A8904SLB datasheet(13/18 Pages) ALLEGRO

13 / 18 page

8904

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

www.allegromicro.com

the write cycle, the CHIP SELECT goes high, the serial port is

disabled and no more data can be transferred. In addition, the

data written to the serial port is latched and becomes active.

If a word of less than 29 bits is sent, the unused most

significant bits that are not programmed, are reset to zero. There

are no compatibility issues when using the A8904 in an existing

A8902-A application as the five MSBs are reset to zero, which

is the default condition for A8902-A operation. The only

consideration when using the A8904 in an A8902-A application

is to ensure the minimum CHIP SELECT high time is at least

500 ns.

D0 - Sleep/Run Mode; LOW = Sleep, HIGH = Run. This

bit allows the device to be powered down when not in use.

D1 - Step Mode; LOW = Normal Operation, HIGH = Step

Only. When in the step-only mode the back-EMF commutation

circuitry is disabled and the start-up oscillator commutates the

power outputs. This mode is intended for device and system

testing.

D2 - Brake; LOW = Run, HIGH = Brake.

D3, D4, and D28 - The output current limit is set by D3 &

D4; D28 sets the transconductance gain.

Current limit Transconductance

D28

(typical)

gain

1.2 A

500 mA/V

1.0 A

500 mA/V

600 mA

500 mA/V

250 mA

500 mA/V

600 mA

250 mA/V

500 mA

250 mA/V

300 mA

250 mA/V

125 mA

250 mA/V

D5 to D18 - 14-bit word, active low. Programs the count

number to produce the corresponding REF signal, which

indicates the desired motor speed.

Bit number

Count number

128

256

D10

512

D11

1,024

D12

2,048

D13

4,096

D14

8,192

D15

16,384

D16

32,768

D17

65,536

D18

131,072

D19 - Speed control mode; LOW = internal, once-around

speed signal, HIGH = external sector data.

D20 and D21 - Programs the number of motor poles for the

once-around FCOM counter.

D20

D21

Motor poles

D22 and D23 - Controls the multiplexer for DATA OUT.

See DATA OUT Section for status definitions.

D22

D23

DATA OUT

TACH (once around or sector) signal

Thermal shutdown

SYNC signal

FCOM signal

D24 - Speed Reference. LOW = Internal, using back-EMF

technique, HIGH = External (internal control disabled).

D25 - Direction. LOW = Forward, HIGH = Reverse.

D26 and D27 - Programs the charging current for the

watchdog capacitor. This function is used for adjusting the

blanking duration and also the watchdog commutation period.

D26

D27

Watchdog charge current (typical)

-10 µA

-20 µA

-30 µA

-40 µA

Functional Description (cont’d)

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