User manual

MTD6505 3-Phase BLDC Sensorless Fan Controller Demo Board User’s Guide
DS52009A-page 18 © 2011 Microchip Technology Inc.
2.3 HOW TO DEFINE THE CORRECT R
PROG
VALUE
This section explains how to define the correct K
M
value for a specific fan. The K
M
is
linked to the R
PROG
(see Ta bl e 2- 1). An incorrect K
M
selection can create issues, or
reduce the efficiency.
2.3.1 Operation
Follow the next steps to define the right R
PROG
value:
1. Apply a constant stream of air to a fan that is not connected.
2. Using an oscilloscope, measure the waveform between two phases, when the
fan is rotating.
3. Measure the generated peak-to-peak voltage (V
p-p
) value and the frequency (f).
4. Compute K
M
based on the measured V
p-p
and f (in mV/Hz):
EQUATION 2-1: K
M
COMPUTE
K
M
should be constant, for all fan rotation speeds, but for the K
M
measurement, the fan
rotation speed due to the air stream, should be close to the nominal fan rotation speed.
Ta bl e 2- 1 shows the corresponding K
M
for different R
PROG
values.
TABLE 2-1: R
PROG
VALUE
R
PROG
K
M
K
M
Range
(mV/Hz)
V
BIAS
(0Ω) 3 26 - 52
3.9 kΩ 2 13 - 26
24 kΩ 16.5 - 13
GND 0 3.25 - 6.5
K
m
V
pp–
2f
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