MAX6651EEE datasheet(12/25 Pages) MAXIM

12 / 25 page

Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

Table 8. Alarm Status Register Bit Assignments

7 (MSB) to 5

GPIO1

—

BIT

GPIO2 (MAX6651 only)

NAME

MIN

0 (LSB)

POR

(DEFAULT)

STATE

MAX

Always 0

Minimum Output Level Alarm

GPIO1 Alarm. Set when GPIO1 is low.

GPIO2 Alarm. Set when GPIO2 is low (MAX6651 only).

FUNCTION

Maximum Output Level Alarm

Tachometer Overflow Alarm

TACH

The first 6 bits of the Tachometer Count-Time Register

are always zero, and the last 2 bits set the count time

(Table 9). The count time may be determined from the

following equation:

tCOUNT = 0.25s x 2KCOUNT

where KCOUNT is the numerical value of the two 2LSBs.

The 0.25 factor has a ±10% uncertainty.

Upon power-up, the Tachometer Count Registers reset

to 00h and the Tachometer Count-Time Register sets a

1s integration time.

Digital-to-Analog Converter

When using the open-loop mode of operation, the DAC

internal operational amplifier compares the feedback

voltage (VFB) with the reference voltage set by the 8-bit

DAC, and adjusts the output voltage (VOUT) until the

two input voltages are equal. The voltage at the FB pin

may be determined by the following equation:

VFB = (10 x VREF x KDAC) / 256

and the voltage across the fan is:

where KDAC is the numerical value of the DAC Register

and VREF = 1.5V. The minimum feedback voltage is

limited by the voltage drop across the external MOS-

FET (RON x IFAN), and the maximum voltage is limited

by the fan’s supply voltage (VFAN). For linear opera-

FAN

DAC

REF

–

256

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

Table 9. Tachometer Count-Time Register

(Assumes two pulses per revolution)

1 = Alarm condition

VALUE

(KCOUNT)

COUNT

TIME

(s)

MAXIMUM

FAN SPEED

(RPS)

MINIMUM

RESOLUTION

(Hz/COUNT)

0000 0000

0.25

512

256

0.5

0000 0001

0000 0010

1.0

128

0.5

0.25

2.0

0000 0011

Table 7. Alarm-Enable Register Bit Masks

7 (MSB) to 5

GPIO1

—

BIT

GPIO2 (MAX6651 only)

NAME

MIN

0 (LSB)

POR

(DEFAULT)

STATE

MAX

TACH

Always 0

Minimum Output Level Alarm Enable/Disable

GPIO1 Alarm Enable/Disable

GPIO2 Alarm Enable/Disable (MAX6651 only)

FUNCTION

Maximum Output Level Alarm Enable/Disable

Tachometer Overflow Alarm Enable/Disable

1 = Enabled

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