MAX6639ATE+ Maxim Integrated Products, MAX6639ATE+ Datasheet - Page 17

• Manufacturer Part Number: MAX6639ATE+
• Description: IC TEMP MONITOR 2CH 16-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX6639ATE+

• Function: Fan Control, Temp Monitor
• Topology: ADC, PWM Generator, Tach Counter
• Sensor Type: External & Internal
• Sensing Temperature: 0°C ~ 150°C, External Sensor
• Output Type: I²C™/SMBus™
• Output Alarm: Yes
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 3.6 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

the circuit in Figure 8 can be used to eliminate pulse
stretching while still allowing accurate tachometer feed-
back. The diode connects the fan to a low-voltage
power supply, which keeps the fan’s internal circuitry
powered even when the PWM drive is zero. Therefore,
the tachometer signal is always available and pulse
stretching can be turned off. Note that this approach
prevents the fan from turning completely off, so even
when the duty cycle is 0%, the fan may still spin.
Figure 8. High-Side PWM Drive with “Keep-Alive” Supply
Figure 9. High-Side Linear Drive Circuit
V
CC
TACH1
PWM1
2-Channel Temperature Monitor with Dual,
4.7kΩ
4.7kΩ
TACH OUTPUT
3V TO 5V
V
CC
3.3V
______________________________________________________________________________________
TACH1
PWM1
100kΩ
Automatic, PWM Fan-Speed Controller
4.7kΩ
4.7kΩ
3V TO 5.5V
OUTPUT
TACH
100kΩ
2.2μF
(12V OR 5V)
V
(5V OR 12V)
FAN
2N3904
91kΩ
33kΩ
V
FAN
OUTPUT
TACH
5V
10μF
While many fans are compatible with PWM power-supply
drive, some are excessively noisy with this approach.
When this is the case, a good alternative is to control the
fan’s power-supply voltage with a variable DC power-sup-
ply circuit. The circuit in Figure 10 accepts the PWM sig-
nal as an input, filters the PWM, and converts it to a DC
voltage that then drives the fan. To minimize the size of
the filter capacitor, use the highest available PWM fre-
quency. Pulse stretching is not necessary when using a
linear fan supply. Note that this approach is not as effi-
cient as PWM drive, as the fan’s power-supply current
flows through the MOSFET, which can have an apprecia-
ble voltage across it. The total power is still less than
that of a fan running at full speed. Table 11 is a summa-
ry of fan-drive options.
Some fans have an additional, fourth terminal that
accepts a logic-level PWM speed-control signal as
shown in Figure 10. These fans require no external
power circuitry and combine the low noise of linear
drive with the high efficiency of PWM power-supply
drive. Higher PWM frequencies are recommended
when using 4-wire fans.
Figure 10. 4-Wire Fan with PWM Speed-Control Input
V
CC
TACH1
PWM1
Linear Fan Supply Drive
4.7kΩ
4.7kΩ
3V TO 5.5V
3V TO 5.5V
OUTPUT
TACH
(5V OR 12V)
V
FAN
4-Wire Fans
17