Datasheet

Data Sheet ADT7470
Rev. E | Page 21 of 40
Figure 19 shows a fan drive circuit using an NPN transistor
such as a general-purpose MMBT2222. While these devices
are inexpensive, they tend to have much lower current handling
capabilities and higher on resistance than MOSFETs. When
choosing a transistor, care should be taken to ensure that it
meets the fan’s current requirements. This is the only major
difference between a MOSFET and NPN transistor fan driver
circuit.
When using transistors, ensure that the base resistor is
chosen such that the transistor is fully saturated when the fan
is powered on. Otherwise, there are power inefficiencies in the
implementation.
12V
Q1
MMBT2222
PWM
3.3V
470Ω
12V
FAN
TACH/AIN
ADT7470
12V
10k
Ω
TACH
10kΩ
4.7k
Ω
1N4148
04684-0-023
Figure 19. Driving a 3-Wire Fan Using an NPN Transistor
Low Frequency
SETTING THE FAN DRIVE FREQUENCY
Configuration Register 2 Bits[6:4] configure the fan drive
frequency in both high and low frequency drive mode.
Table 16. Fan Drive Frequency
Register
0x74[6:4]
High Frequency Drive
(0x40[6] = 0)
Low Frequency Drive
(0x40[6] = 1)
000 1.4 kHz 11 Hz
001 22.5 kHz 14.7 Hz
010 22.5 kHz 22.1 Hz
011 22.5 kHz 29.4 Hz
100 22.5 kHz 35.3 Hz
101 22.5 kHz 44.1 Hz
110 22.5 kHz 58.8 Hz
111 22.5 kHz 88.2 Hz
INVERTED PWM OUTPUT
The PWM duty cycle can be inverted by writing to the PWM
Configuration registers. If the PWM duty cycle is inverted, then
a PWM duty cycle setting of 33% results in an output duty cycle
of 66%, as the PWM waveform is inverted.
Table 17. PWM1/PWM2 Configuration (Register 0x68)
Bit No. Mnemonic Description
5 INV1 0 = PWM1 duty cycle not inverted
(default).
1 = PWM1 duty cycle inverted.
4
INV2
0 = PWM2 duty cycle not inverted
(default).
1 = PWM2 duty cycle inverted.
Table 18. PWM3/PWM4 Configuration (Register 0x69)
Bit No. Mnemonic Description
5 INV3 0 = PWM3 duty cycle not inverted
(default).
1 = PWM3 duty cycle inverted.
4 INV4 0 = PWM4 duty cycle not inverted
(default).
1 = PWM4 duty cycle inverted.
FAN FULL SPEED FUNCTION
When Pin 13 is configured for full speed operation, pulling the
pin low will cause all fans to run at the maximum PWM duty
cycle. A Logic 1 is output on the PWM pins in this case.