Rainbow Electronics MAX6660 - Manual (Page 18)

MAX6660
Remote-Junction Temperature-Controlled
Fan-Speed Regulator with SMBus Interface
18 ______________________________________________________________________________________
REGISTER/
ADDRESS
FL (18h = READ, 1Ch = WRITE)
COMMAND READ LIMIT/FAILURE REGISTER
BIT
7
(MSB)
6 5 4 3 2 1 0
POR STATE 1 1 1 1 1 1 1 1
Table13. Fan Tachometer Count Limit (RFTCL/WFTCL)
Fan Conversion Rate Byte
The FCR register (Table 8) programs the fan’s update
time interval in free-running autonomous mode (
RUN/
STOP = 0). The conversion rate byte’s POR state is 02h
(0.25Hz). The MAX6660 uses only the 3LSBs of this
register. The 4MSBs are “don’t cares.” The update rate
tolerance is ±25% (max) at any rate setting.
Fan Closed Loop
In the thermal open loop but fan closed-loop mode, the
feedback loop can be broken and the temperature data
read directly. After performing external manipulations,
the result can be injected back into the fan control loop
by writing to the FSC register to control fan speed. Fan
closed-loop mode is selected by setting bit 0 of the FG
to zero.
Fan Open Loop
In fan control open-loop mode, selected by setting bit 0
of the FG register to 1, the gain block is bypassed and
the FSC register is used to program the fan voltage
rather than the fan speed. In the fan open-loop mode,
both the temperature feedback loop and fan-speed
control loop are broken, which results in the TACH IN
input becoming disabled. A direct voltage can be
applied after reading the temperature, using the FSC
register, to the fan that provides more flexibility in exter-
nal control algorithms. By selecting fan open-loop
mode, the MAX6660 automatically invokes thermal
open-loop mode.
Fan Driver
The fan driver consists of an amplifier and low-side
NMOS device whose drain is connected to FAN and is
the input from the low side of the fan. The FET has a
typical 4
Ω on-resistance with a typical 320mA maxi-
mum current limit. The driver has a thermal shutdown
sensor that senses the driver’s temperature. It shuts
down the driver if the temperature exceeds +150°C.
The driver is reactivated once the temperature has
dropped below +140°C.
TACH IN
The TACH IN input connects directly to the tachometer
output of a fan. Most commercially available fans have
two tachometer pulses per revolution. The tachometer
input is fully compatible with tachometer signals, which
are pulled up to V
VFAN .
Commutating Current Pulses
When a fan does not come equipped with a tachometer
output, the MAX6660 uses commutating generated cur-
rent pulses for speed detection. This mode is entered
by setting the FG register’s bit 1 to 1. An internal cur-
rent pulse is generated whenever a step increase
occurs in the fan current. Connecting an external resis-
tor between the GAIN pin and V
CC can reduce the sen-
sitivity of current pulses to changes in fan current. In
general, the lower the resistor value, then the lower the
sensitivity, and the fan is easier to turn ON and can use
a smaller external capacitor across its terminals. A suit-
able resistor range is 1k
Ω to 5kΩ.
Fan-Failure Detection
The MAX6660 detects fan failure by comparing the
value in the Fan Tachometer Count (FTC) register, a
READ ONLY register, with a limit stored in the Fan
Tachometer Count Limit (FTCL) register (Table 13). A
counter counts the number of on-chip oscillator pulses
between successive tachometer pulses and loads the
FTC register every time a tachometer pulse arrives. If
the value in FTC is greater than the value in FTCL, a
failure is indicated. In fan closed loop, a flag is activat-
ed when the fan is at full speed.
Set the Fan Tachometer Limit Byte to:
f L = 8415/[N ✕ f]
where N = fan fail ratio and f = frequency of fan
tachometer.
The factor N is less than 1 and produces a fan failure
indication when the fan should be running at full speed
but is only reaching a factor N of its expected frequen-
cy. The factor N is typically set to 0.75 for all fan
Note: The Fan Limit register is programmed with the maximum speed that is compared against the value in the FS register (Address
17) to produce an error output to the Status register.