Hardware Design Considerations using the MC34929

Transcription

1 Freescale Semiconductor Application Note AN3319 Rev. 1.0, 9/2006 Hardware Design Considerations using the MC34929 By: Juan Sahagun RTAC Americas Mexico 1 Introduction This Application Note describes how to implement the hardware design when using the MC This document provides recommendations for the best way to select the external components and methods to design a hardware configuration which supports a good performance by the MC34929 Brushless DC motor driver. This document also gives a guideline to develop the PCB footprint for the QFN 24 pin package, getting an improved Thermal dissipation, and thus a better thermal performance. 2 Features The MC34929 is a complete BLDC (Brushless DC) motor driver embedded in one chip, its main features are: Operation Supply Voltage (8V-28V) Capability of controlling the BLDC motor autonomously Contents 1 Introduction Features Input Signal Characteristics External Components Selection TACH and 3XTACH Management Hall Effect Sensors and 3 Phase Output Signals Connections RISENS Considerations Recommended Schematic Thermal Analysis Footprint File Description Conclusions References Freescale Semiconductor, Inc., All rights reserved.

2 Features 3-Phase Hall Effect sensors interface Two tachometer outputs (TACH and 3XTACH) Adjustable Maximum Current Limit Short Circuit Detection and Protection Over-Temperature Detection and Thermal Shutdown Under voltage Detection and Shutdown 2 Freescale Semiconductor

3 Input Signal Characteristics 3 Input Signal Characteristics The MC34929 BLDC motor driver has 3 main logic input signals, which have an internal 100 k Ω pull up resistance connected to an internal 5V power supply: RUN/STOP: It is an inverted input signal which controls the RUN ( L ) or STOP ( H ) state of the motor. In STOP state, the internal oscillator, counters, charge pump, stall detection, and protection circuitry are disabled. The response time to wake up from a STOP state depends on the time it takes to provide the voltage at the High-Side Gate Driver (Typically 1ms) at 90% of its rated value (VGHS). DIR: It is an inverted input signal which controls the Direction of the stator movement (CW = L, CCW = H ) Freescale Semiconductor 3

4 External Components Selection This logic input could be supplied with a voltage range between 2.0 and (V+ + VF) 0.0 to +5.0 V. PWM: It is an inverted input signal which controls the speed of the motor by the variation of the pulse width and duty cycle of the PWM input signal. 4 External Components Selection The MC34929 needs 3 external capacitors as mentioned in the data sheet as follow: a) CP: is defined as a charged pumping cap and is located between CP+ and CP-, it s recommended value is 0.1 µf. b) CRES: is defined as a reservoir cap, its function with CP is to proportion a stable charge pump voltage, the recommended value of this capacitor is also 0.1 µf. Note: The switching frequency of the charge pump is approx. 250 KHz. c) CT: This capacitor is for Stall Detect Timing; the typical stall detection and protection timing is ~2s with a CT value of 0.1 µf. d) It s important to add a tantalum or electrolytic capacitor with a value of 1 uf between V+ and GND, to filter any high frequency noise. 5 TACH and 3XTACH Management These two feedback outputs are configured as follows: TACH output is the inverted logic version of the HAB hall sensor signal 3XTACH is the result of an inverted EXOR of the three hall sensors signals (HAB, HAC, HBC) Notes: * TACH and 3XTACH are open drain type outputs * The maximum voltage the output devices for the TACH and 3XTACH can tolerate is 42 Volts 4 Freescale Semiconductor

5 Hall Effect Sensors and 3 Phase Output Signals Connections 6 Hall Effect Sensors and 3 Phase Output Signals Connections This driver IC needs to have the feedback from the motor s 3 Hall Effect sensors to function properly. The Hall sensor input pins have to be connected to the motor as follows: Motor s Hall Sensor Number Hall Sensor 1 Hall Sensor 2 Hall Sensor 3 BLDC Driver ICs Input Signal Name HAB HBC HCA These feedback signals give information about the rotor position of the motor to the IC s internal logic, so that the driver IC gives the correct output sequence to move the motor. NOTE: When two of these Hall Sensor input signals are in a static condition for more than 2s the IC driver interprets this condition as a Stall detection and enters suspend mode. Using the Hall sensor s connection as shown in the previous table, the corresponding way to connect the motor s winding to the driver is as follow: Motor s Winding Number Motor Winding 1 Motor Winding 2 Motor Winding 3 BLDC Driver IC s Output Signal Name PHC PHB PHA 7 R ISENS Considerations R ISENS is located between pins LSS, ISENS and PGND. Its function is to limit the maximum amount of current delivered by the driver to the motor, and sets the maximum torque of the motor to be applied to its load. LSS is the common connection for the lower half of the 3-phase bridge that is implemented internally in the IC. The Current limit calculation depends on the follow values: Current through the motor during operation mode. Voltage between ISENS and PGND Freescale Semiconductor 5

6 RISENS Considerations Note: When the current limit exceeds 0.1V, the phase that is currently low will be brought high. The output will be released ~40ms later. The output will then follow the PWM input once again. To calculate the R ISENS value, divide the value of the maximum permitted voltage in ISENS and the current to configure the protection. R ISENS PGND = ISENS I PROTECTED PROTECTED The maximum recommended current to use as I PROTECTED, should be the bridge output continuous output current (1 Amp.) Using 1 Ampere as I PROTECTED current the R ISENS value should be: R ISENS V 0.1Volts = I V 0.1Volts ISENS PGND = = = 0. 1Ω I 1Amp PROTECTED Note: In order to properly select the I protected current, a power dissipation and thermal analysis is recommended. This will help understand the power dissipation of the IC during operating conditions. 6 Freescale Semiconductor

7 Recommended Schematic 8 Recommended Schematic 9 Thermal Analysis The thermal analysis includes some variables to determine the Junction Temperature of the IC, these variables are: RDSON: This value is = 25 C, 14V =< V+ >=28V, Io = 1.0Amps MAX VOLTAGE: Is the motor supply voltage (V+). TOTAL IOUT (TotalIout): Maximum continuous motor current limit. DUTY CYCLE: Pulse width percentage of the PWM signal. RISING AND FALLING TIME: These values are calculated for a 5.6Ω load (approximates 1.0A output current at 12V V+) Freescale Semiconductor 7

8 Thermal Analysis FREQUENCY: It is important to determine an appropriate output frequency because this value could quickly change the power dissipation, and thus the IC Junction Temperature. All of these values can now be used to accurately calculate the Power dissipation associated with switching the output signals, the DC power dissipation, and the Single Output Power, thereby obtaining the TOTAL DEVICE POWER. The corresponding Single Output Power Dissipation calculation for each MOSFET is: PowerPWM TotalIout = MaxVoltage Frequency 2 ( Ri sin gtime + FallingTime ) PowerDC TotalIout = 2 RDSON 100 DutyCycle SingleOutp utpower = PowerPWM + PowerDC Since only two MOSFET are working together at the same time, the Total Device Power is obtaining as follows: TotalDevic epower = SingleOutputPower + SingleOutputPower 1 2 With TOTAL DEVICE POWER, R THJ -AMB and the Ambient Temperature you can obtain the IC Junction Temperature. JunctionTemperature = (TotalDevicePower*RthjAmb) + Ambient Temperature The following figures show how the Junction Temperature varies depending on the duty cycle and the Frequency of the PWM input signal. 8 Freescale Semiconductor

9 Thermal Analysis TJ vs PWM Duty Cycle Figure 1. Junction Temperature vs. Duty Cycle TJ vs PWM Frequency Figure 2. Junction Temperature vs. Frequency Freescale Semiconductor 9

10 Footprint File Description Figures 1 and 2 used the following constants to make the calculations: RDSON = 0.3 ohm. TotalIout = 1 Amp. Max Voltage = 24 Volts. Rising Time = 2.5 x Falling Time = 1.75 x R THJ -AMB = 125 C/W. Ambient Temperature = 25 C. Figure 1 shows how the Junction Temperature changes when the PWM Duty Cycle varies from 1% to 100%, maintaining a constant PWM Frequency of 20kHz. Figure 2 shows how the IC Junction Temperature changes when the PWM Frequency varies from 1KHz to 100KHz, maintaining a maximum duty cycle of 99%. NOTE: If the Junction Temperature is more than 150 C (maximum permissible value) you must reduce the IC's Junction Temperature below 150 C (i.e. reduce either the duty cycle or the frequency of the PWM input signal). 10 Footprint File Description Following is shown the MC34929 package: The recommended Top Copper Area and Vias for getting the best thermal dissipation area are displayed next: 10 Freescale Semiconductor

11 Conclusions The Top copper area and thru-holes are design to get a thermal resistance (RTHJ-AMB) Junction to Ambient <125 C/W and to be able to achieve a power dissipation of 1.0W. Freescale recommends that the pad dimensions and thermal dissipation area: Have a bottom thermal ground plane of > 9cm 2 Maintain the same pitch (0.5mm) between all the pads 11 Conclusions The MC34929 is a complete BLDC driver which integrates not only the H-Bridges to control each of the motor windings, it also integrates all the circuitry required to autonomously control a BLDC motor, with little amount of external components in the system. The MC34929 driver provides the protection and diagnostics that these type of applications require. 12 References 1. Power Electronics, Converters, Applications and Design. Mohan, Undeland and Robbins. Second Edition Page John Wiley & Sons, Inc. 2. MC34939 Datasheet 3. At Use the Enter Keyword search and type "98ARH99033A" to download the 24 pin QFN package drawing Freescale Semiconductor 11

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