downtime. The system should be designed so that the
number of fans used is one more than are actually
needed. This way, there is sufficient cooling even if a
fan fails. With all fans operating correctly, it is unneces-
sary to run the fans at their maximum speed. Reducing
fan speed can reduce noise and increase the life of the
fans. However, once a fan fails, it is important that the
remaining fans spin at their maximum speed.
In Figure 9, all the GPIO0s are configured as ALERT
outputs, and all the GPIO1s are configured as
FULL ON inputs. If any MAX6650 generates an ALERT
(indicating failure), the remaining MAX6650s will auto-
matically turn their fans on full.
Temperature Monitoring and Fan Control
The circuit shown in Figure 10 provides complete tem-
perature monitoring and fan control. The MAX1617A (a
remote/local temperature serial interface with SMBus)
monitors temperature with a diode-connected transis-
tor. Based on the temperature readings provided by
the MAX1617A, the µC can adjust the fan speed pro-
portionally with temperature. Connecting the ALERT
output of the MAX1617A to the FULL ON input of the
MAX6650/MAX6651 (see the
General-Purpose Input/
Output
section) allows the fan to turn on fully if the
MAX1617A detects an overtemperature condition.
MAX6501 Hardware Fail-Safe
Figure 11 shows an application using a MAX6501 as a
hardware fail-safe. The MAX6650 has its GPIO1 config-
ured as FULL ON input. The MAX6501 TOVER pin goes
low whenever its temperature goes above a preset value.
This pulls the FULL ON pin (GPIO1) low, forcing the fan to
spin at its maximum speed. Figure 12 shows the use of
multiple MAX6501s. The MAX6501 has an open-drain
output, allowing multiple devices to be wire ORed to the
FULL ON input. This configuration allows fail-safe moni-
toring of multiple locations around the system.
Hot-Swap Application
Hot swapping of a fan can be detected using the circuit
in Figure 13 where GPIO2 is configured to generate an
alert whenever it is pulled low. As long as the fan card
is connected, GPIO2 is high. However, when the fan
card is removed, a 2.2kΩ resistor pulls GPIO2 low,
causing an interrupt. This signals to the system that a
hot swap is occurring.
Step-by-Step Part Selection
and Software Setup
Determining the Fan System Topology
The MAX6650/MAX6651 support three fan system
topologies. These are single fan control, parallel fan
control, and synchronized fan control.
Single Fan Control
The simplest configuration is a single MAX6650 for
each fan. If two or more fans are required per system,
then additional MAX6650 controllers are used (one per
fan). The advantage of this configuration is the ability to
Fan-Speed Regulators and Monitors
with SMBus/I
2
C-Compatible Interface
16 Maxim Integrated
MAX6650/MAX6651
