A single-chip power IC using third generation BCD technology drives small BLDC motors for applications that run from power supplies up to 60V and loads up to 3A.
Brushless dc (BLDC) motors consist of a stator with phase winding and a permanent magnet rotor. In the typical configuration, a rotating field is generated through the control system that switches current to two phases while maintaining the third phase inactive. The magnetic attraction due to the rotating field produces the active torque. The associated motor drive system must manage the phase angle between the stator and rotor field to minimize the torque ripple. This involves use of position feedback based on Hall Effect sensors.
Designers favor these 3-phase brushless dc motors because of their high efficiency (up to 95%) and small size for a given delivered power. To meet this need, the L6235 power IC provides a complete motor drive system for a 3-phase BLDC motor. The L6235 integrates a six transistor power DMOS stage, the position control system for BLDC Hall Effect-sensored motor, the PWM current control loop for torque control, plus other functions for safe operation and flexibility.
Ic Description Fig. 1 is the L6235's block diagram. Its supply section includes a bandgap reference for the 5V regulator, which supplies all the internal circuitry. It also generates 10V used to supply the bootstrap oscillator output for the charge-pump circuit and low-side driver. The power stage has three half-bridge outputs using power n-channel DMOS transistors.
For current control, the Ic includes a high-speed programmable off-time PWM circuit (up to 100kHz). In operation, the sensing comparator monitors the motor current as it flows through the external resistor and triggers the programmable monostable that sets the off-time on the positive going edge of the sensing comparator output. The logic section consists of a state machine capable of decoding the logic signals produced by the motor's three Hall Effect sensors that detect rotor position.
Besides current control, the Ic also provides built-in overcurrent detection and protection by sensing the current in the high-side power devices. If an overcurrent condition (typically .5.6A) occurs, the device will signal this to the diagnostic output, which in turn can be used to implement overcurrent protection. There are several other protection functions, including undervoltage lockout and dead time control on the power bridges to avoid high-current shoot-through and thermal shutdown. The inhibit function puts the device into a low power consumption (typical 1mA). Other specialized functions included in the Ic are the brake and tachometer.
DMOS Power Half-Bridge The half-bridge output consists of power n-channel DMOS with a breakdown voltage in excess of 60V and capability to deliver 53Adc (6A peak). Each power DMOS has a typical R subscript DS(on) of 300mV.
There are several advantages for driving dc motors with power DMOS: - Low driver forward voltage drop,
- Very high-peak current, and
- Significantly lower power dissipation using synchronous rectification.
The high-side driver stage is shown in Fig. 2. The power DMOS is switched off by an n-channel MOS (M3) connected between its gate and source. In this way, gate charge is not dissipated to ground, but delivered to the output increasing efficiency. In addition, this circuit topology (for low threshold power DMOS) avoids the risk of the output DMOS turning on when its source becomes negative due to an inductive load.
When input logic signal IN goes high the switches S1 and S3 are ON and S2 is OFF, so the input capacitance of M2 and the power DMOS are charged respectively by currents I and Ic. When the power DMOS is fully on, only a small current, just enough to overcome the gate leakage, is absorbed from V subscript boot. As soon as logic signal IN goes low, S1 and S3 are off and S2 on. In this condition M3 is turned on to quickly discharge the gate equivalent capacitance of the power DMOS.
PWM Current Control These Ics incorporate a constant off-time PWM circuit to control the load current in each motor winding. It consists of a sensing comparator that monitors the motor current flowing through the external resistor and a programmable t subscript off-time used by a monostable circuit triggered on the positive edge of the sensing comparator output. When the current through the motor exceeds the threshold (fixed by the ratio between the reference voltage and the sensing resistor, R subscript sense), the current sense comparator switches off the output driver. Then, the current path during the 1sec dead time protection decreases in fast decay. Then, the load current recirculates in a slow decay mode until the monostable voltage decreases to the valley threshold of 2.5V. During this recirculation the current decreases until the fixed off time expires. Then, the appropriate output drivers are switched on again and the cycle is repeated.
In the slow decay mode the sink driver is disabled, both high-source drivers are enabled and the current recirculates via synchronous rectification through the sink and source driver of the same half-bridge. Fig. 3 shows the current control circuitry and the current waveform. During the time t subscript off in chopping current control, the current flows in the high-side loop and does not flow through the sensing resistor. If t subscript off is set so small that the winding current cannot decrease below the threshold during the off time, current regulation is still guaranteed. The monostable maintains the reset state until the current falls below the fixed threshold even if the off time has expired. To avoid false triggering due to current spikes when the power MOS transistors are switched on, a digital blanking time is imposed after the power state is turned back on.
Overcurrent Detection Besides PWM current control, a novel overcurrent detection circuit (OCD) is integrated for full protection. This circuit ensures protection against a short circuit to ground and between two phases of each bridge. A logic output masking time avoids the unwanted turnoff due to current spikes when the power MOS transistors are switched on. Fig. 4, on page 50, shows the overcurrent detection block circuit and Fig. 5, on page 50, shows the circuit details.
The power MOS is composed of thousands of individual, identical transistors connected in parallel, so the overall drain-source current is distributed between the power and power sense according to the cell ratio "n." Since "n" can be more than of 1/10000, this current mirroring allows an easy power dissipation. The auto biasing operational amplifier allows a very low differential input offset current, high-slew rate and input common mode voltage range in excess of 60V. The output stage is realized with an appropriate ratio "m" between the n-channel MOS M1-M2, which wastes silicon area. The OCD comparator compares the internal reference current, I subscript ref (V subscript bg/R) with I subscript sense (I subscript load/ n), therefore, the overcurrent protection, I subscript CC, is given by:
I subscript CC4I subscript ref1n1m
If a fault condition is detected, the ENABLE pin is pulled below the turn off threshold (1.27V typical) by an internal open drain M3 with a pull down capability of 4 mA. By using an external R-C on the ENABLE pin, the off time before recovering normal operation can be easily programmed by means of the accurate thresholds of the logic inputs. The trip point of this protection is internally set at 5.6A. Figs. 6, on page 50, and 7, on page 52, show the OCD operation.
Decoding Logic The logic section consists of a state machine capable of decoding the logic signals produced by three Hall Effect sensors that detect rotor position in a 3-phase BLDC motor. This circuit can discriminate between the actual sensor positions for sensors spaced at 60, 120, 240 and 300 electrical degrees. This decoding method allows the implementation of a universal Ic, without dedicating pins to select the sensor configuration.
There are eight possible input combinations for three sensor inputs. Six combinations are valid for rotor positions with 60 electrical degrees sensor phasing; the remaining two are valid for rotor positions with 120 electrical degrees phasing. Therefore, the decoder can resolve motor rotor position to within a window of 60 electrical degrees for both 60 and 120 electrical degrees sensor phasing.
The sequence of the Hall codes for 300 electrical degrees phasing is the reverse of 60 and the sequence of the Hall codes for 240 phasing is the reverse of 120. So, by decoding the 60 and the 120 codes it is possible to drive the motor with all the four conventions by just changing the direction set.
If 1 means ON and 0 means OFF for each power switch (top and bottom) of each half-bridge, the decoding logic the Table, on page 54, defines the six steps for both 120 and 60 electrical degrees Hall Effect sensors. The minus sign on the top of the phase number means that the current is sunk from the phase, otherwise it is sourced to the phase.
Special Functions Special protection circuitry has been implemented to guarantee the fully protection of the Ic.
Undervoltage lockout (UVLO) monitors the supply voltage turn on and off thresholds (7V and 6V, respectively, independent of temperature). This always guarantees a valid supply to the control logic circuits of the device. All outputs are disabled when the voltage is below these thresholds.
Overtemperature protection (OVT) turns off the power outputs if the junction temperature exceeds 170C with a thermal hysteresis of 20C. In case of an overtemperature condition the DIAG open drain output is activated.
An inhibit function is available for low power conditions. You can activate it by forcing a voltage less than 1.2V on the pin used to program the PWM off-time. If the inhibit is activated, the Ic goes to a standby mode with very low current consumption (1mA, typical).
You obtain the brake function by switching on the upper DMOS of the enabled half-bridges. This causes the current to decrease to zero. After a demagnetization time given by the L/R constant of the motor, the motor changes direction. The current rises to a value given by the back electromotive force (BEMF) divided by the resistance of the motor winding. This reverse current produces a torque, in opposition to the rotation of the motor, which brakes the motor. The reverse current, depending on BEMF and resistance values, might reach high values; however, the Ic will work in a safe operation mode because it can detect and limit current to 5.6A - thanks to the OCD function activated even during braking.
A tachometer function consists of a monostable, with constant t subscript off, whose input is a Hall Effect signal. It allows development of an easy speed control loop by using an external op amp, as shown in Fig. 11. The monostable output drives an open drain output pin. At each positive transition of the Hall Effect sensors it maintains the pulse for a constant time (Fig. 8).
The off-time can be set through an external RC network connected to a dedicated pin (RC pulse). The tachometer pin with an external pull-up provides an output signal, whose average value is proportional to the frequency of the Hall Effect signal - and so to the motor speed. An op amp, configured as an integrator, filters the signal and compares it with a reference voltage which sets the speed of the motor.
An ENABLE pin allows you to disable the device to achieve PWM current control in fast decay mode.
The FWD/REV input selects the forward or reverse rotation of the motor.
Packaging, Thermal Management The Ic is available in different packages, depending on the power dissipation required by the application. The PowerS036 package may be used for higher power applications, whereas the Power DIP24 and S024 packages are suitable for lower current applications. Fig. 9 shows the rms output current that the Ic can deliver to the motor vs the Ic power dissipation for different switching frequencies, considering a supply of V subscript s424V.
When the PowerSO36 package is used and the slug soldered on board (FR4 with a thickness of 1.5 mm) and a dissipating footprint of 6 sq cm (copper thickness of 70m), the Rq subscript ja is about 35C/W. Fig. 10, on page 52, shows mounting methods for this package. Using a multilayer board with vias to a ground plane, you can reduce the thermal impedance to as low as 15C/W superscript . At a temperature of 50C/W, the Ic can dissipate more than 5W, delivering a dc load to the motor of 2.2A using the Power SO package-without additional heat sink.
The Ic is realized in a third generation BCD (Bipolar, CMOS, DMOS) technology. This process allows the integration of bipolar, low- and medium-voltage CMOS for analog and logic circuits and power DMOS transistors with a breakdown voltage of 60V and 80V, as well as lateral DMOS transistors up to 40V, and HV p-channel MOS able to withstand 85V.
Application Fig. 11, on page 52, shows an application using the L6235 with torque and speed control loops. Here, few external components are required to implement a complete speed control. One analog input (V subscript refset) sets the speed, then a filtering circuit closes the speed loop on the tachometer output.
You implement overcurrent protection with a simple RC discharged by the diagnostic output if an overcurrent occurs. This disables the chip (by taking the enable input low) for a period determined by the RC time constant.
Other than the op amp for the speed control, the only additional active components are the two diodes used in the bootstrap circuit. References: 1. P. Cassati and C. Cognetti, "A New High Power Ic Surface Mount Package. PowerSO-20E Power," AN66810894, STMicroelectronics, 1994.