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.

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 2.2k

Ω 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

16

______________________________________________________________________________________

MAX6650

GPIO1

MAX6650

GPIO1

MAX6650

GPIO1

FAN

1

FAN

2

FAN

3

MAX6650

GPIO1

FAN

4

GPIO0

GPIO0

GPIO0

GPIO0

ALERT

FULL ON

ALERT

ALERT

ALERT

FULL ON

FULL ON

FULL ON

TO INT PIN

ON NC

Figure 9. N + 1 Application