MAX6651EEE datasheet(13/25 Pages) MAXIM

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tion, use DAC values between 08h and B0h (see

Typical Operating Characteristics). When using the

closed-loop mode of operation, the contents of the

DAC Register are ignored. When writing to the DAC,

wait at least 500µs before attempting to read back.

Power-on Reset (POR)

The MAX6650/MAX6651 have volatile memory. To pre-

vent ambiguous power-supply conditions from corrupt-

ing the data in the memory and causing erratic

behavior, a POR voltage detector monitors VCC and

clears the memory if VCC falls below 1.6V. When power

is first applied and VCC rises above 1.6V, the logic

blocks begin operating (though reads and writes at

VCC levels below 3V are not recommended).

Power-up defaults include the following:

• All alarms are disabled.

• Prescale divider is set to 4.

• Fan speed is set in full-on mode.

See Table 2 for the default states of all registers.

Applications Information

MOSFET and Bipolar Transistor

Selection

The MAX6650/MAX6651 drive an external N-channel

MOSFET that requires five important parameters for

proper selection: gate-to-source conduction threshold,

maximum gate-to-source voltage, drain-to-source

breakdown voltage, current rating, and drain-to-source

on-resistance (RDS(ON)). Gate-to-source conduction

threshold must be compatible with available VCC. The

maximum gate-to-source voltage and the drain-to-

source breakdown voltage rating should both be at

least a few volts higher than the fan supply voltage

(VFAN). Choose a MOSFET with a maximum continuous

drain current rating higher than the maximum fan cur-

rent. RDS(ON) should be as low as practical to maxi-

mize the feedback voltage range. Maximum power

dissipation in the power transistor can be approximat-

ed by P = (VFAN X IFAN(MAX)) / 4. Bipolar power transis-

tors are practical for driving small and midsize fans

(Figure 6). Very-high-current fans may require output

transistor base current greater than the MAX6650’s

50mA drive capability. Bipolar Darlington transistors

will work but have poor saturation characteristics and

could lose up to 2V to 3V of drive voltage.

Resistor Selection

The tachometer input voltages (VTACH_) and feedback

voltage (VFB) cannot exceed 13.2V (see Absolute

Maximum Ratings). When using a fan powered by a

13.2V or greater supply (VFAN), protect these inputs

from overvoltage conditions with series resistors. The

resistance required to protect these pins may be calcu-

lated from the following equation:

RPROTECT = [(VFAN(MAX) - 13.2V) x RIN] / 13.2V

where VFAN(MAX) is the worst-case maximum supply

voltage used to power the fan and RIN is the input

Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

MAX6650

VCC

SCL

10k

SDA

GPIO0

OUT

ADD

GND

TACH0

VCC

3V TO 5.5V

VFAN

5V OR 12V

CCOMP

μF

SMBus/I2C

INTERFACE

GPIO1

FAN

FULL ON

ALERT

Figure 6. Fan Control with a Bipolar Transistor

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