Datasheet

MTD6505
DS20002281B-page 12  2011-2013 Microchip Technology Inc.
4.7 Internal Voltage Regulator
V
BIAS
voltage is generated internally and is used to
supply internal logical blocks. The V
BIAS
pin is used to
connect an external decoupling capacitor (1 µF or
higher). Notice that this pin is for IC internal use, and is
not designed to supply DC current to external blocks.
4.8 Back Electromotive Force (BEMF)
Coefficient Setting
K
M
is the electro-mechanical coupling coefficient of the
motor (also referred to as “motor constant” or “BEMF
constant”). Depending on the conventions in use, the
exact definition of K
M
and its measurement criteria can
vary among motor manufacturers. To accommodate
various motor applications, the MTD6505 provides
options to facilitate diverse BEMF coefficients.
The MTD6505 defines BEMF coefficient (K
M
) as the
peak value of the phase-to-phase BEMF voltage,
normalized to the electrical speed of the motor. The
following table offers methods to set the K
M
value for
the MTD6505 device.
The R
PROG
sensing is actually a sequence that is
controlled by the firmware. For any given R
PROG
, the
internal control block will output the corresponding K
M
range.
4.9 Defining 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 RPROG
(see Table 4-2). An incorrect K
M
selection can create
issues, or reduce the efficiency.
4.9.1 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 4-1: 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.
TABLE 4-2: Km SETTINGS
K
M
Option
K
M
(mV/Hz) Range
Phase-to-Phase
R
PROG
Min Max
K
M0
3.25 6.5 GND
K
M1
6.5 13 24 kΩ
K
M2
13 26 3.9 kΩ
K
M3
26 52 V
BIAS
Note: This is a theoretical procedure that does
not take care of the harmonics generated
by the BEMF. This information has to be
taken for indication only.
K
M
V
pp–
2f
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