MAX6651EEE datasheet(22/25 Pages) MAXIM

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We will start with KSCALE = 2 (to increase stability, a 4

could be tried, or to improve response time, a 1 could

be tried).

Second, calculate the appropriate value for the Speed

• Set FanSpeed = 25RPS (1500PRM/60).

• Solving for equation 2 gives KTACH = 78 for KSCALE

= 2, KTACH = 39 for KSCALE = 1, or KTACH = 158 for

K = 4.

Determining the Tachometer Count Time

To monitor the fan speed using the SMBus/I2C, the next

step is to determine the tachometer count time. In sys-

tems running in open-loop mode, this is necessary. In

closed-loop or full-speed mode, reading the tachome-

ter can serve as a valuable check to ensure the fan and

the control loop are operating properly.

The MAX6650/MAX6651 use an 8-bit counter to count

the tachometer pulses. This means the device can

count from 0 to 255 tachometer pulses before overflow-

ing. The MAX6650/MAX6651 can accommodate a large

range of fan speeds by allowing the counting interval to

be programmed. Smaller/faster fans should use smaller

count times. Although larger fans could also use small-

er count times, resolution would suffer. Choose the

slowest count time that will not overflow under worst-

case conditions. Fans are mechanical devices, and

their speeds are subject to large tolerance variations. If

an overflow does occur, the counter will read 255. The

MAX6650/MAX6651 can be configured to generate an

alert if an overflow is encountered (see

Setting Up

Alarms). Note that the prescaler value has no effect on

the TACH0 register.

Enter the appropriate count-time value in the tachometer

count-time register.

Example:

Assume a 12V fan rated at 2000 RPM.

To accommodate large tolerance variations, choose a

count time appropriate for a maximum speed of

3000RPM; 3000RPM is 50RPS and generates a 100Hz

(2 pulses/revolution) tachometer signal. Table 9 indi-

cates a count time of 2s will optimize resolution. With a

2s count time, speeds as fast as 3825RPM can be

monitored without overflow. The minimum resolution will

be 15RPM or 0.75% of the rated speed of 2000RPM.

Setting Up the GPIOs

To increase versatility, the MAX6650/MAX6651 have

two and five general-purpose digital inputs/outputs,

respectively. These GPIOs can be configured through

the SMBus/I2C.

Digital Out Low

All GPIOs can be configured to output a logic-level low.

The MAX6650/MAX6651 are designed to sink up to

10mA. This high sink current can be especially useful

for driving LEDs.

On the MAX6651, for GPIO3 and GPIO4, write a zero to

the appropriate location in the GPIO definition register.

For GPIO0, GPIO1, and (MAX6551 only) GPIO2, write a

10 to the appropriate location in the GPIO definition

Digital Out High

All GPIOs can be configured to generate a logic-level

high. An output high is generated using an open-drain

output stage with an internal pullup resistor of nominally

100k

Ω. The MAX6650/MAX6651 power-up default state

is with all GPIOs configured as output highs.

On the MAX6651, for GPIO3 and GPIO4, write a 1 to

the appropriate location in the GPIO definition register.

For GPIO0, GPIO1, and (MAX6551 only) GPIO2, write

an 11 to the appropriate location in the GPIO definition

Digital Input

Since a logic-level high output is open drain with an

internal pullup, an external device can actively pull this

pin low. The MAX6650/MAX6651 allow the user to read

the GPIO value through the GPIO status register.

• Configure the GPIO as an output logic level high (see

above).

• Read the state of the GPIO by reading the GPIO sta-

tus register.

Alert Output

GPIO0 can also serve as an ALERT output. The ALERT

output is designed to drive an interrupt on a µC. The

ALERT output goes low whenever an enabled alarm

condition occurs (see

Setting Up Alarms).

Configure GPIO0 as an ALERT output by writing a 01 to

bits 1 and 0 of the GPIO definition register.

Full-On Input

GPIO1 can also be configured as a full-on input. When

the full-on pin is pulled low, the MAX6650/MAX6651

apply the full available voltage across the fan. This hap-

pens independently of the software mode of operation.

This is a particularly valuable feature in high-reliability

systems, designed to prevent software malfunctions

from causing system overheating.

Configure GPIO1 as a full-on input by writing a 01 to

bits 3 and 2 of the GPIO definition register.

Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

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