ON Semiconductor MC642 Technical data

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MC642

PWM Fan Speed Controller
with Fault Detection

The MC642 is a pulse width modulation (PWM) fan speed
controller for use with DC motors. It provides temperature
proportional speed control. A thermistor connected to the V
furnishes the required control voltage of 1.25V to 2.65V for 0% to
100% PWM duty cycle. Minimum fan speed is set by a simple resistor
divider on the V

input. An integrated Start–Up Timer ensures

MIN

reliable motor start–up at turn–on, coming out of Shutdown Mode, or
following a transient fault. A stalled, open, or unconnected fan causes
the MC642 to trigger its start–up timer once. If the fault persists, the
FAULT

output goes low, and the device is latched in Shutdown Mode.

Features

• Shutdown Mode for Power Saving

• Supports Low Cost NTC/PTC Thermistors

• T emperature Proportional Speed for Acoustic Control /

Longer Fan Life

• Fan Voltage Independent of MC642 Supply Voltage

• Fault Detection Circuits Protect Against Fan Failure and

Aid System T esting

• Operating T emperature Range: 0°C to +85°C

Typical Applications

• Power Supplies

• Personal Computers

• UPS’s, Power Amplifiers, etc.

TYPICAL APPLICATION DIAGRAM

V

DD

D1

IN

input

+12 V

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SO–8

D SUFFIX

PRELIMINARY INFORMATION

PRELIMINARY INFORMATION

CASE TBD

8–Pin DIP
P SUFFIX

CASE TBD

PIN CONFIGURATION

V

V

MIN

GND

1

IN

C

2

MC642D

F

MC642P

3
4

8
7
6
5

V

DD

V

OUT

FAULT
SENSE

+5 V

From
Temp
Sensor

V

16

8

V

DD

FAULT

in

MC642

V

min

3

C

2

(Optional)

 Semiconductor Components Industries, LLC, 1999

February , 2000 – Rev. 0

+

SENSE

F

GND

4

D2

V

out

7

5

C

SENSE

MC642

Reset

1
0

Detected

Fault

FAN

Q1

R

BASE

MC642DR2 8–Pin SOIC 2500 Tape/Reel

R

SENSE

1 Publication Order Number:

MC642P 8–Pin Plastic DIP 50 Tape/Reel

ORDERING INFORMATION

Device Package Shipping

MC642/D

V

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

Á

MC642

FUNCTIONAL BLOCK DIAGRAM

V

IN

C

MIN

GND

V

OTF

–
+

–
+

F

Clock

Generator

+

V

SHDN

–

MC642

70 mV (typ.)

+
–

SHDN

+
–

Control

Logic

3 X T

PWM

Timer

Start–Up

Timer

Missing

Pulse

Detector

OTF

10 k

W

V

DD

V

OUT

FAULT

SENSE

PIN DESCRIPTION

Pin No. Symbol Description

1

ÁÁ

2
3

ÁÁ

ÁÁ

4
5

6

ÁÁ

ÁÁ

7

ÁÁ

8

V

IN

Á

C

F

V

MIN

Á

Á

GND

SENSE

FAULT

Á

Á

V

OUT

Á

V

DD

The thermistor network (or other temperature sensor) connects to this input. A voltage range of 1.25V to 2.65V
(typical) on this pin drives an active duty cycle of 0% to 100% on the V

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Positive terminal for the PWM ramp generator timing capacitor. The recommended CF is 1µF for 30Hz PWM operation.
An external resistor divider connected to this input sets the minimum fan speed by fixing the minimum PWM duty

cycle (1.25V to 2.65V = 0% to 100%, typical). The MC642 enters Shutdown mode when 0 ≤ V

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Shutdown, the FAULT output is inactive, and supply current falls to 25µA (typical). The MC642 exits Shutdown
mode when V

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MIN

≥ V

REL

. See

Applications

section for more details.
Ground Terminal
Pulses are detected at this pin as fan rotation chops the current through a sense resistor. The absence of pulses

indicates a fault.
Fault (open collector) output. This line goes low to indicate a fault condition. When FAULT goes low due to a fan

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fault, the device is latched in Shutdown Mode until deliberately cleared or until power is cycled. FAULT
connected to V

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cycle, however the device will not latch itself off unless FAULT

if a hard shutdown is desired. FAULT

MIN

will also be asserted when the PWM reaches 100% duty

PWM signal output. This active high complimentary output connects to the base of an external NPN motor drive

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transistor. This output has asymmetrical drive. – See

Electrical Characteristics

Power Supply Input. May be independent of fan power supply. See

pin.

OUT

is tied to V

externally.

MIN

section.

Electrical Characteristics

≤ V

MIN

section.

SHDN

may be

. During

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2

MC642

ABSOLUTE MAXIMUM RATINGS*

Parameter Value Unit

Package Power Dissipation (TA ≤ 70°C)

Plastic DIP

Small Outline (SOIC)
Derating Factors 8.0 mW/°C
Supply Voltage 6.0 V
Input Voltage, Any Pin (GND – 0.3) to (VCC + 0.3) V
Operating Temperature Range 0 to +85 °C
Maximum Chip Temperature 150 °C
Storage Temperature Range –65 to +150 °C
Lead Temperature (Soldering, 10 Seconds) +300 °C

* Maximum Ratings are those values beyond which damage to the device may occur.

ELECTRICAL CHARACTERISTICS (T

Symbol Characteristic Min Typ Max Unit

V

DD

I

DD

I

DD(SHDN)

I

IN

V

Output

OUT

t

R

t

F

t

(SHDN)

I

OL

I

OH

VIN, V

V

C(MAX)

V

C(SPAN)

V

SHDN

V

REL

Pulse–Width Modulator

F PWM Frequency (CF = 1.0µF) 26 30 34 Hz

Sense Input

V

TH(SENSE)

Fault Output

V

OL

t

MP

t

STARTUP

t

DIAG

1. Guaranteed by design, not tested.

MIN

, Inputs

, V

Supply Voltage 3.0 — 5.5 V
Supply Current, Operating

Pins 3, 5, 7 Open, CF = 1µF, VIN = V

Supply Current, Shutdown Mode

Pins 1, 5, 6, 7 Open, CF = 1µF, VIN = 0.35V

VIN, V

V

OUT

V

OUT

Pulse Width (On V

Sink Current at V

Source Current at V

Input Voltage at VIN or V

OTF

V

C(MAX)

Voltage Applied to V
Voltage Applied to V

SENSE Input Threshold Voltage with Respect to GND 50 70 90 mV

Output Low Voltage (IOH = 2.5mA) — — 0.3 Hz
Missing Pulse Detector Timeout — 32/F — Sec
Startup Time — 32/F — Sec
Diagnostic Timer Period — 3/F — Sec

Input Leakage (Note 1.)

MIN

Rise Time (IOH = 5.0mA) (Note 1.)
Fall Time (IOH = 1.0mA) (Note 1.)

, V

MIN

Specifications

HYST

OUT

DD

DD

C(MIN)

V

SHDN

VOL = 10% of V

VOH = 80% of V

– V

VDD = 5V

< TA < T

MIN

) to Clear Fault Mode

Output

Output

OUT

for 100% PWM Duty Cycle 2.5 2.65 2.8 V

MIN

to Guarantee Shutdown Mode — — VDD x 0.13 V

MIN

to Release Shutdown Mode

MIN

, VDD = 3.0V to 5.5V, unless otherwise noted.)

MAX

(CMAX)

(Note 1.)

— 0.5 1.0

— 25 —

–1.0 — 1.0 µA

— — 50 µsec
— — 50 µsec

30 — —

1.0 — —

5.0 — —

1.3 1.4 1.5 V

VDD x 0.19 — —

730
470

mW

mA

µA

µsec

mA

mA

V

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3

MC642

DET AILED OPERATING DESCRIPTION

PWM

The PWM (Pulse Width Modulation) circuit consists of a
ramp generator and threshold detector. The frequency of the
PWM is determined by the value of the capacitor connected
to the C

input. A frequency of 30Hz is recommended (C

F

= 1µF). The PWM is also the timebase for the startup and
fault timer (see below). The PWM voltage control range is

1.25V to 2.65V (typical) for 0% to 100% output duty cycle.

V

Output

OUT

The V

pin is designed to drive a low–cost transistor or

OUT

MOSFET as the low side power switching element in the
system. V arious examples of driver circuits are shown in the
following pages. This output has asymmetric
complementary drive and is optimized for driving NPN
transistors or N–channel MOSFET’s. Since the system
relies on PWM rather than linear power control, the
dissipation in the power switch is kept to a minimum.
Generally , very small devices (TO–92 or SOT package) will
suffice. (See Output Drive Transistor Selection paragraph in
Applications Information section.)

Start–Up Timer

T o ensure reliable fan startup, the StartUp Timer turns the
V

output on for 32 cycles of the PWM whenever the fan

OUT

is started from the off–state. This occurs at power–up and
when coming out of shutdown mode. If the PWM frequency
is 30Hz (C

= 1µF), the resulting start–up time will be about

F

one second. If a Fault is detected (see below), the Diagnostic
Timer is triggered once, followed by the Startup–Up T imer .
If the fault persists, the device is shut down. See FAULT
Output below .

Shutdown Control (Optional)

When V

(pin 3) is pulled below V

MIN

, the MC642 will

SHDN

go into Shutdown mode. This can be accomplished by
driving V

with an open drain logic signal or using an

MIN

external transistor as shown in Figure 1. All functions are
suspended until the voltage on V
V

(0.85V @ VDD = 5.0V). Pulling V

REL

becomes higher than

MIN

below V

MIN

SHDN

will
always result in complete device shutdown and reset. The
FAULT output is unconditionally inactive in Shutdown
mode.

A small amount of hysteresis, typically one percent of V
(50mV at VDD = 5.0V), is designed into the V
threshold. The levels specified for V

SHDN

and V

DD

SHDN/VREL

in the

REL

Electrical Characteristics section include this hysteresis
plus adequate margin to account for normal variations in the
absolute value of the threshold and hysteresis.

CAUTION: Shutdown mode is unconditional. i.e., the

fan will not activate regardless of the voltage on V
The fan should not be shut down until all heat–producing
activity in the system is at a negligible level.)

F

SENSE Input

The SENSE input, pin 5, is connected to a low–value
current sensing resistor in the ground return leg of the fan
circuit. During normal fan operation commutation occurs as
each pole of the fan is energized. This commutation causes
brief interruptions in the fan current, which is seen as pulses
across the sense resistor. When the device is not in
Shutdown Mode and pulses are not appearing at the SENSE
input, a fault condition exists.

The short, rapid changes in fan current (high dI/dt) cause
corresponding dV/dt pulses across the sense resistor, R
The waveform on R
a logic–level pulse–train by C

is differentiated and converted to

SENSE

and the internal signal

SENSE

processing circuitry (See Figure 1). The presence and
frequency of this pulse–train is a direct indication of fan
operation. See the Applications Information section for
more details.

FAULT Output

The MC642 detects faults in two ways:
(1) Pulses appearing at SENSE due to the PWM turning on
are blanked and the remaining pulses are filtered by a
missing pulse detector. If consecutive pulses are not
detected for 32 PWM cycles (1 Sec if CF = 1µF), the
Diagnostic Timer is activated and V

OUT

continuously for three PWM cycles (100msec if CF = 1µF).
If a pulse is not detected within this window, the
Startup–Timer is triggered. This should clear a transient
fault condition. If the Missing Pulse Detector times out
again, the PWM is stopped and FAULT

goes low. When
FAULT is activated due to this condition, the device is
latched in Shutdown mode and will remain off indefinitely .
(Diodes D1, D2 and resistor R5 (See Figure 1) are provided
to ensure that fan restarting is the result of a fan fault, and not
an over–temperature fault. A CMOS logic OR gate may be
substituted for these components if available).

When F AUL T is activated due to this condition, the device

is latched in Shutdown mode and will remain off
indefinitely. Important: At this point, action must be

taken to restart the fan by momentarily pulling V
below V

, or by cycling system power. In either case

SHDN

the fan cannot be permitted to remain disabled due to a
fault condition, as severe system damage could result. If
the fan cannot be restarted, the system should be shut
down. The MC642 may be configured to continuously

attempt fan restarts if so desired.

. (Note:

IN

SENSE

is driven

MIN

.

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4

0.1 mF

MC642

V

DD

R5

10 k

D1

+12 V

From
System
Shutdown
Controller

+5 V

R1

(Optional)

* The parallel combination of R3 and R4 must be > 10 k.

R3

R4

From
Temp
Sensor

0.01
C

B

+

V

in

16

V

min

3

C

F

2

C

F

1 mF

Figure 1. T ypical Fan Control Application

Continuous restart mode is enabled by connecting the

FAULT

output to V

through a 0.1µF capacitor as shown

MIN

in Figure 1. When so connected, the MC642 automatically
attempts to restart the fan whenever a fault condition occurs.
When the fault output is driven low, the V
momentarily pulled below V

, initiating a reset and

SHDN

input is

MIN

clearing the fault condition. Normal fan startup is then
attempted as previously described. The F AUL T

output may
be connected to external logic (or the interrupt input of a
microcontroller) to shut down the MC642 if multiple fault
pulses are detected at approximately one second intervals.

(2) FAULT

is also asserted when the PWM control
voltage applied to VIN becomes greater than that needed to
drive 100% duty cycle (see Electrical Characteristics). This
indicates that the fan is at maximum drive and the potential
exists for system overheating. Either heat dissipation in the
system has gone beyond the cooling system’ s design limits
or some other fault exists such as fan bearing failure or an
airflow obstruction. This output may be treated as a System
Overheat warning and used to trigger system shutdown.
However in this case, the fan will continue to run even when
F AUL T
connected to V

is asserted. If a shutdown is desired, F AUL T may be

outside the device. This will latch the

MIN

MC642 in Shutdown Mode when any fault occurs.

8

DD

FAULT

SENSE

4

D2

V

out

7

5

MC642

Reset

1
0

Fault Detected

C

SENSE

R

BASE

FAN

Q1

R

SENSE

+5 V

V

MC642

GND

SYSTEM BEHAVIOR

The flowcharts describing the MC642’s behavioral
algorithm are shown in Figure 3. They can be summarized
as follows:

Power–Up

(1) Assuming the device is not being held in Shutdown

mode (V

> V

MIN

(2) Turn V

):

REL

output on for 32 cycles of the PWM

OUT

clock. This ensures that the fan will start from a dead stop.

(3) During this Start–up time, if a fan pulse is detected

then branch to Normal Operation; if none are received.

(4) Activate the 32–cycle Start–up Timer one more
time and look for fan pulses; if a fan pulse is detected,

proceed to Normal Operation; if none are received....

(5) Proceed to Fan Fault

(6) End

After this period elapses, the MC642 begins normal

operation.

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