US90A / US91A
24V Hall IC Fan Driver
Features and Benefits
Low cost
High sensitivity Hall Sensor
Locked rotor protection and auto-restart
Power-efficient CMOS and power MOSFETs
Built-in output protection clamping diode
Integrated tachometer (US90A) or alarm (US91A) signal output
2-coil fan driver with FG/RD output in 4-pin VK / VA
and 8-pin narrow SOIC packages (all RoHS Compliant)
Application Examples
12V/24V fans
Peak current up to 500mA
Ordering Information
Part No.
US90A
US90A
US90A
US91A
US91A
US91A
Temperature Code
E (-40° to 85°
C
C)
E (-40° to 85°
C
C)
E (-40° to 85°
C
C)
E (-40° to 85°
C
C)
E (-40° to 85°
C
C)
E (-40° to 85°
C
C)
Package Code
VK (4-pin TO-92)
DC (8-pin narrow SOIC)
VA (4-pin TO-92 flat)
VK (4-pin TO-92)
DC (8-pin narrow SOIC)
VA (4-pin TO-92 flat)
1 Functional Diagram
2 General Description
The US90A/91A is a one-chip solution for driving
two-coil brushless DC cooling fans.
Based on the advanced Melexis CMOS process,
the IC contains a Hall-effect sensor, dynamic
offset correction and low side output drivers.
Frequency Generator or Rotation Detection is
available. The open-drain output makes the
connectivity with any external interface such as
hardware monitoring or Super I/O IC easier.
These features are combined with the Melexis
patented no-V
DD
design to fit the IC in small 4-pin
VK and VA package.
The device is also delivered in DC package for
automatic assembly.
390109026302
Rev 002
Page 1 of 12
Data Sheet
Apr/07
US90A / US91A
24V Hall IC Fan Driver
Table of Contents
1 Functional Diagram ........................................................................................................ 1
2 General Description........................................................................................................ 1
3 Glossary of Terms .......................................................................................................... 3
4 Absolute Maximum Ratings ........................................................................................... 3
5 Pin Definitions and Descriptions................................................................................... 3
6 General Electrical Specifications .................................................................................. 4
7 Magnetic Specifications ................................................................................................. 4
8 Driver Output vs Magnetic Pole..................................................................................... 4
9 Detailed General Description ......................................................................................... 5
10 Unique Features............................................................................................................ 5
11 Performance Graphs .................................................................................................... 6
11.1 R
DSON
vs T
J
...................................................................................................................................................................... 6
11.2 R
DSON
vs V
DD
.................................................................................................................................................................. 6
11.3 Magnetic parameters vs T
J
.................................................................................................................................. 6
11.4 Magnetic parameters vs V
DD
............................................................................................................................... 6
11.5 I
DD
vs T
J
............................................................................................................................................................................. 6
11.6 I
DD
vs V
DD
......................................................................................................................................................................... 6
11.7 V
OL
vs T
J
........................................................................................................................................................................... 7
11.8 Power Dissipation vs. T
A
....................................................................................................................................... 7
12 Application Information................................................................................................ 7
12.1 Typical application with pull-up resistor.....................................................................
7
12.2 Configuration for reverse voltage / ground hot-switch protection and enhanced ESD
module protection
............................................................................................................. 7
13 Application Comments ................................................................................................. 7
14 Standard information regarding manufacturability of Melexis products with
different soldering processes........................................................................................... 8
15 ESD Precautions ........................................................................................................... 8
16 Package Information..................................................................................................... 9
16.1 VK Package Information (4-pin TO92)
......................................................................................................... 9
16.2 DC Package Information (8-pin narrow SOIC)
..................................................................................... 10
16.3 VA Package Information (4-pin TO92 flat)
.............................................................................................. 11
17 Disclaimer.................................................................................................................... 12
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Rev 002
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Data Sheet
Apr/07
US90A / US91A
24V Hall IC Fan Driver
3 Glossary of Terms
Two-coil fan
MilliTesla (mT), Gauss
V
DD
I
DD
Peak output current
Continuous output current
Locked rotor
FG
RD
LFPM
A fan with two-coil windings where current alternates from 1 coil to the other
depending on the direction of the magnetic field.
Units of magnetic flux density :
1mT = 10 Gauss
Voltage on the coils common node.
Current supplying the chip which flows through the coil connected to the
switched off output driver.
The current flowing in the coil at start-up, only limited by the coil resistance
R
COIL
and the output driver resistance R
DSON.
The current flowing in the coil when the fan is spinning normally.
The state when the fan stopped spinning due to mechanical blockage.
Frequency generator or tachometer output
Rotation detection or alarm output
Linear Feet Per Minute – Unit of airflow velocity
4 Absolute Maximum Ratings
Parameter
Symbol
Fan Supply Voltage
V
DD
Peak Output Current
I
OUTp
Continuous Output Current
I
OUTc
FG / RD Output Current
I
FG (RD)
Operating Temperature Range
T
A
Junction Temperature
T
J
Storage Temperature Range
T
S
Magnetic Flux Density
B
Table 1: Absolute maximum ratings
Value
30
500
250
20
-40 to 85
125
-55 to 150
Unlimited
Units
V
mA
mA
mA
°C
°C
°C
mT
Exceeding the absolute maximum ratings may cause permanent damage. Exposure to absolute-maximum-
rated conditions for extended periods may affect device reliability.
5 Pin Definitions and Descriptions
Pin Name Pin number (VK) Pin number (DC) Pin number (VA)
FG (RD)
1
1
1
OUT1
2
2
2
OUT2
3
4
4
GND
4
3
3
-
-
5, 6, 7, 8
-
Table 2: Pin definitions and descriptions US90A (US91A)
Function
FG (RD) open drain output signal
Open Drain Coil Driver 1
Open Drain Coil Driver 2
Ground pin
Not Connected
390109026302
Rev 002
Page 3 of 12
Data Sheet
Apr/07
US90A / US91A
24V Hall IC Fan Driver
6 General Electrical Specifications
DC Operating Parameters T
A
= 25 C, V
DD
= 24V (unless otherwise specified)
Parameter
Symbol Test Conditions
Fan Supply Voltage
V
DD
Operating, R
COIL
= 50Ω
Supply Current
I
DD
Output Saturation Voltage
V
DSON 1,2
I
OUT
= 150mA
Output Saturation Voltage
V
DSON 1,2
I
OUT
= 250mA
Output Clamp Voltage
V
OUT 1,2
FG / RD Output Low Voltage
V
OL
I
OL
= 10mA
FG / RD Output Clamp Voltage
V
CLAMP
FG / RD Output Leakage Current I
LEAK
V
FG
(V
RD
) = 18V
DC Thermal Resistance
R
THja
One-sided PCB, zero LFPM
VK Thermal Resistance
R
THja
One-sided PCB, zero LFPM
VA Thermal Resistance
R
THja
One-sided PCB, zero LFPM
Locked Rotor Period
T
ON
Locked Rotor Period
T
OFF
Output Switching Delay
T
DELAY
“Dead time” when both drivers are off
Table 3: General electrical specifications
Min
4.7
(1)
Typ
2
375
625
60
250
28
10
150
200
170
0.25
1.5
200
500
Max
30
4
Units
V
mA
mV
mV
V
mV
V
A
°C/Watt
°C/Watt
°C/Watt
S
S
us
o
Note 1:
The minimal value of VDD should be determined using the following equation: V
DD
= 4.5V + R
COIL
* I
DD
7 Magnetic Specifications
DC Operating Parameters T
A
= 25 C, V
DD
= 24V (unless otherwise specified)
Parameter
Symbol
Test Conditions
Operate point
B
OP
Release point
B
RP
Hysteresis
B
HYST
Table 4: Magnetic specifications
Min
-6
2
Typ
3
-3
6
Max
6
Units
mT
mT
mT
o
8 Driver Output vs Magnetic Pole
Parameter
Test Conditions
North pole
B < Brp – all packages
South pole
B > Bop - all packages
Table 5: Driver output vs. magnetic pole
OUT1
High
Low
OUT2
Low
High
Note 3:
The magnetic pole is applied facing the branded side of the package
390109026302
Rev 002
Page 4 of 12
Data Sheet
Apr/07
US90A / US91A
24V Hall IC Fan Driver
9 Detailed General Description
The US90A/91A is a one-chip solution for driving two-coil brushless DC fans. Based on advanced Melexis
CMOS process, the IC contains a Hall-effect sensor, dynamic offset correction and low side output drivers.
The output drivers OUT1 and OUT2 are fully protected against switching transients. So there is no need for
an external zener diode to cut the high voltage spikes induced by the fan coils.
The US90A has an open-drain integrated tachometer FG output that follows the Hall signal.
In the US91A, the open-drain rotation detection output RD is active low during normal spinning of the motor.
It goes high when the flux switching frequency becomes too low, which means the motor is blocked.
10 Unique Features
The absence of a V
DD
pin enables the two low side output drivers and FG/RD signal output to fit in a four-pin
VK package. The lack of a V
DD
pin decreases also the probability to damage the chip due to reverse voltage
connection, using the coil resistance to limit the reverse current. In this condition, the total reverse current is
twice the peak current value of the fan used.
The built-in locked-rotor protection will automatically shut off the coil current when the rotor is mechanically
blocked, or the rotational speed drops below 60 RPM (4-pole rotor magnet). The fan will try to restart every
1.5 seconds until the obstruction is clear. The On / Off cycling reduces the average stall current to 1/7 normal;
this is enough to prevent overheating or damage to most fans. Both the US90A and US91A have this feature.
390109026302
Rev 002
Page 5 of 12
Data Sheet
Apr/07
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