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July 2000
ML4426* Bi-directional Sensorless BLDC Motor Controller
GENERAL DESCRIPTION
The ML4426 PWM motor controller provides all of the functions necessary for starting and controlling the speed of delta or wye wound Brushless DC (BLDC) motors without Hall Effect sensors. Back EMF voltage is sensed from the motor windings to determine the proper commutation phase sequence using a PLL. This patented sensing technique will commutate a wide range of 3Phase BLDC motors and is insensitive to PWM noise and motor snubbing circuitry. The ML4426 limits the motor current using a constant offtime PWM control loop. The velocity loop is controlled with an onboard amplifier. The ML4426 has circuitry to ensure that there is no shoot-through in directly driven external power MOSFETs. The timing of the start-up sequence is determined by the selection of three timing capacitors. This allows optimization for a wide range of motors and loads.
FEATURES
s Motor starts and stops with power to IC
s Bi-directional motor drive for applications requiring forward/ reverse operation s On-board start sequence: Align (R) Ramp (R) Set Speed s Patented Back-EMF commutation technique provides jitterless torque for minimum "spin-up" time s Onboard speed control loop s PLL used for commutation provides noise immunity from PWM spikes, compared to noise sensitive zero crossing technique s PWM control for maximum efficiency s Direct FET drive for 12V motors; drives high voltage motors with IC buffers from IR, IXYS, Harris, Power Integrations, Siliconix, etc. (* Indicates Part Is End Of Life As Of July 1, 2000)
BLOCK DIAGRAM (Pin Configuration Shown for 28 Pin Version)
17 VDD 750nA FB A 22 FB B 23 FB C 24 BACK EMF SAMPLER 1.5V
- +
CAT 750nA
VDD
19 CRT
-
21 CRR
20 SPEED CVCO FB
15
16 RVCO
1.5V
+
VDD 500nA VOLTAGE CONTROLLED OSCILLATOR VCO/TACH 13
VCO OUT
F/R 12
+
VCO OUT
R A F B
COMMUTATION STATE MACHINE HA HB GATING LOGIC & OUTPUT DRIVERS UVLO HC LA LB LC UV FAULT 4k REFERENCE 2 3 4 9 10 11 18
8 SPEED SET 5 SPEED COMP
-
3.9V
- +
E D
C
1.7V 6 CT 20kHz 1 ISENSE 1.7V x5 VREF 16k
- +
-
ILIMIT 1-SHOT
1.4V VDD
+
8k 26 CIOS BRAKE 25 14 VDD
GND 28 27
RREF
7
VREF
1
ML4426
PIN CONFIGURATION
ML4426 28-Pin Narrow PDIP (P28N) 28-Pin SOIC (S28)
ISENSE HA HB HC SPEED COMP CT VREF SPEED SET LA 1 2 3 4 5 6 7 8 9 28 27 26 25 24 23 22 21 20 19 18 17 16 15 GND RREF CIOS BRAKE FB C FB B FB A CRR SPEED FB CRT UV FAULT CAT RVCO CVCO
LB 10 LC 11 F/R 12 VCO/TACH 13 VDD 14
TOP VIEW
ML4425 32-Pin TQFP (H32-7)
ISENSE BRAKE GND CIOS RREF HA NC HB
H3 NC SPEED COMP CT VREF SPEED SET LA LB
32 31 30 29 28 27 26 25 24 1 2 3 4 5 6 7 8 9 23 22 21 20 19 18 17 10 11 12 13 14 15 16
FB C FB B FB A CRR SPEED FB CRT UV FAULT CAT
LC
F/R
VCO/TACH
VDD
NC
NC
CVCO
TOP VIEW
2
RVCO
ML4426
PIN DESCRIPTION
PIN NAME
(Pin number in parenthesis is for TQFP package)
PIN NAME FUNCTION
FUNCTION
1(30)
I SENSE
Motor current sense input. When ISENSE exceeds 0.2 ILIMIT, the output drivers LA, LB, and LC are shut off for a fixed time determined by CIOS Active low output driver for the phase A high-side switch Active low output driver for the phase B high-side switch Active low output driver for the phase C high-side switch
17(17) CAT
A capacitor to GND sets the time that the controller stays in the align mode This output goes low when VDD drops below the UVLO threshold, and indicates that all output drivers have been disabled A capacitor to GND sets the time that the controller stays in the ramp mode Output of the back-EMF sampling circuit and input to the VCO. An RC network connected to SPEED FB sets the compensation for the PLL loop formed by the back-EMF sampling circuit, the VCO, and the commutation state machine A capacitor to between CRR and SPEED FB sets the ramp rate (acceleration) of the motor when the controller is in ramp mode The motor feedback voltage from phase A is monitored through a resistor divider for back-EMF sensing at this pin The motor feedback voltage from phase B is monitored through a resistor divider for back-EMF sensing at this pin The motor feedback voltage from phase C is monitored through a resistor divider for back-EMF sensing at this pin A logic low input activates motor braking by shutting off the highside output drivers and turning on the low-side output drivers A capacitor to GND sets the time that the low-side output drivers remain off after ISENSE exceeds its threshold An 137kW resistor to GND sets a current proportional to VREF that is used to set all the internal bias currents except for the VCO Signal and power ground
18(18) UV FAULT
2(31) 3(32) 4(1) 5(3)
HA HB HC
19(19) CRT
20(20) SPEED FB
SPEED COMP Speed control loop compensation is set by a series resistor and capacitor from SPEED COMP to GND CT V REF SPEED SET A capacitor from CT to GND sets the PWM oscillator frequency 6.9V reference voltage output Speed loop input which ranges from 0 (stopped) to VREF (maximum speed) Active high output driver for the phase A low-side switch Active high output driver for the phase B low-side switch Active high output driver for the phase C low-side switch 24(24) FB C This TTL level input selects the direction of the motor by changing the sequence of the commutation state machine This TTL level output corresponds to the signal used to clock the commutation state machine. The output frequency is proportional to the motor speed when the backEMF sensing loop is locked onto the rotor position 12V power supply input A capacitor to GND sets the voltage-to-frequency ratio of the VCO An resistor to GND sets up a current proportional to the input voltage of the VCO 27(27) R REF 23(23) FB B 21(21) C RR
6(4) 7(5) 8(6)
22(22) FB A
9(7) 10(8) 11(9)
LA LB LC
12(10) F/R
25(25) BRAKE
13(11) VCO/TACH
26(26) CIOS
14(12) V DD 15(15) CVCO
16(16) RVCO
28(28) GND
3
ML4426
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. V DD .......................................................................... 14V Logic Inputs (BRAKE, F/R) ...................... GND - 0.3 to 7V All Other Inputs and Outputs .. GND -0.3V to VDD + 0.3V Output Current (LA, LB, LC, HA, HB, HC) ............ 50mA Junction Temperature .............................................. 150C Storage Temperature Range ...................... -65C to 150C Lead Temperature (Soldering 10 sec.) ..................... 260C Thermal Resistance (qJA) 28-Pin Narrow PDIP ......................................... 48C/W 28-Pin SOIC ..................................................... 75C/W 32-Pin TQFP ..................................................... 80C/W
OPERATING CONDITIONS
Temperature Range ML4426CX................................................. 0C to 70C ML4426IX ............................................... -40C to 85C V DD ......................................................... 10.8V to 13.2V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified,VDD = 12V 10%, RSENSE = 1W, CVCO = 10nF, CIOS = 100pF, RREF = 137kW, TA = Operating Temperature Range (Notes 1, 2)
SYMBOL REFERENCE
VREF Total Variation Line, Temp 6.5 6.9 7.5 V
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PWM OSCILLATOR
Total Variation Ramp Peak Ramp Valley Ramp Charging Current CT = 1nF 28 3.9 1.7 ? kHz V V A
SPEED CONTROL LOOP
SPEED SET Input Voltage Range SPEED FB Input Voltage Range SPEED COMP Output Current SPEED SET Error Amp Transconductance VSPEED SET = xV, VSPEED FB = yV 0 0 5 144 VREF VREF 20 V V A
START-UP
CAT Charging Current C Suffix I Suffix CAT Threshold Voltage CRT Charging Current C Suffix I Suffix CRT Threshold Voltage 0.68 0.5 1.4 0.68 0.5 1.4 0.98 1.1 1.7 0.98 1.1 1.7 A A V A A V
VOLTAGE CONTROLLED OSCILLATOR
Frequency Range Frequency vs. SPEED FB RVCO = 5V, SPEED FB = 6V RVCO = 5V, 0.5V SPEED FB 7V 1.5 1.85 300 2.2 kHz Hz/V
CURRENT LIMIT
ISENSE Gain One Shot OFF-Time CIOS = 100pF C Suffix I Suffix 4.5 9 9 5.0 5.5 18 20 V/V s s
4
W
ML4426
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER LOGIC INPUTS (BRAKE, F/R) (Note 3)
V IH VIL IIH IIL Input High Voltage Input Low Voltage Input High Current Input Low Current VIH = 2.4V VIL = 0.4V 2.4 2.9 2 0.8 V V mA mA
(Continued)
CONDITIONS MIN TYP MAX UNITS
LOGIC OUTPUTS (VCO/TACH, UV FAULT) (Note 3)
VCO/TACH Output High Voltage VCO/TACH Output Low Voltage UV FAULT Output High Voltage IOUT = -100A IOUT = 400A IOUT = -10A C Suffix I Suffix UV FAULT Output Low Voltage IOUT = 400A 3.4 3.2 4.5 2.2 0.6 5.4 5.6 0.6 V V V V V
BACK-EMF SAMPLER
SPEED FB Align Mode Voltage SPEED FB Ramp Mode Current C Suffix I Suffix SPEED FB Run Mode Current State A, CRT = 5V, VPHB = VDD/3 C Suffix I Suffix 500 500 30 27 -15 -90 -90 125 250 720 750 90 90 15 -30 -27 mV nA nA A A A A A
State A, CRT = 5V, VPHB = VDD/2 State A, CRT = 5V, VPHB = 2VDD/3 C Suffix I Suffix
OUTPUT DRIVERS
High Side Driver Output Low Current High Side Driver Output High Voltage Low Side Driver Output Low Voltage Low Side Driver Output High Voltage VHX = 2V IHX = -10A ILX = 1mA V(ISENSE) = 0V C Suffix VDD - 2.2 I Suffix VDD - 2.9 Phase C Cross-conduction Lockout Threshold VDD - 3.0 0.5 VCC - 1.3 0.2 0.7 1.2 mA V V V V V
SUPPLY
I DD VDD Current UVLO Threshold C Suffix I Suffix UVLO Hysteresis
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions. Note 2: For explanation of states, see Figure 4 and Table 1. Note 3: The BRAKE and UV FAULT pins each have an internal 4kW resistor to the internal reference.
32 8.8 8.6 150 9.5
50 10.2 10.3
mA V V mV
5
ML4426
FUNCTIONAL DESCRIPTION
GENERAL
The ML4426 provides all the circuitry for sensorless speed control of 3-phase Brushless DC (BLDC) motors. Controller functions include start-up circuitry, back-EMF commutation control, Pulse Width Modulation (PWM) speed control, fixed OFF-time current limiting, braking, and undervoltage protection. The start-up circuitry aligns the motor to a known position, then ramps up the motor speed to generate a back-EMF signal. A back-EMF sampling circuit controls commutation timing by forming a Phase Locked Loop (PLL). The commutation control circuitry also outputs a speed feedback (SPEED FB) signal used in the speed control loop. The speed control loop consists of an error amplifier and PWM comparator that produce a PWM duty cycle for speed regulation. Motor current is limited by a fixed OFF-time PWM shutdown comparator that is controlled by an external sense resistor. Commutation control, PWM speed control, and current limiting are combined to produce the output driver signals. Six output drivers are used to provide gating signals to an external 3 phase bridge power stage sized for the BLDC motor voltage and current requirements. Additional functions include a braking function and undervoltage protection circuit to shut down the output drivers in the event of a low voltage condition on VDD of the ML4426. If one or more of the above values is not known, it is still possible to pick components for the ML4426, but some experimentation may be necessary to determine the optimal values. All quantities are in SI units unless otherwise specified. The following formulas should be considered as a starting point for optimization. All calculations for capacitors and resistors should be used as the first approximation for selecting the closest standard value.
POWER SUPPLY AND REFERENCE
The supply voltage (VDD) is nominally 12V 10%. A 100nF bypass capacitor to ground should be placed as close as possible to VDD. A 6.9V voltage reference output (VREF) is provided to set the speed command and current limit of the ML4426. A 137kW from RREF to GND is required to set up a reference current for internal functions.
OUTPUT DRIVERS
The output drivers LA, LB, LC, HA, HB, and HC provide totem pole output drive signals for a 3 phase bridge power stage. All control functions in the ML4426 translate to outputs at these pins. LA, LB, and LC provide the low-side drive signals for phases A, B, and C of the 3 phase power stage and are 12V active high signals. HA, HB, and HC provide the high-side signals and are 12V active low signals.
COMPONENT SELECTION
Selecting external components for the ML4426 requires calculations based on the motor's electrical and mechanical parameters. The following is a list of the motor parameters needed for these calculations : DC motor supply voltage - VMOTOR (V) Maximum operating current - IMAX (A) Number of magnetic poles - N Back EMF constant - Ke (V-s/Rad) Motor torque constant - Kt (Nm/A) (Kt = Ke in SI units) Maximum speed of operation RPMMAX (RPM) Moment of inertia of the motor and load - J (Kg-m2) Viscous damping factor of the motor and load - z
VMOTOR 12V
DC SUPPLY CAPACITOR
HA
HB MOTOR PHASE A
HC MOTOR PHASE B LC MOTOR PHASE C
LA
LB
RSENSE
Figure 1. Using RSENSE in a 3-Phase 12V Power Stage
6
ML4426
FUNCTIONAL DESCRIPTION
CURRENT LIMITING IN THE POWER STAGE
The current sense resistor (RSENSE) shown in Figure 1 regulates the maximum current in the power stage and the BLDC motor. Current regulation is accomplished by shutting off the output drivers LA, LB, and LC for a fixed amount of time if the voltage across RSENSE exceeds the current limit threshold. (Continued)
FROM RSENSE ISENSE x5 VREF 16k
2.9V STOP START 30A
- +
PWM ON/OFF S R Q Q
R SENSE
The function of RSENSE is to provide a voltage proportional to the motor current to set the current limit trip point. The default trip voltage across RSENSE is 460mV, set by the internal ILIMIT divider ratio. The current sense resistor should be a low inductance resistor such as a carbon composition. For resistors in the milliohms range, wire-wound resistors tend to have low values of inductance. RSENSE should be sized to handle the power dissipation (IMAX2 RSENSE).
8k
0V
CIOS
Figure 2. Current Sense Circuitry
ISENSE Filter
The ISENSE RC lowpass filter is placed in series with the current sense signal as shown in Figure 2. The purpose of this filter is to remove the diode reverse recovery shootthrough current. This current causes a voltage spike on the leading edge of the current sense signal which may falsely trigger the current limit. The current sense voltage waveform is shown before and after filtering in Figure 3. The recommended starting values for this circuit are R = 1kW and C = 330pF. This gives a time constant of 330ns, and will filter out spikes of shorter duration. C can be increased to as much as 2.2nF, but should not exceed a time constant of more than a few microseconds.
460mV
0V (a) (b)
Figure 3. Current Sense Resistor Waveforms (a) Without Filtering, and (b) With Filtering
C IOS
When ISENSE exceeds 0.2 ILIMIT, the current limit oneshot is activated, turning off LA, LB, and LC for a fixed amount of time (tOFF). tOFF is set by the amount of capacitance connected to CIOS. CIOS is usually set for a fixed off time equal to or less than the PWM period. For a 25kHz PWM frequency, the PWM period is 40s; tOFF should be between 20s and 40s. The lower limit of tOFF is dictated by the minimum on time of the power stage; a safe approximation is 5s or less. The equation for finding the CIOS capacitance value is as follows: COS = t OFF 50mA 2.4V (1) one N output from one phase and one P output from another phase. There are six combinations of N and P outputs (six switching states) that constitute a full commutation cycle. These combinations are illustrated in Table 1 and Figure 4, and are labeled states A through F. This sequence is programmed into the commutation state machine. Clocking of the commutation state machine is provided by a voltage controlled oscillator (VCO).
Forward/Reverse Control
The commutation sequence is reversed by pulling the F/R pin to GND. This allows the motor to operate in the opposite direction. This pin should change state only when the motor is at rest. Either remove and restore power to the ML4426, or pull the BRAKE pin and CAT pin to 0V to stop the motor prior to changing the voltage on F/R. Either method resets the internal commutation state machine, and initiates a new start-up sequence.
COMMUTATION CONTROL
A 3-phase BLDC motor requires electronic commutation to achieve rotational motion. Electronic commutation requires the switching on and off of the power switches of a 3-phase half bridge. For torque production to be achieved in one direction, the commutation is dictated by the rotor position. Electronic commutation in the ML4426 is achieved by turning on and off, in the proper sequence,
7
ML4426
OUTPUTS STATE
R A B C D E F
LA
OFF OFF OFF ON ON OFF OFF
LB
ON OFF OFF OFF OFF ON ON
LC
OFF ON ON OFF OFF OFF OFF
HA
ON ON OFF OFF OFF OFF ON
HB
OFF OFF ON ON OFF OFF OFF
HC
ON OFF OFF OFF ON ON OFF
INPUT SAMPLING
N/A FB B FB A FB C FB B FB A FB C
Table 1. Commutation State Functions (Forward Direction)
A HA HIGH SIDE DRIVE OUTPUTS B C D E F A B C D E F
HB
HC
LA LOW SIDE DRIVE OUTPUTS
LB
LC
Figure 4. Output Commutation Sequence Timing Diagram (Forward Direction) Cycle 1 - Full Commutation, Cycle 2 - Commutation with 50% PWM Duty Cycle
FUNCTIONAL DESCRIPTION
Voltage Controlled Oscillator (VCO)
(Continued) frequency corresponds to the maximum commutation frequency or maximum motor speed when the VCO input is equal to or slightly less than VREF. CVCO is calculated using the following equation: 6.5V 3101 10 -6 . 0.05 Hz Farad V
The VCO provides a TTL compatible clock output on the VCO/TACH pin proportional to the VCO input voltage at the SPEED FB pin. The proportion of frequency to voltage (VCO constant, Kv) is set by an 80.6kW resistor on RVCO and a capacitor on CVCO as shown in Figure 5. RVCO sets up a current proportional the VCO input voltage at SPEED FB. This current is used to charge and discharge CVCO between the threshold voltages of 2.3V and 4.3V. The resulting triangle wave on CVCO corresponds to the clock on VCO. Kv should be set so that the VCO output
C VCO =
Hz N SPEEDMAX RPM
(2)
The closest standard value that is equal to or less than the calculated CVCO should be used.
8
ML4426
FUNCTIONAL DESCRIPTION
fMAX = 0.05 N RPMMAX
(Continued)
The maximum frequency on the VCO pin is found by:
CVCO RVCO
(3)
SPEED CVCO FB RVCO
The voltage at the VCO/TACH pin is equal to the rotor speed. The voltage at SPEED FB is controlled by the back EMF sampler.
BACK EMF SAMPLER
The input to the voltage controlled oscillator is the back EMF sampler. The back EMF sense pins FB A, FB B, and FB C inputs to the back EMF sampler require a signal from the motor phase leads that is below the VDD of the ML4426. The phase sense input impedance is 8kW. This requires a series resistor RES1 from the motor phase lead as shown in Figure 6 based on the following equation: RES1 = 670W / V VMOTOR - 10V
FROM BACK EMF SAMPLER & RAMP GENERATOR VOLTAGE CONTROLLED OSCILLATOR VCO/TACH
RESET (FROM CAT)
1
6
4.3V CVCO 2.3V
(4)
The back EMF sampler takes the motor phase voltages divided down to signals that are less than VDD (12V nominal) and calculates the neutral point of the motor by the following equation: Neutral = PH1+ PH2 + PH3 3 (5)
5V VCO/TACH 0V
Figure 5. External VCO Component Connections
This allows the ML4426 to compare the back EMF signal to the motor's neutral point without the need for bringing out an extra wire on a WYE wound motor. For DELTA wound motors there is no physical neutral to bring out, so this reference point must be calculated in any case. The back EMF sampler measures the motor phase that is not driven (i.e. if LA and HB are on, then phase A is driven low, phase B is driven high, and phase C is
sampled). The sampled phase provides a back EMF signal that is compared against the neutral of the motor. The sampler is controlled by the commutation state machine. The sampled back EMF is compared to the neutral through an error amplifier. The output of the error amplifier outputs a charging or discharging current to SPEED FB, which provides the control voltage to the VCO.
MOTOR A MOTOR B MOTOR C
RES1 RES2 RES3
FB A FB B FB C NEUTRAL SIMULATOR A + B + C 6 4k 4k MULTIPLEXER SIGN CHANGER gm =
+ -
1 8k TO SPEED FB
4k
4k
4k
4k
COMMUTATION STATE MACHINE
Figure 6. Back EMF Sampler Detailed Block Diagram
9
ML4426
FUNCTIONAL DESCRIPTION
(Continued)
CSPEEDFB1
BACK EMF SENSING PLL COMMUTATION CONTROL
RSPEEDFB CSPEEDFB2 20 FB A 22 FB B 23 FB C 24 500nA BACK EMF SAMPLER SPEED FB VDD VOLTAGE CONTROLLED OSCILLATOR VCO/TACH 13
Three blocks form a phase locked loop that locks the commutation clock onto the back EMF signal: the commutation state machine, the voltage controlled oscillator, and the back EMF sampler. The complete phase locked loop is illustrated in Figure 7. The phased locked loop requires a lead lag filter that is set by external components on SPEED FB. The components are selected as follows:
CSPEEDFB1
R SPEEDFB
CSPEEDFB2
K N = 0.25 M d ln f 100 f d = 2 M ln 100 N K 01- M5 =C 0M - 15
O1 S 2 2 VCO 2
(6a)
F/R 12
R A F B
VCO
S
O1
(6b) (6c)
PHASE LOCKED LOOP
SPEEDFB1
E D
C
COMMUTATION STATE MACHINE
START-UP SEQUENCE
When power is first applied to the ML4426 and the motor is at rest, the back EMF is equal to zero. The motor needs to be rotating for the back EMF sampler to lock onto the rotor position and commutate the motor. The ML4426 uses an open loop start-up technique to bring the rotor from rest up to a speed fast enough to allow back EMF sensing. Start-up is comprised of three modes: align mode, ramp mode, and run mode.
Figure 7. Back EMF Commutation Phase Locked Loop
Ramp Mode
At the end of align mode the controller goes into ramp mode. Ramp mode starts commutating through the states A through F as shown in Table 1. This ramps up the commutation frequency, and therefore the motor speed, for a fixed length of time. This allows the motor to reach a sufficient speed for the back EMF sampler to lock commutation onto the motor's back EMF. The amount of time the ML4426 stays in ramp mode is determined by a capacitor connected to the CRT pin as shown in Figure 8. CRT is charged by a constant 750A current from GND to 1.5 V until the ramp comparator trips to end the ramp mode. This gives a fixed ramp time. CRT is calculated as follows: CRT = 2p J 5 10 -7 amp K V IMAX K t 3 N (8)
Align Mode (RESET)
Before the motor can be started, the rotor must be in a known position. When power is first applied to the ML4426, the controller is reset into the align mode. Align mode turns on the output drivers LB, HA, and HC which aligns the motor into a position 30 electrical degrees before the center of the first commutation state. This is shown as state R in the commutation states of Table 1. Align mode must last long enough to allow the motor and its load to settle into this position. The align mode time is set by a capacitor connected to the CAT pin as shown in Figure 8. CAT is charged by a constant 750A current from GND to 1.5 V until the align comparator trips to end the align mode. A starting point for CAT is calculated as follows: C AT = t S 7.5 10 -7 amp 15V . (7)
If the align time is not long enough to allow the rotor to settle for reliable starting, then increase CAT until the desired performance is achieved.
The rate at which the ML4426 ramps up the motor speed is determined by a fixed 500A current source on the SPEED FB pin. The current sources charges up the PLL filter components causing the VCO frequency to ramp up. During ramp mode, the back EMF sampler is disabled to allow control of the ramping to be set only by the 500A current source. The ramp based on the SPEED FB filter is generally too fast for the motor to keep up, so a capacitor from CRR to SPEED FB can be added to slow down the ramping rate. The optimal ramp rate is based on the motor and load parameters and is can be adjusted by varying the value of CRR.
10
ML4426
CRR TO SPEED FB FILTER CAT CRT
VDD 750nA
CAT 750nA
-
VDD
CRT
-
CRR
SPEED CVCO FB
RVCO
FB A FB B FB C BACK EMF SAMPLER
1.5V
+
1.5V
+
VDD 500nA VOLTAGE CONTROLLED OSCILLATOR VCO/TACH
TO RESET INPUT OF COMMUTATION STATE MACHINE
Figure 8. ML4426 Start-up Circuitry for Controlling the Align and Ramp Times
Run Mode (Back EMF Sensing)
At the end of ramp mode the controller goes into run mode. In run mode, the back EMF sensing is enabled and commutation is now under the control of the phase locked loop. Motor speed is now regulated by the speed control loop.
VREF
FROM SPEED FB TO GATING LOGIC & OUTPUT DRIVERS
-
+
10k SPEED SET
-
3.9V
PWM SPEED CONTROL
Speed control is accomplished by setting a speed command at SPEED SET with an input voltage from 0 to 6.9V (VREF). The accuracy of the speed command is determined by the external components RVCO and CVCO. There are a number of methods that can be used to control the speed command of the ML4426. One is to use a 10kW potentiometer from VREF to ground with the wiper connected to SPEED SET. If SPEED SET is controlled from a microcontroller, one of its DACs can be used with VREF as its input reference. The speed command is compared with the sensed speed from SPEED FB through a transconductance error amplifier. The output of the speed error amplifier is SPEED COMP. SPEED COMP is clamped between one diode drop above 3.9V (approximately 4.6V) and one diode drop below 1.7V (approximately 1V) to prevent speed loop "wind-up". Speed loop compensation components are connected to this pin as shown in Figure 9. The speed loop compensation components are calculated as follows:
RSC CSC CT
SPEED COMP 1.7V
+
CT 1.7V
20kHz
PWM ON/OFF FROM ILIMIT ONE-SHOT
Figure 9. Speed Control Loop Component Connections
The voltage on SPEED COMP is compared with a ramp oscillator to create a PWM duty cycle. The PWM ramp oscillator creates a sawtooth function from 1.7V to 3.9V as shown in Figure 9. A negative clamp at one diode drop below 1.7V (approximately 1V) starts the oscillator on power up. The frequency of the ramp oscillator is set by a capacitor to ground CIOS and is selected using the following equation:
CSC =
R SC =
26.9 N VMOTOR C VCO . fSB K e 25 + 98.696 tm fSB
10 2p fSB CSC
2 2
(9a) (9b)
CT =
50mA fPWM 2.4V
1
(10)
Where fSB is the speed loop bandwidth in Hz.
Where fPWM is the PWM frequency in Hz. The PWM duty cycle from the speed control loop is gated the current limit one shot that controls the LA, LB, and LC output drivers.
11
ML4426
FUNCTIONAL DESCRIPTION
CROSS CONDUCTION COMPARATOR
When the ML4426 goes from align mode into ramp mode, there is a possibility of cross conduction in phase 3 of the bridge power stage. This cross conduction can happen when HC is on in the align mode shown as state R in Table 1, and the controller transitions to state A in ramp mode where HC is turned off and LC is turned on. Cross conduction can appear due to the differences in turn on and turn off times of the power devices. To solve this problem, the LC output driver is gated off until the HC is equal to VDD - 3V as shown in Figure 10. (Continued)
FROM COMMUTATION STATE MACHINE
FROM SPEED CONTROL LOOP & CURRENT LIMIT
HA HB GATING LOGIC & OUTPUT DRIVERS HC LA LB LC
2 3 4 9 10 11
-
1.4V
+
9.5V VDD
+ -
UV FAULT 4k REFERENCE BRAKE 25 14 VDD GND 28 27 RREF VREF
BRAKING
When the BRAKE pin is pulled below 1.4V, the low side output drivers LA, LB, and LC are turned on and the high side output drivers HA, HB, HC are turned off. Braking causes rapid deceleration of the motor and current limiting is de-activated, and care should be taken when using the BRAKE pin. BRAKE is has an internal 4kW pullup as shown in Figure 10, and can be driven by a switch to ground, an open collector or drain logic signal, or a TTL logic signal.
18
7
Figure 10. Cross Conduction, Brake, and UVLO Circuits
UNDERVOLTAGE LOCKOUT
Undervoltage lockout is used to protect the 3-phase bridge power stage from a low VDD condition. Undervoltage is triggered at VDD of 9.5V or less and is indicated by a TTL low output on the UV FAULT pin. Undervoltage lockout also turns off all output drivers (LA, LB, LC, HA, HB, and HC). The comparator that triggers undervoltage lockout has 150mV of hystresis.
DESIGN CONSIDERATIONS
INTERFACING TO A 3-PHASE BRIDGE POWER STAGE
The ML4426 output drivers are configured to drive a 3 phase bridge power stage. For applications with buss voltages from 12V up to 80V, level shifting circuitry can be used to drive higher voltage P-channel MOSFETS for the high side switches as shown in Figure 11. The most flexible configuration is to use high side drivers to control N-Channel MOSFETs (or IGBTs) which allows applications from less than 12V up to 600V. Figure 12 shows the interface between the ML4426 and IR2118 high side drivers from International Rectifier. This configuration is capable of driving motors from busses of up to 320V. The BRAKE pin can be pulsed prior to startup with an RC circuit. This charges the bootstrap capacitors (C19, C20, and C21) for the three high side drivers, allowing the reset phase to operate normally. These capacitors must be sized so that they stay sufficiently charged during the align mode. Refer to AN-43 for additional applications information on the ML4426.
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ML4426
VBUSS 24V-80V C2 330F 100V R2 10k R3 10k R4 10k
C1 100nF 100V
Q4 IRFR9120 Q1 2N6718 C3 1F Q2 2N6718
Q5 IRFR9120 Q3 2N6718
Q6 IRFR9120
12V
Q7 IRFR120 R1 470m 2W
Q8 IRFR120
Q9 IRFR120 MOTOR
R12 2k
R13 2k
R14 2k
R15 1k
C5 2.2nF
ML4425
ISENSE HA HB HC SPEED COMP R16 10k C9 100nF CT C17 1nF C12 R18 10k R21 787 VREF SPEED SET LA LB LC R5 100 R6 100 FORWARD C4 REVERSE C14 1F 12V C13 100nF F/R VCO/TACH VDD GND RREF CIOS BRAKE FB C FB B R10 (RES1) FB A C14 CRR SPEED FB CRT UV FAULT CAT RVCO CVCO R19 80.5k C15 470nF C8 1F R17 10k C6 1F C7 100nF BRAKE C16 330pF R8 (RES1) R9 (RES1) RUN S1 R20 137k
R7 100
Figure 11. Driving Lower Voltage Motors (12 to 80V)
13
ML4426
12V
IR2118
C16 100nF 25V VCC IN COM NC VB HO VS NC
D1 MUR150
IR2118
C17 100nF 25V VCC IN COM NC VB HO VS NC
D2 MUR150
IR2118
C18 100nF 25V VCC IN COM NC VB HO VS NC
D3 MUR150
C19 2.2F 25V
C20 2.2F 25V
C21 2.2F 25V
VBUSS 24V-80V
C5 330F 400V
R6 100
R7 100
R8 100
Q1 IRF720
Q3 IRF720
Q5 IRF720
Q2 IRF720 R12 470m 2W
Q4 IR720
Q6 IRF720 MOTOR
R1 1k
C1 2.2nF BOOTSTRAP PRE-CHARGE CAPACITOR GND RREF CIOS BRAKE FB C FB B R13 (RES1) FB A C13* RAMP COMP SPEED FB CRT UV FAULT CAT RVCO CVCO C8 10nF 12V R16 80.6k C9 470nF C10 1F R17 10k C12 1F BRAKE C14 330pF R15 (RES1) R14 (RES1) RUN S1 R18 137k
ML4425
ISENSE HA HB HC SPEED COMP R5 10k C3 100nF CT C4 1nF C15 100nF R20 10k R19 787 R9 100 R10 100 D4 D5 D6 (3x1N5819) R11 100 FORWARD VREF SPEED SET LA LB LC F/R VCO/TACH VDD
C11 100nF
REVERSE C6 1F
C7 100nF
Figure 12. ML4426 High Voltage Motor Drive Application Circuit
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ML4426
PHYSICAL DIMENSIONS
inches (millimeters
Package: H32-7 32-Pin (7 x 7 x 1mm) TQFP
0.354 BSC (9.00 BSC) 0.276 BSC (7.00 BSC) 25 0 - 8 0.003 - 0.008 (0.09 - 0.20)
1 PIN 1 ID 0.276 BSC (7.00 BSC) 0.354 BSC (9.00 BSC)
17
0.018 - 0.030 (0.45 - 0.75)
9 0.032 BSC (0.8 BSC) 0.012 - 0.018 (0.29 - 0.45) 0.048 MAX (1.20 MAX) 0.037 - 0.041 (0.95 - 1.05) SEATING PLANE
Package: P28N 28-Pin Narrow PDIP
1.355 - 1.365 (34.42 - 34.67) 28
PIN 1 ID
0.280 - 0.296 0.299 - 0.325 (7.11 - 7.52) (7.60 - 8.26)
1 0.045 - 0.055 (1.14 - 1.40) 0.100 BSC (2.54 BSC) 0.020 MIN (0.51 MIN)
0.180 MAX (4.57 MAX)
0.125 - 0.135 (3.18 - 3.43)
0.015 - 0.021 (0.38 - 0.53)
SEATING PLANE
0 - 15
0.008 - 0.012 (0.20 - 0.31)
15
ML4426
PHYSICAL DIMENSIONS
inches (millimeters)
Package: S28 28-Pin SOIC
0.699 - 0.713 (17.75 - 18.11) 28
0.291 - 0.301 0.398 - 0.412 (7.39 - 7.65) (10.11 - 10.47) PIN 1 ID
1 0.024 - 0.034 (0.61 - 0.86) (4 PLACES) 0.050 BSC (1.27 BSC) 0.095 - 0.107 (2.41 - 2.72) 0 - 8
0.090 - 0.094 (2.28 - 2.39)
0.012 - 0.020 (0.30 - 0.51)
SEATING PLANE
0.005 - 0.013 (0.13 - 0.33)
0.022 - 0.042 (0.56 - 1.07)
0.009 - 0.013 (0.22 - 0.33)
ORDERING INFORMATION
PART NUMBER
ML4426CP (End of Life) ML4426CS (End of Life) ML4426CH (End of Life) ML4426IP (End of Life) ML4426IS (End of Life) ML4426IH (End of Life)
TEMPERATURE RANGE
0C to 70C 0C to 70C 0C to 70C -40C to 85C -40C to 85C -40C to 85C
PACKAGE
28-Pin PDIP (P28N) 28-Pin SOIC (S28) 32-Pin TQFP (H32-7) 28-Pin PDIP (P28N) 28-Pin SOIC (S28) 32-Pin TQFP (H32-7)
(c) Micro Linear 1998.
is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners.
DS4426-01 2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com
7/6/98 Printed in U.S.A.
Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897; 5,717,798; 5,742,151; 5,747,977; 5,754,012; 5,757,174; 5,767,653;. Japan: 2,598,946; 2,619,299; 2,704,176. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application.
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