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 fans 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.
Linear Fan Supply Drive
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
fans 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 fans 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.
4-Wire Fans
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.
2-Channel Temperature Monitor with Dual,
Automatic, PWM Fan-Speed Controller
Maxim Integrated
17
MAX6639/MAX6639F
V
CC
PWM1
4.7k?/DIV>
4.7k?/DIV>
TACH1
3V TO 5.5V
3V TO 5.5V
TACH
OUTPUT
V
FAN
(5V OR 12V)
Figure 10. 4-Wire Fan with PWM Speed-Control Input
V
CC
PWM1
4.7k?/DIV>
4.7k?/DIV>
TACH1
3V TO 5.5V
TACH
OUTPUT
V
FAN
(12V OR 5V)
5V
Figure 8. High-Side PWM Drive with Keep-Alive Supply
V
CC
PWM1
TACH1
4.7k?/DIV>
3V TO 5V
4.7k?/DIV>
100k?/DIV>
9.1k?/DIV>
33k?/DIV>
100k?/DIV>
3.3V
2N3904
2.2糉
10糉
TACH
OUTPUT
V
FAN
(5V OR 12V)
TACH OUTPUT
Figure 9. High-Side Linear Drive Circuit
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相关PDF资料
MAX6641AUB92+
IC TEMP MONITOR SMBUS 10UMAX
MAX6642ATT94+T
IC TEMP SENSOR SMBUS 6TDFN
MAX6643LBFAEE+
IC CTLR PWM FAN-SPEED 16QSOP
MAX6649MUA/V+
IC SENSOR REMOTE SMBUS 8UMAX
MAX6652AUB+T
IC TEMP SENS/MON 10-UMAX
MAX6659MEE+T
IC TEMP SENSOR SMBUS 16-QSOP
MAX6661AEE+T
IC REG FAN SPEED 16-QSOP
MAX6664AEE+T
IC TEMP MON FAN CNTRL 16-QSOP
相关代理商/技术参数
MAX6639FATE+T
功能描述:板上安装温度传感器 2Ch Temperature Monitor RoHS:否 制造商:Omron Electronics 输出类型:Digital 配置: 准确性:+/- 1.5 C, +/- 3 C 温度阈值: 数字输出 - 总线接口:2-Wire, I2C, SMBus 电源电压-最大:5.5 V 电源电压-最小:4.5 V 最大工作温度:+ 50 C 最小工作温度:0 C 关闭: 安装风格: 封装 / 箱体: 设备功能:Temperature and Humidity Sensor
MAX6639YAEE+
功能描述:板上安装温度传感器 2Ch Temperature Monitor RoHS:否 制造商:Omron Electronics 输出类型:Digital 配置: 准确性:+/- 1.5 C, +/- 3 C 温度阈值: 数字输出 - 总线接口:2-Wire, I2C, SMBus 电源电压-最大:5.5 V 电源电压-最小:4.5 V 最大工作温度:+ 50 C 最小工作温度:0 C 关闭: 安装风格: 封装 / 箱体: 设备功能:Temperature and Humidity Sensor
MAX6639YAEE+T
功能描述:板上安装温度传感器 2Ch Temperature Monitor RoHS:否 制造商:Omron Electronics 输出类型:Digital 配置: 准确性:+/- 1.5 C, +/- 3 C 温度阈值: 数字输出 - 总线接口:2-Wire, I2C, SMBus 电源电压-最大:5.5 V 电源电压-最小:4.5 V 最大工作温度:+ 50 C 最小工作温度:0 C 关闭: 安装风格: 封装 / 箱体: 设备功能:Temperature and Humidity Sensor
MAX663C/D
功能描述:低压差稳压器 - LDO RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20
MAX663CJA
功能描述:低压差稳压器 - LDO RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20
MAX663CPA
功能描述:低压差稳压器 - LDO 5V Prog uPower Voltage Regulator RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20
MAX663CPA+
功能描述:低压差稳压器 - LDO 5V Prog uPower Voltage Regulator RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20
MAX663CSA
功能描述:低压差稳压器 - LDO 5V Prog uPower Voltage Regulator RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20