LM93 datasheet(39/95 Pages) TI1

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LM93

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SNAS210E – DECEMBER 2003 – REVISED MARCH 2013

3.16.10.6 Fan Control Priorities

The automatic fan control is not the only function that influences PWM duty cycle. There are several other

functions that influence the PWM duty cycle. All the functions can be classified into several categories:

SPACER

Category #

Category Name

PWM to 100% conditions

VRDx_HOT ramp-up/ramp-down

PROCHOT ramp-up/ramp-down function

Manual PWM Override

Fan Spin-Up Control

Automatic Fan Control Algorithm

The ultimate PWM duty cycle that is chosen can be described by the following formula:

If (Manual PWM Override is active)

PWM = max(1,2,3,4)

Else

PWM = max(1,2,3,5,6)

So in general, categories 1, 2 and 3 are always active. In addition to that, either category 4 or categories 5

and 6 are active depending on whether manual override is enabled. In this sense the manual override,

when enabled, replaces category 5 and 6.

3.16.10.7 PWM to 100% Conditions

There are several conditions that cause the duty cycles of all PWM outputs to immediately get set to

100%. They are:

1. Any of the four temperature zones has exceeded the programmed Fan Boost Limit setting but has not

yet cooled down enough to drop below the hysteresis point.

2. The OVRID bit is set in the LM93 Status/Control.

3.16.10.8 VRDx_HOT Ramp-Up/Ramp-Down

This function causes the duty cycle of the PWM outputs to gradually increase over time if VRD1_HOT or

VRD2_HOT are asserted.

When VRDx_HOT is asserted, the ramp function is enabled. The enabling process involves two steps:

1. The current duty cycle being requested by other PWM functions is memorized.

2. The ramp function immediately adds one PWM duty cycle step to the memorized value and requests

this duty cycle.

Once the function is enabled, it gradually adds additional duty cycle steps every X milliseconds whenever

VRDx_HOT is asserted (X is programmable via the PWM Ramp Control register). If VRDx_HOT remains

asserted for a long enough time, the duty cycle eventually reaches 100%.

Whenever VRDx_HOT is de-asserted, the ramp function begins to ramp down by subtracting one PWM

duty cycle step every X milliseconds. If VRDx_HOT is currently de-asserted, and the ramp function is less

than to the PWM duty cycle being requested by other functions, the ramp function is disabled.

As long as the function is enabled, it continues to ramp up or ramp down depending on the state of

VRDx_HOT. The ramp enabling process described above can only re-occur after the ramp function has

been disabled. Rapid assertion/de-assertion of VRDx_HOT does not trigger the enabling process unless

VRDx_HOT was de-asserted long enough for the ramp function to disable itself.

Functional Description

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