Absolute Maximum Ratings
Load Supply Voltage, VBB . . . . . . . . . . . . 15 V
Output Current1, IOUT . . . . . . . . . . . . . . . . . . . . . ±1.4 A
Peak
Output
Current
(Brake)2,
IOUT(BRK) . ±3.0 A
Period2 for IOUT(BRK) to fall from
±3.0 A to ±1.4 A . . . . . . . . . . . . . . . 800 ms
Logic Supply Voltage, VDD . . . . . . . . . . . 7.0 V
Logic Input Voltage Range, VIN
(continuous) . . . . . . -0.3 V to VDD + 0.3 V
(tw <30 ns) . . . . . . . -1.0 V to VDD + 1.0 V
Package Power Dissipation, PD. . See Graph
Operating Temperature, TA. . . -20°C to +85°C
Junction Temperature3, TJ. . . . . . . . . +150°C
Storage Temperature,TS. . . . -55°C to +150°C
1Output current rating may be restricted to a value
determined by system concerns and factors. These
include: system duty cycle and timing, ambient
temperature, and use of any heatsinking and/or forced
cooling. For reliable operation, the specified maximum
junction temperature should not be exceeded.
2Peak output current is a transient condition that
occurs during braking when the motor acts as a
generator. The 3 A level is based on the maximum
peak of a sine wave that is damped. The maximum
period between the initial brake being applied and the
current through the drivers falling to 1.4 A should not
exceed 800 ms. See Braking section for more
information.
3Fault conditions that produce excessive junction
temperature will activate device thermal shutdown
circuitry. These conditions can be tolerated, but should
be avoided.
Data Sheet
26301.5D
The A8904SLB and A8904SLP are three-phase brushless dc motor
controller/drivers designed for applications where accurate control of high-
speed motors is required. The three half-bridge outputs are low on-resistance
n-channel DMOS devices capable of driving up to 1.2 A. The A8904 provides
complete, reliable, self-contained back-EMF sensing, motor startup and
running algorithms. A programmable digital frequency-locked loop speed
control circuit together with the linear current control circuitry provides
precise motor speed regulation.
A serial port allows the user to program various features and modes of
operation, such as the speed control parameters, startup current limit, sleep
mode, direction, and diagnostic modes.
The A8904 is fabricated in Allegro’s BCD (Bipolar CMOS DMOS)
process, an advanced mixed-signal technology that combines bipolar, analog
and digital CMOS, and DMOS power devices. The A8904SLB is provided in
a 24-lead wide-body SOIC batwing package. The A8904SLP is provided in a
thin (<1.2 mm), 28-lead TSSOP package with an exposed thermal pad. Each
package type is available in a lead-free version (100% matte tin leadframe).
Features
Pin-for-pin replacement for A8902CLBA
Startup commutation circuitry
Sensorless commutation circuitry
Option of external sector data tachometer signal
Option of external speed control
Oscillator operation up to 20 MHz
Programmable overcurrent limit
Transconductance gain options: 500 mA/V or 250 mA/V
Programmable watchdog timer
Directional control
Serial Port Interface
TTL-compatible inputs
System diagnostics data out ported in real time
Dynamic braking through serial port or external terminal
3-PHASE BRUSHLESS DC MOTOR
CONTROLLER/DRIVER WITH BACK-EMF SENSING
8904
A8904SLB (SOIC)
*Pb-based variants are being phased out of the product line. The variants cited in this footnote
are in production but have been determined to be NOT FOR NEW DESIGN. This classification
indicates that sale of this device is currently restricted to existing customer applications. The
variants should not be purchased for new design applications because obsolescence in the
near future is probable. Samples are no longer available. Status change: May 1, 2006. These
variants include: A8904SLB, A8904SLBTR, A8904SLP, and A8904SLPTR.
Part Number Pb-f r ee* Package Packing
A8904SLB-T Yes 24-pin SOIC 31 per tube
A8904SLBTR- T Yes 24- pin SO IC 450 per r eel
A8904SLP-T Yes 28-pin TSSOP 50 per tube
A8904SLPTR- T Yes 28-pin TSSOP 4000 per r ee
l
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
2
Functional Block Diagram
(A8904SLB terminal numbers shown)
Copyright © 2003 Allegro MicroSystems, Inc.
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
3
A8904SLP (TSSOP)
* Measured on “High-K” multi-layer PWB per JEDEC Standard
JESD51-7.
† Measured on typical two-sided PWB with power tabs (LB
package) or thermal pad (LP package) connected to copper foil
with an area of three square inches (1935 mm2). See Applica-
tion Note 29501.5, Improving Batwing Power Dissipation, for
additional information.
LB (SOIC) Package LP (TSSOP) Package
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
4
ELECTRICAL CHARACTERISTICS at TA = +25°C, VDD = 5.0 V
Limits
Characteristic Symbol Test Conditions Min. Typ. Max. Units
Logic Supply Voltage VDD Operating 4.5 5.0 5.5 V
Logic Supply Current IDD Operating — 7.5 10 mA
Sleep mode — 250 500 µA
Undervoltage Threshold UVLO Decreasing VDD — 3.6 — V
Increasing VDD — 3.9 — V
Load Supply Voltage VBB Operating 4.0 — 14 V
Load Supply Current IBB Operating — 4.0 8.0 mA
Sleep mode — 20 30 µA
Thermal Shutdown TJ— 165 — °C
Thermal Shutdown Hysteresis ∆TJ—20—°C
Output Drivers
Output Leakage Current IDSX VBB = 14 V, VOUT = 14 V, sleep mode — 200 300 µA
VBB = 14 V, VOUT = 0 V — -2.0 -15 µA
Total Output ON Resistance rDS(on) IOUT = 600 mA — 1.0 1.4 Ω
(source + sink + RS)
Output Sustaining Voltage VDS(sus) VBB = 14 V, IOUT = IOUT(MAX), L = 3 mH 14 — — V
Clamp Diode Forward Voltage VFIF = 1.0 A — 1.25 1.5 V
Control Logic
Logic Input Voltage VIN(0) SECTOR DATA, RESET, CLK, — — 0.8 V
VIN(1) CHIP SELECT, OSC 2.0 — — V
Logic Input Current IIN(0) VIN = 0 V — — -0.5 µA
IIN(1) VIN = 5.0 V — — ±1.0 µA
BRAKE Threshold VBRK 1.5 1.75 2.0 V
BRAKE Hysteresis Current IBRKL VBRK = 750 mV — 4.0 — µA
BRAKE Current IBRK Brake set, D2 = 1, IBRK = 750 mV — 20 — µA
DATA Output Voltage VOUT(0) IOUT = 500 µA — — 1.5 V
VOUT(1) IOUT = -500 µA 3.5 — — V
CST Current ICST Charging -9.0 -10 -11 µA
Discharging, VCST = 2.5 V — 500 — µA
CST Threshold VCSTH High 2.25 2.5 2.75 V
VCSTL Low 0.85 1.0 1.15 V
Filter Current IFILTER Charging -9.0 -10 -11 µA
Discharging 9.0 10 11 µA
Leakage, VFILTER = 2.5 V — — ±5.0 nA
Filter Threshold VFILTERTH 1.57 1.85 2.13 V
CD Current ICD Charging -18 -20 -22 µA
(CD1 or CD2)Discharging 32 40 48 µA
CD Current Matching — ICD(DISCHRG)/ICD(CHRG) 1.8 2.0 2.2 —
CD Threshold VCDTH 2.25 2.5 2.75 V
CD Input Leakage ICDIL — — 1.0 µA
Continued next page …
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
5
Limits
Characteristic Symbol Test Conditions Min. Typ. Max. Units
CWD Current ICWD Charging, D26 = 0, D27 = 0 -9.0 -10 -11 µ A
Charging, D26 = 0, D27 =1 -18 -20 -22 µA
Charging, D26 = 1 D27 = 0 -27 -30 -33 µA
Charging, D26 = 1, D27 =1 -36 -40 -44 µA
CWD Threshold Voltage VTL 0.22 0.25 0.28 V
VTH 2.25 2.5 2.75 V
Max. FLL Oscillator Frequency fOSC 20* — — MHz
Oscillator High Duration ton 20 — — ns
Oscillator Low Duration toff 20 — — ns
Maximum Output Current IOUT(MAX) D3 = 0, D4 = 0, D28 = 0 1.0 1.2 1.4 A
D3 = 0, D4 = 1, D28 = 0 0.9 1.0 1.1 A
D3 = 1, D4 = 0, D28 = 0 500 600 700 mA
D3 = 1, D4 = 1, D28 = 0 — 250 — mA
D3 = 0, D4 = 0, D28 = 1 500 600 700 mA
D3 = 0, D4 = 1, D28 = 1 415 500 585 mA
D3 = 1, D4 = 0, D28 = 1 — 300 — mA
D3 = 1, D4 = 1, D28 = 1 — 125 — mA
Transconductance Gain gmD28 = 1 210 250 290 mA/V
D28 = 0 420 500 580 mA/V
Centertap Resistors RCT 5.0 10 13 kΩ
Back-EMF Threshold with respect
— 5.0 20 37 mV
to VCTAP at FCOM transition -5.0 -20 -37 mV
ELECTRICAL CHARACTERISTICS continued
Negative current is defined as coming out of (sourcing) the specified device terminal.
* Operation at an oscillator frequency greater than the specified minimum value is possible but not waranteed.
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
6
Serial Port Timing Conditions
A. Minimum CHIP SELECT setup time before CLOCK rising edge ......... 100 ns
B. Minimum CHIP SELECT hold time after CLOCK rising edge .............. 150 ns
C. Minimum DATA setup time before CLOCK rising edge ....................... 150 ns
D. Minimum DATA hold time after CLOCK rising edge ............................ 150 ns
E. Minimum CLOCK low time before CHIP SELECT.................................. 50 ns
F. Maximum CLOCK frequency .............................................................. 3.3 MHz
G. Minimum CHIP SELECT high time ...................................................... 500 ns
Note: the A8904 can be directly used in an existing A8902–A application, as the five most
significant bits 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.
DATA
CLOCK
Dwg. WP
A
C
B
D C D
CHIP SELECT
E
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
7
Terminal Functions
A8904SLB A8904SLP
Terminal Name Function (SOIC) (TSSOP)
LOAD SUPPLY VBB; the 5 V or 12 V motor supply. 1 15
CD2 One of two capacitors used to generate the ideal commutation points from 2 16
the back-EMF zero crossing points.
CWD Timing capacitor used by the watchdog circuit to blank out the back-EMF 3 17
comparators during commutation transients, and to detect incorrect motor
position.
CST Startup oscillator timing capacitor. 4 18
NC No( internal) connection. – 19
OUTAPower amplifier A output to motor. 5 20
NC No (internal) connection. – 21
GROUND Power and logic ground and thermal heat sink. 6-7 –
POWER GROUND Power ground. – 22*
NC No (internal) connection. – 23
OUTBPower amplifier B output to motor. 8 24
OUTCPower amplifier C output to motor. 9 25
CENTERTAP Motor centertap connection for back-EMF detection circuitry. 10 26
BRAKE Active low turns ON all three sink drivers shorting the motor windings to 11 27
ground. External capacitor and resistor at BRAKE provide brake delay.
The brake function can also be controlled via the serial port.
CRES External reservoir capacitor used to hold charge to drive the source drivers’ 12 28
gates. Also provides power for brake circuit.
ANALOG GROUND Analog ground. – 1*
FILTER Analog voltage input/output to control motor current. Also, compensation node
for internal speed control loop. 13 2
SECTOR DATA External tachometer input. Can use sector or index pulses from disk to 14 3
provide precise motor speed feedback to internal frequency-locked loop.
LOGIC SUPPLY VDD; the 5 V logic supply. 15 4
OSCILLATOR Clock input for the speed reference counter. 16 5
DATA OUT Thermal shutdown indicator, FCOM, TACH, or SYNC signals available in 17 6
real time, controlled by 2-bit multiplexer via serial port.
NC No (internal) connection. – 7
GROUND Power and logic ground and thermal heat sink. 18-19 –
DIGITAL GROUND Logic ground. –8*
RESET When pulled low forces the chip into sleep mode; clears all serial port bits. 20 9
NC No (internal) connection. – 10
CHIP SELECT Strobe input (active low) for data word. 21 11
CLOCK Clock input for serial port. 22 12
DATA IN Sequential data input for the serial port. 2 3 13
CD1 One of two capacitors used to generate the ideal commutation points from 24 14
the back-EMF zero crossing points.
* For the A8904SLP, ground terminals 1, 8, and 22 must be connected together externally.
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
8
Functional Description
Overview of operation. Each electrical revolution
contains six states that control the three half-bridge outputs.
Optimized switching from state to state is achieved through the
adaptive commutation circuitry. During any state, one output is
high, one is low and the other is high impedance. The back-
EMF at the high-impedance output is sensed and compared to
the voltage of the centertap and when the two signals are
equivalent, the FCOM signal toggles. A controlled delay is then
introduced before the sequencer commutates into the next state.
Linear current-mode control is employed to provide
precision control of the motor speed while maintaining ex-
tremely low electrical noise emissions. The speed control is
realized through a frequency-locked loop that processes the
sensed back-EMF signals from the stator phases to eventually
produce a TACH signal. The TACH signal is then compared to
the desired programmed speed, to produce an error. The error
signal is then used to linearly control the current through the
low-side DMOS power devices to obtain the correct speed.
Alternative control schemes can be introduced, giving the
user maximum flexibility and optimization for each application.
An external tachometer signal applied to the SECTOR DATA
input, along with the internal speed reference can be used for
high-precision speed control. As another alternative, the user can
introduce external speed control by driving the FILTER terminal
directly.
Start-up routines are inherent in the solution to guarantee
reliable start-up. During start-up, a YANK feature allows rapid
transition to the nominal operating condition on the FILTER
terminal. This feature is also available when the external speed
control is used.
Dynamic braking can be introduced by either the external
BRAKE terminal or through the brake bit in the serial port.
A serial port allows the user to program various features and
modes of operation, such as motor speed, internal or external
speed control, internal or external speed reference, current limit,
sleep mode, direction, charge current (for blanking pulse), motor
poles, transconductance gain, and various diagnostic outputs.
Full device protection is incorporated, including program-
mable overcurrent limit, thermal shutdown, and undervoltage
shutdown on the logic supply.
Power outputs. The power outputs of the A8904 are n-
channel DMOS transistors with a total source plus sink rDS(on) of
typically 1 Ω. An internal charge pump provides a voltage rail
above the load supply for driving the high-side DMOS gates.
Intrinsic ground clamp and flyback diodes provide protection
when switching inductive loads. These diodes will also rectify
the motor back-EMF during power-down conditions. If neces-
sary, a transient voltage supply can be provided, by connecting
an external Schottky power diode or pass FET in series, between
the power source and the load supply (VBB). This FET or diode
effectively isolates the low impedance path through the power
source. A filter capacitor is also required to ‘hold up’ the
rectified signal, and is connected between the load supply and
ground.
Back-EMF sensing motor startup and running
algorithm. The A8904 provides a complete self-contained
back-EMF sensing, startup and running commutation scheme. A
state machine with six states, (shown in the tables below for
both forward and reverse direction) controls the three half-
bridge outputs. In each state, one output is high (sourcing
current), one low (sinking current), and one is OFF (high
impedance or ‘Z’). Motor back-EMF is sensed at the output that
is OFF.
Sequencer State
(forward direction) O UTAOUTBOUTC
1 High Z Low
2 High Low Z
3 Z Low High
4 Low Z High
5 Low High Z
6 Z High Low
Sequencer State
(reverse direction) OUTAOUTBOUTC
1 High Z Low
6 Z High Low
5 Low High Z
4 Low Z High
3 Z Low High
2 High Low Z
At start-up, the outputs are always enabled in state 1. The
back-EMF is examined at the OFF output by comparing the
output voltage to the motor centertap voltage at CENTERTAP.
The motor will then either step forward, step backward or
remain stationary (if in a null-torque position).
If the motor does not move during the initial start-up state,
the outputs are commutated automatically by the start-up
oscillator. When suitable back-EMF signals are detected, the
start-up oscillator is overridden and the corresponding timing
clock is generated, providing synchronous back-EMF commuta-
tion. The start-up oscillator period is determined by
tCST = (VCSTH - V CSTL) x CST / IST(charge)
where CST is the start-up capacitor.
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
9
If the motor moves, the back-EMF detection and direction
circuit waits for the correct polarity of back-EMF zero crossing
(output crossing through centertap). If the correct polarity of
back-EMF is not detected, a watchdog circuit commutates the
output until the correct back-EMF is detected. Correct back-
EMF sensing is indicated by the FCOM signal, which toggles
every time the back-EMF completes a zero crossing (see
waveforms below). FCOM is available at the DATA OUT
terminal.
True back-EMF zero crossings are used by the adaptive
commutation delay circuit to advance the state sequencer
(commutate) at the proper time to synchronously run the motor.
See next section.
Adaptive commutation delay. The adaptive commuta-
tion delay circuit uses the back-EMF zero-crossing indicator
signal (FCOM) to determine an optimal commutation time for
efficient synchronous switching of the output drivers. When the
FCOM signal changes state, one of the delay capacitors (CD1 or
CD2) is discharged at approximately twice the rate of the
charging current. When the capacitor reaches the 2.5 V thresh-
old, a commutation occurs. During this discharge period, the
other delay capacitor is being charged in anticipation of the next
FCOM state change. In addition, there is an interruption to the
charging, which is set by the blanking duration (see waveform
below, VCWD, and next section). This additional charging delay
causes the commutation to occur at slightly less than 50% of the
FCOM on or off duration, to compensate for delays caused by
winding inductance.
Functional Description (cont’d)
The typical delta voltage change during normal operation in
the commutation capacitors (CD1 & CD2), will range between
1.5 V and 2.0 V. The commutation capacitor values can be
determined from: CDX = ICD x t / VCD
where VCD = 1.5 V, ICD = 20 µA, and t = (60/rpm)/(#motor
poles x 3), duration of each state.
To avoid the capacitors charging to the supply rail, the value
selected should provide adequate margin, taking into account the
effects of capacitor tolerance, charging current, etc.
Blanking and watchdog timing functions. The
blanking and watchdog timing functions are derived from one
timing capacitor CWD .
During normal commutation, at the beginning of each new
sequencer state, a blanking signal is created until the watchdog
capacitor CWD is charged to the threshold VTL (see waveforms
below). This blanking signal prohibits the back-EMF compara-
tors from tripping due to the discharging of inductive energy and
voltage settling transients during sequence state transitions. The
duration of this blanking signal depends on the size of the CWD
capacitor and the programmed charge current, ICWD (via D26-
27). This blanking pulse also interrupts the commutation delay
capacitors CD1 and CD2 from charging (see previous section).
The ability to select the minimum charge current for CWD is
particularly useful during start-up, where the duration of the
diode recirculation current is highest. In applications where
high motor speeds are experienced, the charge current can be
increased so that the blanking period does not encroach signifi-
cantly into the period of each sequencer state and does not cause
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
10
unbalance in the commutation points.
It is recommended to select the value of CWD in the actual
application circuit with the A8904 put into step mode. CST
should be reselected (only for this test), to be between 4.7 µF
and 10 µF, so that the motor comes to rest between steps and the
maximum diode conduction time can be measured. The value of
CWD can be determined as:
CWD = ICWD x td / VTL
where td = measured diode conduction, ICWD = charge current at
start-up, and VTL = 250 mV.
t BLANK
Dwg. WP-022
BLANK
CWD
V
VTL
t BLANK
t WD
VTL
VTH
Dwg. W P-021
BLANK
CWD
V
After the watchdog capacitor CWD charges to the VTL threshold,
and if the correct polarity of back-EMF signal is detected, the
back-EMF detection circuit discharges CWD to zero volts (see
waveform above) and the circuit is ready to detect the next back-
EMF zero crossing.
If the correct polarity of back-EMF is not detected between
the blanking period, tBLANK, and the watchdog period, tWD, then
the back-EMF detection circuit does not allow the watchdog
capacitor CWD to be discharged and the watchdog circuit
commutates the outputs to the next sequencer state (see wave-
form above). This mode of operation continues until a suitable
back-EMF signal is detected. This function is useful in prevent-
ing excessive reverse rotation, and helps in resynchronising (or
starting) with a moving spindle.
The duration of the watchdog-triggered commutation is
determined by: tWD = VTH x CWD / ICWD
where ICWD = normal charge current.
Speed control. The actual speed of the motor is mea-
sured by either internally sensing the back-EMFs or by an
external scheme via the SECTOR DATA terminal. A TACH
signal is produced from these signals, which is then compared
against the desired speed, which is programmed into a 14-bit
counter (see diagram and waveforms below - assumes internal
scheme used). The resulting error signal, ERROR, is then used
to charge or discharge the FILTER terminal capacitor depending
on whether the motor is running too slow or too fast. The
FILTER terminal voltage is used to linearly drive the low-side
MOSFETs to match the desired speed.
Each back-EMF signal detected causes the state of the
FCOM signal to change. The number of FCOM transitions per
mechanical revolution is equal to the number of poles times 3.
For example, with a 4-pole motor (as shown on next page), the
number of FCOM transitions will equal 12 per mechanical
revolution. The number of poles are programmed via serial port
bits D20 and D21. There are six electrical states per electrical
revolution, therefore, for this example, there are 12 commuta-
tions or two electrical revolutions per mechanical revolution.
The TACH signal changes state once per mechanical
revolution and as well as providing information on the actual
motor speed is also used to trigger the REF counter which
contains the information on the desired motor speed. Alterna-
tively an external TACH signal can be used, an explanation of
which is presented in the Sector Mode Section.
The duration of REF is set by programming the counter to
count the desired number of OSCILLATOR cycles, according to
the following:
total count = 60 x fOSC / desired motor speed (rpm)
where the total count (number of oscillator cycles) is equal to the
sum of the count numbers selected through bits D5 to D18 in the
serial port and fOSC corresponds to the OSCILLATOR fre-
quency.
Functional Description (cont’d)
Normal commutation
Watchdog-triggered commutation
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
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11
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.
Functional Description (cont’d)
Speed error detection
Speed error signals
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
12
Sector mode. An external tachometer signal, such as
sector or index pulses, can be used to create the TACH signal,
rather than the internally generated once-around scheme. The
external signal is applied to the SECTOR DATA terminal and
the serial port bit (D19 = 1) must be programmed to enable this
feature.
In applications where both internal and external TACH
signals are used, it is important to only switch between modes
when the SYNC signal on DATA OUT is low. This ensures the
speed control information that is being processed during the
transition, is not corrupted. SYNC is accessed through the
DATA OUT multiplexer, which is controlled by D22 & D23.
DATA OUT. The DATA OUT terminal is the output of a
2-bit input multiplexer controlled by D22 & D23 of the serial
port. Data available are TACH signal (internally or externally
generated), SYNC signal, FCOM signal, and thermal shutdown
(LOW = A8904 operating within thermal limits, HIGH =
thermal shutdown has occurred).
Speed loop initialization (YANK). To ensure rapid
transition from start-up to the normal operating condition, the
FILTER terminal is pulled up to the filter threshold voltage,
Functional Description (cont’d)
VFILTERTH, by the internal YANK command and the initial
output current will be set to the maximum selected current limit.
This condition is maintained until the motor reaches the correct
speed and the first ERROR FAST signal is produced which
removes the YANK and allows linear current control.
The YANK feature is also activated when an external speed
control scheme is used (D24 = 1). To ensure the YANK is
released at start-up by the internal speed control, it is important
to ensure the speed reference is set at a lower speed than what
the motor is designed to run at. Note that when the serial port is
programmed to run initially, the default condition for the speed
is set for the slowest condition so this will guarantee the YANK
to be released. It is important when using external speed control
that, as a minimum, the number of poles, speed control mode,
and speed reference are programmed in the serial port.
Forward/reverse. Directional control is managed
through D25 in the serial port.
Serial port. Control features and diagnostic data selection
are communicated to the A8904 through the 29-bit serial port.
See serial port timing diagrams on page 6. When CHIP SE-
LECT is low, data is written to the serial port on the positive
edge of the clock with the MSB (D28) fed in first. At the end of
Speed and current control
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
13
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
D3 D4 D28 (typical) gain
0 0 0 1.2 A 500 mA/V
0 1 0 1.0 A 500 mA/V
1 0 0 600 mA 500 mA/V
1 1 0 250 mA 500 mA/V
0 0 1 600 mA 250 mA/V
0 1 1 500 mA 250 mA/V
1 0 1 300 mA 250 mA/V
1 1 1 125 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
D5 16
D6 32
D7 64
D8 128
D9 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.
D2 0 D2 1 Motor poles
00 8
01 4
10 16
11 12
D22 and D23 - Controls the multiplexer for DATA OUT.
See DATA OUT Section for status definitions.
D2 2 D2 3 DATA OUT
0 0 TACH (once around or sector) signal
0 1 Thermal shutdown
1 0 SYNC signal
1 1 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.
D2 6 D2 7 Watchdog charge current (typical)
0 0 -10 µA
0 1 -20 µA
1 0 -30 µA
1 1 -40 µA
Functional Description (cont’d)
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
14
D28 - Programs the transconductance gain. LOW = 500 mA/V,
HIGH = 250 mA/V.
Reset. When the RESET terminal is pulled low, all the
serial port bits are reset to LOW and the part operates in sleep
mode.
Undervoltage lockout, VDD. When an undervoltage
condition occurs, all the serial port bits are reset to LOW and the
part operates in sleep mode.
Charge pump. The charge pump is required to provide a
voltage rail above the load supply for driving the high-side
DMOS gates. In addition the charge pump supply capacitor,
CRES, also powers the brake control circuit during power-down
conditions. CRES should be 220 nF.
Braking. A dynamic braking feature of the A8904 shorts
the three motor windings to ground. This is accomplished by
turning the three source drivers OFF and the three sink drivers
ON. Activation of the brake can be implemented through the
BRAKE input or through the D2 bit in the serial port.
During braking, the motor is effectively acting as three sine-
wave voltage generators, 120° out of phase, where the voltage
developed by each of the windings is proportional to the motor
speed and constant. The current through any sink driver is
simply the generated voltage divided by the center tap to OUT
resistance plus the sink driver resistance. As the motor tends to
slow during the braking process, both the generated voltage and
the corresponding current decreases.
When selecting a motor to use where braking will be
applied, it is important to characterize the application to ensure
that when braking is applied, the peak current in the sink drivers
does not exceed 3A and the period from the peak current to the
maximum current limit of the drivers does not exceed 800 ms.
Another consideration is the thermals of the solution, where
repeated spin-up followed by brake cycles could cause excessive
junction temperatures.
The supply voltage for the brake circuit is derived from the
charge pump supply capacitor, CRES. With CRES chosen to be
220 nF, the brake circuit will function for at least 100 ms after a
power failure.
In certain applications such as disk drives, it is desirable to
include a brake delay to allow sensitive circuitry such as the disk
Functional Description (cont’d)
head to retract before activating the spindle motor brake. The
brake delay can be simply implemented by using an external RC
and diode to control the brake terminal.
BRAKE
FAULT
VBRK
V – V
FAULT D BRAKE
ACTIVATED
Dwg. OP-004
RB
CB
t BRK
The brake delay can be set using the equation:
tBRK = –RBCB x ln (VBRK / [VFAULT – VD]).
Once the brake is activated, the three sink drivers will
remain active until the supply rails fall below the operating
range. It is recommended that the part is reset before restarting.
Centertap. It is recommended that the centertap connec-
tion of the motor be connected to the CENTERTAP terminal. If
the centertap of the motor is not connected to the CENTERTAP
terminal, the A8904 internally emulates the centertap voltage of
the motor through a series of 10 kΩ resistors connected between
each output and CENTERTAP. This technique does not provide
ideal commutation points.
External component selection. All capacitors should
be rated to at least 25 V and the dielectric should be X7R, apart
from the start-up capacitor CST, which can be Z5U dielectric or
equivalent and the input capacitor Cfilter, which should be an
electrolytic type of value greater than 100 µF, 35 V, Iripple > 100
mA. If the solution experiences ambient temperatures of greater
than 70°C then Cfilter should be rated for 105°C.
All resistors are at least 1/8 W and have a tolerance of ±5%.
In noise-sensitive systems where electromagnetic interfer-
ence is an issue, or to stabilize the current waveforms with
certain motors, it may be necessary to add RC snubbers across
the motor windings as shown in the application circuit on the
next page. The A8904 solution should be relatively noise
immune from the effects of switching voltage spikes etc. if the
correct watchdog capacitor has been selected for optimum
blanking and good layout practices are implemented.
At the range of operating frequencies that the current pulses
are drawn out of the load supply, it is the capacitance reactance
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
15
Typical application
(A8904SLB)
as opposed to the ESR that dominates the overall impedance of
the input filter, Cfilter. Therefore, it is possible to reduce con-
ducted electromagnetic emissions further, by simply increasing
the value of Cfilter. In extremely sensitive systems, it may be
necessary to introduce a differential mode inductor in series with
the load supply line.
Layout considerations. The TSSOP part (A8904SLP)
has three separate ground connections, analog, digital, and
power that must be connected together externally. A ground
plane should be used to provide heat sinking for the power
switches and the reduction of potential noise pick-up through
inductive loops and radiated emissions. The ground plane
should cover the area beneath the A8904 and extend beyond the
outline to form a plane around all the external components. The
exposed thermal pad of the TSSOP part should be connected
to the ground plane.
Filter components, especially Cfilter, timing, and delay
capacitors should be positioned as close as possible to the device
terminals. It is also imperative that the traces to the serial port
and oscillator are as short and as wide as possible to reduce stray
inductance and prevent potential data corruption. In addition,
these traces should be positioned well away from any noisy
signals.
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
16
A8904SLB (SOIC)
NOTES: 1. Webbed lead frame. Leads 6, 7, 18, and 19 are internally one piece.
2. Lead spacing tolerance is non-cumulative.
3. Exact body and lead configuration at vendor’s option within limits shown.
4. Supplied in standard sticks/tubes of 31 devices or add “TR” to part number
for tape and reel.
Dimensions in Millimeters
(controlling dimensions)
Dimensions in Inches
(for reference only)
0° TO 8°
1
24
13
2
3
0.2992
0.2914
0.6141
0.5985
0.419
0.394
0.020
0.013
0.0926
0.1043
0.0040
MIN.
0.012
0.009
Dwg. MA-008-25A in
0.050
BSC
NOTE 1
NOTE 3
0.050
0.016
0°
TO
8°
1
24
2
3
7.60
7.40
15.60
15.20
10.65
10.00
0.51
0.33
2.65
2.35
0.10
MIN.
0.32
0.23
1.27
BSC
NOTE 1
NOTE 3
1.27
0.40
Dwg. MA-008-25A mm
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
www.allegromicro.com
17
A8904SLP (TSSOP)
Dimensions in Millimeters
(controlling dimensions)
Dimensions in Inches
(for reference only)
NOTES: 1. Exact body and lead configuration at vendor’s option within limits shown.
2. Lead spacing tolerance is non-cumulative.
3. Supplied in standard sticks/tubes of 49 devices or add “TR” to part number for tape and reel.
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
18
The products described here are manufactured under one or more
U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time to
time, such departures from the detail specifications as may be required
to permit improvements in the performance, reliability, or
manufacturability of its products. Before placing an order, the user is
cautioned to verify that the information being relied upon is current.
Allegro products are not authorized for use as critical components
in life-support devices or systems without express written approval.
The information included herein is believed to be accurate and
reliable. However, Allegro MicroSystems, Inc. assumes no responsi-
bility for its use; nor for any infringement of patents or other rights of
third parties which may result from its use.
