will back-off the

PWM drive to limit the maximum current taken by the

motor. To do this the current monitor will internally

apply a correction signal to the SPD pin. If the motor

current is below the set threshold the controller does

not influence the SPD voltage. A suitable voltage range

for the SetTh pin, and thus the sensed voltage on the

Sense pin, would be 50mV to 200mV.

It should be noted that the effectiveness of the control is

determined by the external network used to control the

SPD pin. It will not work where the fan is being

controlled by a low source impedance voltage. If the

fan is being controlled from an external voltage source

then a resistor >1k

should be placed in series with the

SPD pin.

The minimum speed setting on the SMIN pin will

override the current limit feature. A current cannot be

set that is lower than the current taken when the motor

stalls at minimum speed.

Sense

This pin is used by the current sensing circuit, as

described above, to monitor the current taken by the

motor windings. The signal comes from a sense

resistor in the Low-Side ground return of the external

H-Bridge driver.

ZXBM1017

ISSUE 4 - MAY 2007

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