A8904SLB View Datasheet(PDF) - Allegro MicroSystems
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A8904SLB Datasheet PDF : 19 Pages
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8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
Functional Description (cont’d)
Speed error detection
Speed error signals
A speed error signal is created by integrating the differences
between the TACH and REF signal. If the TACH signal goes
low before the REF signal then an ERROR FAST is produced
and if the TACH signal goes low after the REF signal then an
ERROR SLOW is produced. The error signal generated enables
the appropriate current source (see diagram next page) to either
charge or discharge the filter components on the FILTER
terminal.
The FILTER voltage is then used to provide linear current
control in the windings via the transconductance stage (see
diagram next page). The output current is sensed through an
internal sense resistor, RS. The voltage across the sense resistor
is compared to the lowest of either one-tenth of the voltage at the
FILTER terminal, minus the filter threshold voltage, or to the
maximum current limit reference.
Alternatively, external control of the FILTER terminal can
be introduced by disabling the frequency-lock loop circuitry
(D24 = 1).
The transconductance function is defined as:
IOUT = (VFILTER – VFILTERTH) / (10 x RS x G)
where RS is nominally 200 mΩ,
VFILTERTH is approximately 1.85 V,
G = 1, when D28 = 0 and gain = 500 mA/V or
G = 2, when D28 = 1 and gain = 250 mA/V.
The closed loop control response of the overall system is
shaped via the filter components that are introduced at the
FILTER terminal.
Clamping the current to a level defined by the serial port
(D3 & D4) provides output current limit protection. This feature
is particularly useful where high transient currents are experi-
enced, e.g., during start-up. Once normal running conditions are
reached, the current limit can be appropriately reduced. Note
that the current limit is scaled according to the gm value selected.
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