Rainbow Electronics MAX1669 - Manual (Page 8)

MAX1669
Fan Controller and Remote Temperature Sensor
with SMBus Serial Interface
8 _______________________________________________________________________________________
The PWM or DAC fan control circuitry is completely
independent from the temperature measurement, and
software closes the temperature-control feedback loop
(Figure 2).
ADC and Multiplexer
The ADC is an averaging type that integrates over a
62ms period (typ), with excellent noise rejection. The
multiplexer automatically steers bias currents through
the remote diode, measures the forward voltage, and
calculates the temperature.
The DXN input is biased at 0.7V above ground by an
internal diode to set up the analog-to-digital (A/D)
inputs for a differential measurement. The worst-case
DXP-DXN differential input voltage range is 0.21V to
0.95V. Diode voltages that are outside the ADC input
range cause overrange indications rather than non-
monotonic readings. Overrange readings will return
+127°C. Excess resistance in series with the remote
diode causes approximately +1/2°C error/
Ω. Likewise,
200µV of offset voltage forced on DXP-DXN causes
approximately +1°C error.
A/D Conversion Sequence
When the device is taken out of standby mode, the
result of the measurement is available one conversion
time later (78ms max). If the ADC is busy, the results of
the previous conversion are always available. Toggling
the standby mode on and off is a good way to initiate a
new conversion since this action resets the rate timer.
Low-Power Standby Mode
Supply-current drain during the 62ms conversion peri-
od is 500µA. Between conversions, the instantaneous
supply current is 18µA. In standby mode, supply cur-
rent drops to 3µA and the fan output is disabled.
SMBus Digital Interface
From a software perspective, the MAX1669 appears as
a set of byte-wide registers that contain temperature
data, alarm threshold values, or control bits. A standard
SMBus 2-wire serial interface is used to read tempera-
ture data and write control bits and alarm threshold
data.
The MAX1669 employs four standard SMBus protocols:
write byte, read byte, send byte, and receive byte
(Figure 3). The two shorter protocols (receive and
send) allow quicker transfers, provided that the correct
data register was previously selected by a write or read
byte instruction. Use caution with the shorter protocols
in multimaster systems since a second master could
overwrite the command byte without informing the first
master.
The temperature data format is 7 bits plus sign in two’s
complement form for each channel, with the LSB repre-
senting +1°C (Table 1), MSB transmitted first.
Measurements are offset by +1/2°C to minimize internal
rounding errors; for example, +99.5°C to +100.4°C is
reported as +100°C.
Alarm Threshold Registers
Three registers store alarm threshold data, with high-
temperature (T
HIGH
) and low-temperature (T
LOW
) reg-
isters that activate the ALERT output, and a critical
overtemperature register (T
CRIT ) that activates the
OVERT output. If a measured temperature equals or
exceeds the T
HIGH or T LOW threshold value, an ALERT
interrupt is asserted. Do not set the T CRIT register to
values outside of the temperatures in Table 1.
The power-on-reset (POR) state of the T HIGH register is
full scale (0111 1111b or +127°C). The POR state of the
T
LOW register is 1100 1001b or -55°C. The POR state of
the T CRIT register is 0110 0100b or +100°C.
OVERT
Thermostat Output
The OVERT output is a self-clearing interrupt output
that is activated when the temperature equals or
exceeds T
CRIT . OVERT normally goes low when active,
but this polarity can be changed through the configura-
tion register. The latch is cleared when the temperature
reading is equal to or less than T
CRIT minus 5°C, which
provides for 5°C of hysteresis.
The ALERT and OVERT comparisons are made after
each conversion, and at the end of a write command to
their respective temperature limit registers. For exam-
ple, if the limit is changed while the device is in standby
OVERT
FANON
FAN
DAC
PWM
CONTROLLER
DRIVER
CONTROL
LOGIC
(0XF0)
UPPER NIBBLE
4
FREQ
REGISTER
MUX
AND
MAX1669
DUTY
REGISTER
Figure 2. MAX1669 Fan-Control Functional Diagram