Pulse stretching, Figure 2. fan speed example – Rainbow Electronics MAX34441 User Manual

Page 24

PMBus 5-Channel Power-Supply Manager

and Intelligent Fan Controller

MAX34441

Figure 2. Fan Speed Example

One or all of the six available temperature sensors can
be used to control the fan speed. Each temperature
sensor has an offset adjustment that allows monitor-
ing specific temperature zones with different thermal
characteristics. In Figure 1, I

C temperature sensor 0 is

monitoring a zone that is 15NC more sensitive than the
zone that the I

C temperature sensor 3 is measuring.

To keep the audible noise and fan power consumption
as low as possible, the device allows each temperature
sensor to have a temperature offset added. This allows
temperature zones with different thermal profiles to con-
trol the fan at the lowest possible speed to maintain the
required temperature.
If no temperature sensors are assigned to control the fan,
the output fan PWM signal is ramped to 100% duty cycle.
In Figure 2, at temperature sample 1, the required fan
speed is at the level associated with temperature level
2 and since temperature sample 1 is above temperature
level 3, the fan PWM duty cycle needs to be increased
to increase the fan speed. The device increases the fan
PWM duty cycle at a rate controlled by the RAMP bits in
the MFR_FAN_CONFIG command code. If the PWM duty
cycle has not reached the target value before the tem-
perature sample detects that a new PWM target value is
needed, the device stops moving toward the old target

and starts moving to the new target according to the
programmed ramp rate.
At temperature sample 2, the temperature has increased
to greater than temperature level 4 so again the PWM
duty cycle must be increased. At temperature sample
3, the temperature has dropped but not below the ther-
mal hysteresis level (which is set by the HYS bits in the
MFR_FAN_CONFIG command code), so the fan speed
remains at level 4. At temperature sample 4, the tem-
perature has dropped below the hysteresis point so the
PWM duty cycle is decreased.

Pulse Stretching

In some 3-wire fan applications, when the supply power
is interrupted to control fan speed, the tachometer sig-
nal is not available. Some fan controllers periodically
stretch the PWM signal to allow the tachometer to be
accurately detected. Pulse stretching can create audible
noise. This device does not implement pulse stretch-
ing. Hence, the RPM fan modes that require a reliable
tachometer signal to be available at all times should
not be used in applications that switch power to the fan
to control the speed of the fan. For example, low-side
switching of a 3-wire fan should not use either of the RPM
fan modes.

SENSOR

TEMPERATURE

THERMAL

HYSTERESIS

ELAPSED TIME

TEMPERATURE SAMPLE RATE

(ONCE PER SECOND)

FAN SPEED

SAMPLE 1

SAMPLE 2

SAMPLE 3

SAMPLE 4

SAMPLED TEMPERATURES