Microchip MEC1428 Handleiding

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 2017 Microchip Technology Inc. DS00002137B-page 1
INTRODUCTION
Pulse Width Modulation (PWM) modules, which produce basically digital waveforms, can be used in many applications
as a more efficient and lower power alternative to the traditional 8 bit Digital-to-Analog (D/A) converters using only two
inexpensive filter components. A wide variety of microcontroller applications that exist today are better served by utiliz-
ing a PWM generated analog output as opposed to an 8 bit D/A converter. One application where using a high resolution
PWM has value vs. a traditional 8 Bit DAC is in driving the 3-wire voltage controlled fan in a PC.
Audience
This document is written for a reader that is familiar with hardware design. The goal of this application note is to provide
information regarding how to use a PWM to drive 3-wire fans that provides system benefits and an enhanced customer
experience.
References
The following documents should be referenced when using this application note. Please contact your Microchip repre-
sentative for availability.
• Microchip MEC140x/MEC141x/MEC142x Data Sheet
• Microchip MEC140x/MEC141x/MEC142x EVBs
AN2137
PWM Controlled Fan for MEC140x/MEC141x/MEC142x
Author: Tom Tse / Greg VanWesten

Microchip Technology Inc.
AN2137
DS00002137B-page 2  2017 Microchip Technology Inc.
1.0 OVERVIEW
Key potential benefits of using a high resolution PWM generated analog output for 3 wire fan control include:
• Finer RPM resolution (4x) control vs a traditional 8 bit DAC.
• Finer RPM control can potentially solve the issue of having two consumer platforms at the same thermal condition
each running at a different RPM Zoned speed – since the finer control would allow you to control RPM closer to
mechanical resonant points of the box and thus significantly reduce the chance of one machine running at differ-
ent binned zone speeds).
The purpose of this white paper is to show the benefits of using the MEC140x/MEC141x/MEC142x 16 bit PWM vs a
traditional 8 bit DAC to control 3 wire fans. (see typical schematic of a 3 wire fan drive in Figure 1-1 by DAC and Figure 1-
2 by PWM).
Note: A typical PC has up to 10 bands (zones) of RPMs that they run the fan in – stepping around mechanical
resonant frequencies of the platform.
FIGURE 1-1: 3-WIRE FAN DRIVE BY DAC
MEC140x/
MEC141x/
MEC142x
Fan
Driver
(e.g.
APL5606)
VFSM
VIN
VSET
DAC
+5V
+3.3V
TACH
2.2uF
VOUT = 1.6 x VSET
2.2uF 1000pF
Fan Connector
10k
0.01uF
3-Wire Fan
Fan Volt
TACH
GND
 2017 Microchip Technology Inc. DS00002137B-page 3
AN2137
FIGURE 1-2: 3-WIRE FAN DRIVE BY PWM WITH R/C FILTER
MEC140x/
MEC141x/
MEC142x
Fan
Driver
(e.g.
APL5606)
VFSM
VIN
VSET
PWM
(1KHz)
TACH
10k
10uF
2.2uF
VOUT = 1.6 x VSET
2.2uF 1000pF
0.01uF
3-Wire Fan
Fan Volt
TACH
GND
+5V
Fan Connector
+3.3V
10k
AN2137
DS00002137B-page 6  2017 Microchip Technology Inc.
3.0 THEORY AND MATH OF CONVERTING THE PWM TO ANALOG VOLTAGE
Use a simple in expensive R/C Low pass filter: R=10K and C= 10uF cap, sets the pole at ~1.6Hz to filter our 1Khz PWM
output.
With a fan PWM is 1Khz, the ripple is down ~60db (factor of 1000) - see bode plot below in Figure 3-2.
Calculated error with this filter introduced would be ~ 3V /1000 = 3mv ripple on the filtered PWM value (essentially 1 bit
of resolution on 10 bit PWM – which is in noise level for the PC board environment).
FIGURE 3-1: TYPICAL IN-EXPENSIVE R/C LOW PASS FILTER
FIGURE 3-2: BODE PLOT FOR FREQUENCY ANALYSIS
Note: This 3mV is input voltage ripple that is insignificant (i.e. factor of 20 less) compared to the error associated
with the Fan driver chip itself, which shows for a fixed 1V Vset input the output voltage could vary almost
100mV. See Fan driver spec: as shown in Figure 3-3.
FIGURE 3-3: TYPICAL FAN DRIVER SPEC


AN2137
DS00002137B-page 8  2017 Microchip Technology Inc.
3598 2.93 4.692 3667.482
3582 2.916 4.671 3636.364
3566 2.904 4.651 3577.818
3550 2.891 4.631 3571.429
3534 2.878 4.611 3544.005
3518 2.865 4.59 3548.196
3502 2.852 4.57 3496.503
3486 2.839 4.549 3470.214
3470 2.826 4.529 3470.214
3454 2.813 4.508 3418.803
3438 2.8 4.487 3401.361
3422 2.787 4.467 3384.095
3406 2.774 4.446 3407.155
3390 2.761 4.426 3357.583
3374 2.748 4.405 3363.229
3358 2.735 4.385 3346.347
3342 2.723 4.364 3302.146
3326 2.709 4.343 3273.322
3310 2.696 4.323 3264.418
3294 2.684 4.302 3253.796
3278 2.671 4.281 3237.992
3262 2.658 4.261 3243.243
3246 2.645 4.24 3231.018
3230 2.632 4.219 3188.098
3214 2.619 4.199 3224.073
3198 2.606 4.178 3169.572
3182 2.593 4.157 3157.895
3166 2.58 4.137 3169.572
3150 2.567 4.116 3103.983
3134 2.554 4.096 3131.524
3118 2.541 4.075 3089.598
3102 2.528 4.054 3078.502
TABLE 3-1: DAC GENERATED ANALOG OUTPUT (CONTINUED)
DAC
8-bit
(Steps)
VSET Voltage
VOUT (Voltage to Fan)
VOUT = 1.6 VSET +
Inaccuracies of Fan Driver
RPM

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: MEC1428

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