Rainbow Electronics MAX6651 - Manual (Page 19)

MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I 2 C-Compatible Interface
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Even in systems where there is a MAX6650/MAX6651
for each fan, there can still be speed mismatches. This
is primarily due to the oscillator tolerance. The
MAX6650/MAX6651 oscillator tolerance is specified to
be ±10%. In the worst case, this could result in a 20%
(one 10% high, one 10% low) speed mismatch.
The solution is to use a single MAX6651 for each fan,
and configure the parts to use a shared clock. The
shared clock can either be an external system clock or
one of the MAX6651s’ internal clocks. If an external
clock is used, its frequency can range from approxi-
mately 50kHz to 500kHz.
For synchronized fan control, select the MAX6651.
Combination
In more complex systems, a combination of some or all
of the above control types may be needed.
Choosing a Fan
Once the topology is chosen, the next step is to choose
a fan. See the appropriate section.
Enter a zero in bit 3 of the configuration register for a
5V fan and 1 for a 12V fan.
Configuring this bit also adjusts the tachometer input
threshold voltage. This optimizes operation of the
MAX6650/MAX6651 for the operating voltage of the fan
being used.
Setting the Mode of Operation
The MAX6650/MAX6651 have four modes of operation
as determined by bits 5 and 6 of the configuration reg-
ister: full on, full off, open loop, and closed loop.
Full-On
The full-on mode applies the maximum available volt-
age across the fan, guaranteeing maximum cooling.
Full-on mode can be entered through software or hard-
ware control. To enter full-on mode through hardware,
see the Setting Up the GPIOs section. Note that a hard-
ware full-on overrides all other modes.
Configure the MAX6650/MAX6651 to run in software
full-on mode by entering 00 into bits 5 and 4 of the con-
figuration register.
Full-Off
The full-off mode removes all the voltage across the
fan, causing the fan to stop. Because the MAX6650/
MAX6651
V CCIN
SCL
10k
SDA
GPIO0
OUT
ADD
GND
FB
TACH0
V CC
3V TO 5.5V
V FAN
5V OR 12V
C
COMP
10 µF
I 2 C/SMBus
INTERFACE
GPIO1
GPIO2
FULL ON
ALERT
FAN
2.2k
V ID
HOT-SWAP SECTION
Figure 13. Hot-Swap Application