Detailed Description
The MAX6641 temperature sensor and fan controller
accurately measures the temperature of its own die
and the temperature of a remote pn junction. The
device reports temperature values in digital form using
a 2-wire serial interface. The remote pn junction is typi-
cally the emitter-base junction of a common-collector
pnp on a CPU, FPGA, or ASIC. The MAX6641 operates
from supply voltages of 3.0V to 5.5V and consumes
500µA of supply current. The temperature data controls
a PWM output signal to adjust the speed of a cooling
fan. The device also features an over-temperature
alarm output to generate interrupts, throttle signals, or
shut down signals.
SMBus Digital Interface
From a software perspective, the MAX6641 appears as
a set of byte-wide registers that contain temperature
data, alarm threshold values, and control bits. A stan-
dard SMBus-compatible 2-wire serial interface is used
to read temperature data and write control bits and
alarm threshold data. These devices respond to the
same SMBus slave address for access to all functions.
The MAX6641 employs four standard SMBus protocols:
write byte, read byte, send byte, and receive byte
(Figures 1, 2, and 3). The shorter receive byte protocol
allows quicker transfers, provided that the correct data
register was previously selected by a read byte instruc-
tion. Use caution when using the shorter protocols in
multimaster systems, as a second master could over-
write the command byte without informing the first mas-
ter. The MAX6641 has four different slave addresses
available; therefore, a maximum of four MAX6641
devices can share the same bus.
Temperature data within the 0°C to +255°C range can
be read from the read external temperature register
(00h). Temperature data within the 0°C to +125°C range
can be read from the read internal temperature register
(01h). The temperature data format for these registers is
8 bits, with the LSB representing +1°C (Table 1) and the
MSB representing +128°C. The MSB is transmitted first.
All values below 0°C are clipped to 00h.
Table 1 details the register address and function,
whether they can be read or written to, and the power-on
reset (POR) state. See Tables 1–5 for all other register
functions and the Register Descriptions section. Figure 4
is the MAX6641 block diagram.
MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
_______________________________________________________________________________________ 5
PIN NAME FUNCTION
1, 6 I.C. Internally Connected. Must be connected to GND.
2DXN
Combined Remote-Diode Cathode Connection and A/D Negative Input. Connect the cathode of the
remote-diode-connected transistor to DXN.
3 DXP
Combined Remote-Diode Current Source and A/D Positive Input for Remote-Diode Channel. Connect
DXP to the anode of a remote-diode-connected temperature-sensing transistor. DO NOT LEAVE
DXP FLOATING; connect to DXN if no remote diode is used. Place a 2200pF capacitor between DXP
and DXN for noise filtering.
4 GND Ground
5 OT
Active-Low, Open-Drain, Over-Temperature Output. Use OT as an interrupt, a system shutdown
signal, or to control clock throttling. OT can be pulled up to 5.5V, regardless of the voltage on V
CC
.
OT is high impedance when V
CC
= 0.
7 SMBCLK
SMBus Serial-Clock Input. SMBCLK can be pulled up to 5.5V, regardless of V
CC
. Open drain.
SMBCLK is high impedance when V
CC
= 0.
8
