MLX90285

24V Low Noise Two-Coil Fan Driver

3901090285

Rev. 005

Page 5 of 14

DataSheet

Aug/14

7 Magnetic Specifications

Parameter

Symbol

Min

Typ

Max

Units

Operate point

0.5

3

6

mT

Release point

-6

-3

-0.5

mT

Hysteresis

2

6

mT

Table 3: Magnetic specifications

8 Driver Output vs Magnetic Pole

Parameter

Test Conditions

OUT1

OUT2

FG

(6)

North pole

B < Brp – all packages

High

Low

High

South pole

B > Bop - all packages

Low

High

Low

Table 4: Driver output vs. magnetic pole

(7)

9 Detailed General Description

The MLX90285 is a one-chip solution for driving two-coil brushless DC cooling fans.

Based on mixed-signal CMOS design, the device includes Hall-effect sensor with offset cancellation scheme, voltage

regulator, digital logic and power output drivers in a single package.

The MLX90285-FG version has an open-drain tachometer FG output that follows the Hall signal. This signal is used by

hardware monitoring system to determine the rotation speed of the fan.

In the MLX90285-RD version, the open-drain alarm output RD is a safety signal which allows detecting if the fan rotates or

not. It is active low during normal spinning of the motor. It goes high when the magnetic flux switching frequency drops

below nearly 1.15Hz (35RPM for 2 pole-pair fan)

The built-in locked rotor protection automatically shuts off the coil current when the rotor is mechanically blocked for more

than 0.43 second. The fan tries to restart every 3 seconds until the rotor is released. This on/off cycling reduces the average

current by factor of about 7. This protection prevents the fan to be overheated or damaged.

The device is based on the Melexis patented “no-VDD” design which removes the need of separate supply voltage pin for

two-coil fan. During operation, the power outputs are always operating as complementary, meaning also that one power

output is kept inactive while the other is active. Melexis cleverly turned this fact into benefit as “No-VDD” devices such as

MLX90285 recover the power supply through the inactive power output pin. Therefore, a separate supply voltage pin is not

required anymore and the device including logical output FG/RD can fit into a small 4-pin through-hole package VK.

In the application, the “No-VDD” design inherently isolates the IC behind the fan coils. Without direct output to the external

environment, the device is best protected against ESD and over-voltage issue on the power supply voltage line.