MAX6651EEE datasheet(8/25 Pages) MAXIM

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Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

SMBCLK

SMBDATA

tSU:STA

tHD:STA

tLOW

tHIGH

tSU:DAT

tHD:DAT

tSU:STO tBUF

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO SLAVE

H = LSB OF DATA CLOCKED INTO SLAVE

I = SLAVE PULLS SMBDATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO MASTER

K = ACKNOWLEDGE CLOCK PULSE

L = STOP CONDITION, DATA EXECUTED BY SLAVE

M = NEW START CONDITION

SMBCLK

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

SMBDATA

tSU:STA tHD:STA

tLOW

tHIGH

tSU:DAT

tSU:STO

tBUF

E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO MASTER

H = LSB OF DATA CLOCKED INTO MASTER

I = ACKNOWLEDGE CLOCK PULSE

J = STOP CONDITION

K = NEW START CONDITION

Figure 3. SMBus Write Timing Diagram

Figure 4. SMBus Read Timing Diagram

Command-Byte Functions

The 8-bit Command-Byte Register (Table 2) is the mas-

ter index that points to the various other registers within

MAX6650/MAX6651. The register’s power-on reset

(POR) state is 0000 0000, so that a receive-byte trans-

mission (a protocol that lacks the command byte)

occurring immediately after POR returns the current

speed setting.

Fan-Speed Register

In closed-loop mode, the MAX6650/MAX6651 use the

Fan-Speed Register to set the period of the tachometer

signal that controls the fan speed. The Fan-Speed

MAX6650/MAX6651 regulate the fan speed by forcing

the tachometer period (tTACH) equal to the scaled reg-

ister value. One revolution of the fan generates two

tachometer pulses, so the required Fan-Speed Register

value (KTACH) may be calculated as:

tTACH = 1 / (2 x Fan Speed)

KTACH = [tTACH x KSCALE x (fCLK / 128)] - 1

where the fan speed is in rotations per second (RPS),

tTACH is the period of the tachometer signal, fCLK is the

internal oscillator frequency (254kHz ±10%), and

KSCALE is the prescaler value (see Configuration-Byte

to the tachometer period, the Fan-Speed Register value

(KTACH) does not linearly control the fan speed (Table

3). Select the prescaler value so the fan’s full speed is

achieved with a register value of approximately 64

(0100 0000) to optimize speed range and resolution.

The MAX6651 may be controlled by an external oscilla-

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