MAX6620ATI+ Maxim Integrated Products, MAX6620ATI+ Datasheet - Page 25

• Manufacturer Part Number: MAX6620ATI+
• Description: IC QUAD LIN FAN-SPD CTRLR 28TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX6620ATI+

• Applications: Fan Controller
• Number Of Outputs: 4
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN Exposed Pad
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Load: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Match external pass transistors to the fans being used.
Ensure that the pass transistor is capable of handling
the maximum fan current. For best results, the pass
transistor’s maximum current rating should be at least
50% greater than the fan’s nominal supply current.
The transistor should also be capable of dissipating the
worst-case power, which usually occurs when the fan is
being driven to approximately 50% of the nominal sup-
ply voltage. The maximum power dissipation will
depend on the thermal resistance of the transistor, its
case, and the printed-circuit board (PCB) to which it is
soldered. For example, if the worst-case transistor
power dissipation occurs when the fan current is
100mA, and the voltage across the fan is 6.5V, the
maximum power dissipation will be 650mW. A
BCP69T1-D in a SOT223-4 package is rated at 1.5W at
25°C (about 1W at 70°C) when soldered to a 0.93in
(6cm
power dissipation. Larger copper pads, packages with
lower thermal resistance, or different transistors can
give significantly different results.
The MAX6620 uses an advanced output driver design
that eliminates the large external capacitors often con-
nected across the fan’s power-supply terminals. For
stability with a variety of fans, connect a 0.1µF capaci-
tor from DACFB_ to ground.
Voltage regulators can be used instead of discrete tran-
sistors to drive the fans (Figure 7). The voltage feed-
back loop is closed around the regulator to provide the
desired output voltage. When using a voltage regulator,
note the following:
• Most regulators require relatively large capacitors at
• Most regulators have a lower output voltage limit that
their inputs and outputs for stability.
is >0V. If removing the drive from the fan is neces-
sary when using a regulator, choose a regulator that
has an on/off control input and drive that input from
the system microcontroller.
Using a Low-Dropout Voltage Regulator
2
) copper PCB pad, and can easily handle this
Applications Information
______________________________________________________________________________________
External Pass Transistors
(LDO) as the Pass Device
Quad Linear Fan-Speed Controller
2
The MAX6620 has two main modes for controlling fan
speeds. In DAC mode, the MAX6620 produces an out-
put voltage that drives the fan. This voltage is propor-
tional to the main fan power-supply voltage (V
Write the 9-bit desired voltage value in the Fan_ Target
Drive Voltage register.
In RPM mode, the MAX6620 monitors the tachometer
signals from the fans through the TACH_ inputs and
adjusts the drive voltage to yield the desire tachometer
count. The tachometer count is the number of internal
8192 clock cycles that are counted during the selected
number of tachometer pulses.
In DAC mode, the MAX6620 sets the fan’s supply volt-
age to the value selected in the Fan_ Target Drive
Voltage register. Tachometer monitoring is never done
when controlling a 2-wire fan, so the TACH input enable
bit in the Fan_ Configuration register should be set to 0.
Enabling the TACH input when using a 2-wire fan will
result in an erroneous fan failure detection.
Initial Settings:
• Begin with the POR settings. The POR value of the
Starting the Fan:
• Write the desired drive voltage value to the Fan_
Changing Speeds:
• Write the new desired drive voltage value to the Fan_
Stopping the Fan:
• Write a voltage value of 0 to the Fan_ Target Drive
In DAC mode, the MAX6620 sets the fan’s supply volt-
age to the value selected in the Fan_ Target Drive
Voltage register. 3-wire fans with tachometer outputs
allow monitoring of the fan’s speed to detect fan failure.
To monitor a fan’s speed, the TACH input should be
enabled.
fan_ DAC rate-of-change bits (4:2 of the Fan_
Dynamics Register) can yield slower fan speed
changes than desired. If this is the case, choose a
faster value, such as 001.
Target Drive Voltage register.
Target Drive Voltage register.
Voltage register.
Fan-Speed Control (DAC and RPM Modes)
Controlling 2-Wire Fans (DAC Mode)
Controlling 3-Wire Fans (DAC Mode)
FAN
25
).