EMC2302 datasheet(15/70 Pages) MICROCHIP

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DS20006532A-page 15

EMC2301/2/3/5

4.3.1

ERROR WINDOW

The FSC has a controllable error window, allowing the

user to determine how close to the programmed RPM

the fan needs to come before the FSC algorithm stops

attempting to adjust fan speed (see Register 6-14).

This feature helps prevent “motor-boating” when con-

trolling difficult fans. The values for the error range is

shown in Table 4-4.

4.3.2

PROGRAMMING THE RPM-BASED

FAN SPEED CONTROL ALGORITHM

The RPM-based Fan Speed Control algorithm is dis-

abled upon device power-up. The following registers

control the algorithm. The EMC2301/2/3/5 fan control

registers are preloaded with defaults that will work for a

wide variety of fans so only the TACH Target Register

is required to set a fan speed. The other fan control reg-

isters can be used to fine-tune the algorithm behavior

based on application requirements.

Note that steps 1 to 6 below are optional and need only

be performed if the default settings do not provide the

desired fan response.

Set the Spin Up Configuration Register to the

spin up level and spin time desired.

Set the Fan Step Register to the desired step

size.

Set the Fan Minimum Drive Register to the min-

imum drive value that will maintain fan

operation.

Set the Update Time and Edges options in the

Fan Configuration Register.

Set the Valid TACH Count Register to the high-

est tach count that indicates the fan is spinning.

Refer to AN17.4 RPM to TACH Counts Conver-

sion for examples and tables for supported RPM

ranges (500, 1k, 2k, 4k).

Set the TACH Target Register to the desired

tachometer count.

Enable the RPM-based Fan Speed Control

algorithm by setting the ENAG bit.

4.3.3

ADVANCED TOPICS USING THE

FAN SPEED CONTROL ALGORITHM

In addition to the seven steps listed in this section,

there are several other optional settings that may be

used when attempting to control a fan that the previous

steps failed to do.

4.3.3.1

Selecting the Derivative Options

The DPT bits in Register 6-14 allow the user to select

the derivative response applied to the closed loop cal-

culation. Table 4-5 shows the options available. A brief

description is included in the table.

4.3.3.2

Setting the PID Gains

The Gain registers store the gain terms used by the

proportional and integral portions of each of the RPM-

based Fan Speed Control algorithms (see Register 6-

15). These gain terms are used as the KD, KI, and KP

gain terms in a classic PID control solution. The effec-

tive gains are shown in Table 4-6.

4.3.3.3

Fan Drive Max Step

The Fan Max Step registers (see Register 6-17), along

with the Update Time in the Fan Configuration 1 Regis-

ter, control the ramp rate of the fan driver response cal-

culated by the RPM-based Fan Speed Control

algorithm. The value of the register represents the

maximum step size each fan driver will take between

update times.

800 ms

1200 ms

1600 ms

TABLE 4-4:

ERROR RANGE OPTIONS

ERGX[1:0]

Operation

0 RPM (default)

50 RPM

100 RPM

200 RPM

TABLE 4-3:

UPDATE TIME (CONTINUED)

UPDATEX[2:0]

Update

Time

TABLE 4-5:

DERIVATIVE OPTIONS

DPTX[1:0]

Name

Operation

NONE No derivative options used.

BASIC The derivative of the current

error, along with the Propor-

tional and Integral values, are

added to the iterative Fan Drive

STEP Same as the BASIC option, but

the result is not limited by the

MAX STEP SIZE register.

BOTH Uses BASIC and STEP,

effectively providing a gain of 2.

TABLE 4-6:

GAIN DECODE

GAIND or GAINP or GAINI[1:0] Respective Gain

Factor

4x (default)

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