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 V
CC
and
clears the memory if V
CC
falls below 1.6V. When power
is first applied and V
CC
rises above 1.6V, the logic
blocks begin operating (though reads and writes at
V
CC
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 (R
DS(ON)
). Gate-to-source conduction
threshold must be compatible with available V
CC
. 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
(V
FAN
). Choose a MOSFET with a maximum continuous
drain current rating higher than the maximum fan cur-
rent. R
DS(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 = (V
FAN X
I
FAN(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 (V
TACH_
) and feedback
voltage (V
FB
) cannot exceed 13.2V (see
Absolute
Maximum Ratings
). When using a fan powered by a
13.2V or greater supply (V
FAN
), protect these inputs
from overvoltage conditions with series resistors. The
resistance required to protect these pins may be calcu-
lated from the following equation:
R
PROTECT
= [(V
FAN(MAX)
- 13.2V) x R
IN
] / 13.2V
where V
FAN(MAX)
is the worst-case maximum supply
voltage used to power the fan and R
IN
is the input
Fan-Speed Regulators and Monitors
with SMBus/I
2
C-Compatible Interface
Maxim Integrated 13
MAX6650/MAX6651
