MLX90285

24V Low Noise Two-Coil Fan Driver

3901090285

Rev. 005

Page 6 of 14

DataSheet

Aug/14

B

10 Unique Features

10.1 Active Soft Switching

The use of the Melexis Soft Switching techniques in the US168/169 and US651/661 has already proven its efficiency to

noticeably reduce electrical and acoustic noise by precisely control the output slope duration on the fan coil. The US168/168

is the first generation with fixed output slope duration and the US651/661 is the second generation with possibility for the

user to fine-tune the slope duration via an external pin.

The MLX90285 represents the third generation of soft switching device as it goes even a step further into intelligence and

smart driving.

It mainly combines the major advantage of each previous generation:

Adjustable Slope Control  best balance between motor performance and acoustic/electrical noise

No need of external component for slope control  cost efficiency and simplicity

“Adjustable without external component” is possible as the device automatically determine the slope duration depending on

the fan rotation speed.

The MLX90285 determines the fan rotation speed by measuring the half output switching period from the magnetic field

signal. The output slope duration is extrapolated by measuring the duration from the falling edge on OUT1 to the cross over

with OUT2.

leading to two possible options:

The Digital Slope Duration Control register is incremented or decremented accordingly, thus updating the slope duration

value for both outputs OUT1 and OUT2, prior the next rising slope occurring on OUT1.

Magnetic Field

at Hall sensor

B

OP

(from rotor magnet)

RP

Digital Magnetic Field

(North or South)

½ . T

OUT1

= ½ . 7.5ms = 3.75ms

V

OUT1

V

OUT2

Output Voltage

Output

crossover

OUT1 slope duration

measurement signal

½ t

SLOPE

= 0.15ms

½ t

SLOPE

= 0.145ms

Slope Duration to

Output Switching

Period Ratio register

Digital Slope Duration

Control register

( 1

N

96 )

SL

= 0.3ms / 3.75ms = 8%



Slope duration must be decreased

N = N

t

t

1 - All values in green are arbitrary and just given for example purpose

2 - Condition example: 4000RPM, 2-pole pair rotor magnet, initial slope

duration = 0.3ms, target SL

Picture 1 – Active Soft Switching Operation Principle