MTD6505_14 Datasheet, PDF (12/22 Pages) Microchip Technology – 3-Phase Sinusoidal Sensorless Brushless Motor Driver

MTD6505
4.7 Overvoltage Shutdown
The MTD6505 device has an overvoltage protection
function which detects when the V_DD voltage exceeds
V_OV = +7.2V. When this temperature is reached, the
circuit enters Thermal Shutdown mode and outputs
OUT1, OUT2 and OUT3 are disabled
(high-impedance).
4.8 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.
4.9 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.
TABLE 4-2: K_M SETTINGS
K_M
Option
K_M (mV/Hz) Range
Phase-to-Phase
Min.
Max.
R_PROG
K_M0
3.25
K_M1
6.5
K_M2
13
K_M3
26
6.5
GND
13
24 k
26
3.9 k
52
V_BIAS
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.
4.10 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 R_PROG
(see Table 4-2). An incorrect K_M selection can create
issues or reduce efficiency.
4.10.1 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
KM
=
V----_{P----–----P-}
2f
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 nominal.
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.
DS20002281C-page 12
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