Rainbow Electronics MAX6660 - Manual (Page 20)

MAX6660
Remote-Junction Temperature-Controlled
Fan-Speed Regulator with SMBus Interface
20 ______________________________________________________________________________________
the nature of the tachometer output (open collector,
totem pole) and the resultant levels and configure the
connection to the MAX6660. For a fan with an open
drain/collector output, a pullup resistor of typically 5k
Ω
must be connected between FAN and VFAN. Note how
many pulses per revolution are generated by the
tachometer output (this varies from model to model and
among manufacturers, though two pulses per revolu-
tion is the most common). Table 14 lists the representa-
tive fan manufacturers and the model they make
available with tachometer outputs.
Low-Speed Operation
Brushless DC fans increase reliability by replacing
mechanical commutation with electronic commutation.
By lowering the voltage across the fan to reduce its
speed, the MAX6660 is also lowering the supply volt-
age for the electronic commutation and tachometer
electronics. If the voltage supplied to the fan is lowered
too far, the internal electronics may no longer function
properly. Some of the following symptoms are possible:
• The fan may stop spinning.
• The tachometer output may stop generating a signal.
• The tachometer output may generate more than two
pulses per revolution.
• The problems that occur and the supply voltages at
which they occur depend on which fan is used. As
a rule of thumb, 12V fans can be expected to expe-
rience problems somewhere around 1/4 and 1/2
their rated speed.
Chip Information
TRANSISTOR COUNT: 22,142
PROCESS: BiCMOS
Table 14. Fan Manufacturers
MANUFACTURER FAN MODEL OPTION
Comair Roton
All DC brushless models can be
ordered with optional tachometer
output.
EBM-Papst
Tachometer output optional on
some models.
NMB
All DC brushless models can be
ordered with optional tachometer
output.
Panasonic
Panaflo and flat unidirectional
miniature fans can be ordered with
tachometer output.
Sunon
Tachometer output optional on
some models.
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
VFAN TACH IN
STBY
SMBCLK
GAIN
SMBDATA
ALERT
ADDO
OVERT
TOP VIEW
MAX6660
QSOP
V
CC
DXP
ADD1
DXN
FAN
PGND
AGND
Pin Configuration