User guide

on the amount of time between status read operations.
In these circumstances, it is best not to rely on the sta-
tus bits to indicate reversals in long-term temperature
changes. Instead, use a current temperature reading to
establish the trend direction.
Manufacturer and Device ID Codes
Two ROM registers provide manufacturer and device
ID codes. Reading the manufacturer ID returns 4D,
which is the ASCII code M (for Maxim). Reading the
device ID returns 09h, indicating the MAX6661 device.
POR and UVLO
The MAX6661 has a volatile memory. To prevent unreli-
able power-supply conditions from corrupting the data
in memory and causing erratic behavior, a POR voltage
detector monitors V
CC
and clears the memory if V
CC
falls below 1.91V (see Electrical Characteristics). When
power is first applied and V
CC
rises above 2.0V (typ),
the logic blocks begin operating, although reads and
writes at V
CC
levels below 3.0V are not recommended.
A second V
CC
comparator, the ADC UVLO comparator
prevents the ADC from converting until there is suffi-
cient headroom (V
CC
= 2.89V typ).
The software POR (SPOR) command can force a
power-on reset of the MAX6661 registers through the
serial interface. This can be done by writing F8h to the
MAX6661.
Power-up defaults include:
• Interrupt latch is cleared.
• ADC begins autoconverting.
• Command register is set to 00h to facilitate quick-
internal Receive Byte queries.
• T
HIGH
and T
LOW
registers are set to +127°C and
-55°C, respectively.
• T
HYST
and T
MAX
are set to +95°C and +100°C,
respectively.
Fan Control
The fan-control function can be divided into the thermal
loop, the fan-speed-regulation loop (fan loop), and the
fan-failure sensor. The thermal loop sets the desired fan
speed based on temperature while the fan-speed-regu-
lation loop uses an internally divided down reference
oscillator to regulate the fan speed. The fan-speed-reg-
ulation loop includes the fan driver and the tachometer
sensor. The fan-failure sensor provides a FAN FAIL
alarm that signals when the value in the fan tachometer
period register is greater than the fan tachometer peri-
od limit register value, which corresponds to a fan
going slower than the limit. The fan driver is an N-chan-
nel, 4Ω MOSFET with a 13.5V maximum V
DS
whose
drain terminal connects to the low side of the fan. The
tachometer sensor (TACH IN) of the MAX6661 is driven
from the tachometer output of the fan and provides the
feedback signal to the fan-speed regulation loop for
controlling the fan speed. For fans without tachometer
outputs, the MAX6661 can generate its own tachometer
pulses by monitoring the commutating current pulses
(see the Commutating Current Pulses section).
Thermal Loop
Thermal Closed Loop
The MAX6661 can be operated in a complete closed-
loop mode, with both the thermal and fan loops closed,
where the remote-diode sensor temperature directly
controls fan speed. Setting bit 3 of the configuration
register to zero places the MAX6661 in thermal closed
loop (Figure 6). The remote-diode temperature sensor
is updated every 500ms. The value is stored in a tem-
porary register (TEMPDATA) and compared to the pro-
grammed temperature values in the T
HIGH
, T
LOW
,
T
HYST
, T
MAX
, and T
FAN
registers to produce the error
outputs OVERT and ALERT.
The fan conversion rate (FCR) register (Table 6) can be
programmed to update the TEMPDATA register every
0.5s to 32s. This enables control over timing of the ther-
mal feedback loop to optimize stability.
The fan threshold (TFAN) register value is subtracted
from the UPDATE register value. If UPDATE exceeds
T
FAN
temperature, then the fan-speed control (FSC)
register (Table 7), stores the excess temperature in the
form of a 7-bit word with an LSB of 0.5°C. If the differ-
ence between the T
FAN
and UPDATE registers is high-
er than 32°C, then bits 6-0 are set to 1. In thermal
closed loop, the FSC register is READ ONLY.
The fan gain (FG) register (Table 8) determines the
number of bits used in the fan-speed control register.
This gain can be set to 4, 5, or 6. If bits 6 and 5 are set
to 10, all 6 bits of TEMPDATA are used directly to pro-
gram the speed of the fan so that the thermal loop has
a control range of 32°C with 64 temperature steps from
fan off to full fan speed. If bits 6 and 5 are set to 01, the
thermal control loop has a control range of 16°C with 32
temperature steps from fan off to full fan speed. If bits 6
and 5 are set to 00, the thermal control loop has a con-
trol range of 8°C with 16 temperature steps from fan off
to full fan speed.
Thermal Open Loop
Setting bit 3 of the configuration register (Table 4) to 1
places the MAX6661 in thermal open loop. In thermal
open-loop mode, the FSC register is read/write.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
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