Circuit Note CN-0196 Devices Connected/Referenced Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0196. ADuM7234 Isolated Precision Half-Bridge 4 A Driver ADG787 2.5 Ω CMOS Low Power Dual 2:1 Switch ADuC7061 ARM7 Based Microcontroller with Dual 24-Bit Σ-Δ ADCs ADuM3100 Digital Isolator ADP1720 50 mA Linear Regulator ADCMP350 Comparator with 0.6 V Reference H-Bridge Driver Circuit Using Isolated Half-Bridge Drivers EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0196 Circuit Evaluation Board (EVAL-CN0196-EB1Z) ADuC7061 MiniKit (EVAL-ADuC7061MKZ) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit, shown in Figure 1, is an H-bridge composed of high power switching MOSFETs that are controlled by low voltage logic signals. The circuit provides a convenient interface between logic signals and the high power bridge. The bridge uses low cost N-channel power MOSFETs for both the high and low sides of the H-bridge. The circuit also provides galvanic isolation between the control side and power side. This circuit can be used in motor control, power conversion with embedded control interface, lighting, audio amplifiers, and uninterruptable power supplies (UPS). Modern microprocessors and microconverters are generally low power and operate on low supply voltages. Source and sink current for 2.5 V CMOS logic outputs ranges from µA to mA . Driving an H-bridge switching 12 V with a 4 A peak current requires the use of carefully selected interface and level translation components, especially if low jitter is needed. The ADG787 is a low voltage CMOS device that contains two independently selectable single-pole double-throw (SPDT) switches. With a 5 V dc power supply, a voltage as low as 2 V is a valid high input logic voltage. Therefore, the ADG787 provides appropriate level translation from the 2.5 V controlling signal to the 5 V logic level needed to drive the ADuM7234 half-bridge driver. The ADuM7234 is an isolated, half-bridge gate driver that employs Analog Devices’ iCoupler® technology to provide independent and isolated high-side and low-side outputs making it possible to use N-channel MOSFETs exclusively in the H-bridge. There are several benefits in using N-channel MOSFETs: N-channel MOSFETs typically have one third of the on resistance of P-channel MOSFETs and higher maximum current; they switch faster, thereby reducing power dissipation; and the rise time and fall time is symmetrical. The 4 A peak drive current of the ADuM7234 ensures that the power MOSFETs can switch on and off very fast, thereby minimizing the power dissipation in the H-bridge stage. The maximum drive current of the H-bridge in this circuit can be up to 85 A, which is limited by the maximum allowable MOSFET current. The ADuC7061 is a low power, ARM7 based precision analog microcontroller with integrated pulse width modulated (PWM) controllers that have outputs that can be configured to drive an H-bridge after suitable level translation and conditioning. CIRCUIT DESCRIPTION Level Translation of 2.5 V PWM Control Signal to 5 V The EVAL-ADuC7061MKZ delivers 2.5 V logic level PWM signals, while the minimum logic high input threshold for the ADuM7234 is 3.5V with a 5 V power supply. Because of this incompatibility, the ADG787 switch is used as an intermediate level translator. The minimum input logic high control voltage to the ADG787 is 2 V, which is compatible with the 2.5 V logic from the ADuC7061. The output of the ADG787 switches between 0 V and 5 V, which is more than sufficient to drive the Rev.0 Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2011 Analog Devices, Inc. All rights reserved. CN-0196 Circuit Note 12V 5V 5V NC VIA ADG787 5V GND 2.5V PWM0 PWM 2 ADuM7234 ENCODE 16 DECODE 15 3 14 4 13 5 12 D1 R1 VDDA Z1 13V VOA C1 R5 330µF + 100kΩ 25V R2 D2 10Ω 10Ω C1 + 330µF R6 25V 100kΩ Z2 13V GNDA 5V VDD1 D1 S1B GND1 IN1 IN2 PWM1 ADuC7061MKZ VIB S1A 1 DISABLE S2A VDD1 D2 S2B 11 6 7 ENCODE DECODE 10 NC 9 8 NC NC 12V VDDB VOB R7 GNDB 15Ω GND NC VIA VIB 1 2 ADuM7234 ENCODE 16 DECODE 15 3 14 4 13 5 12 6 11 Q1 R9 Q2 15Ω D5 D6 VDDA VOA M GNDA 5V GND1 DISABLE VDD1 NC 7 ENCODE DECODE 10 9 8 NC Q3 Q4 R10 R8 NC 15Ω 12V D7 15Ω D8 VDDB R4 470Ω VOB GNDB R3 470Ω Q1, Q2, Q3, Q4: FDP5800 D1, D2: SSC54 D5, D6, D7, D8: 15ETH06FPPBF Z1, Z2: SMAZ5928B 09644-001 VDD1 Figure 1. H-Bridge Using ADuM7234 Isolated Half-bridge Driver (Simplified Schematic: All Connections and Decoupling Not Shown) ground. On the other hand, the source voltages of the high side MOSFETs (Q1, Q2) switch as the pairs of MOSFETs are turned on and off. Therefore the optimum gate drive signals should be referenced or "bootstrapped" to this floating voltage. input of the of the ADuM7234, which has a 3.5 V threshold. Two jumpers are provided, which makes it easy to configure the polarity of the controlling PWM signal. H-Bridge Introduction The gate drive signals from the ADuM7234 offer the benefit of true galvanic isolation between each input and each output. Each output may be operated up to ±350 V peak relative to the input, thereby supporting low-side switching to negative voltages. As a result, the ADuM7234 provides reliable control of the switching characteristics of various MOSFET configurations over a wide range of positive or negative switching voltages. For safety and ease of testing, a 12 V dc supply was chosen as the power supply for this design. There are four switching elements in the H-bridge shown in Figure 1 (Q1, Q2, Q3, Q4). The switches are turned on in pairs, either high left side (Q1) and low right side (Q4) or low left side (Q3) and high right side (Q2). Keep in mind that the switches on the same side of the bridge are never on at the same time. The switches can be implemented using MOSFETs or IGBTs (insulated gate bipolar transistors) and use pulse width modulation (PWM) or other control signals from the controller to turn the switches on and off, thereby changing the polarity of the load voltage. Bootstrapped Gate Drive Circuit The sources of the low side MOSFETs (Q3, Q4) are connected to ground, therefore, their gate drive signal is also referenced to The power supplies to the gate drivers for the high-side and low-side are different. The low-side gate drive voltage is Rev. 0 | Page 2 of 6 Circuit Note CN-0196 bootstrap capacitor after the circuit is powered off and serves no function when the circuit is switching. ground-referenced, so it is developed directly from the groundreferenced dc supply. However, the high-side is floating, so a bootstrap drive circuit is useful and operates as follows. Bootstrap Current Limit Resistors (R1, R2) Let us look at the left side of the H-bridge circuit shown in Figure 1. The bootstrap drive circuit is implemented with the capacitor C1, the resistors R1 and R3, and the diode D1. Immediately after power on, the PWM does not come instantly, and all the MOSFETs are in the high impedance state until all dc voltages are settled. During this time capacitor C1 is charged by the dc supply through the path R1, D1, C1, and R3. The charged capacitor C1 provides the voltage for high-side gate drive. The time constant for C1 charging is τ = (R1 + R3) C1. The series resistor R1 provides the current limit when charging the bootstrap capacitor. If R1 is too high, the dc quiescent current from the high-side driver supply of the ADuM7234 will cause too much voltage drop across R1, and the ADuM7234 may go into undervoltage lockout. The maximum dc supply current for the ADuM7234 is IMAX = 30 mA. If the voltage dropped across R1 due to this current is limited to VDROP = 1 V, then R1 should be less than VDROP/IMAX, or 33 Ω. Therefore, a resistor of 10 Ω was selected as the bootstrap resistor. When the MOSFETs switch due to the PWM signal, the lowside switch Q3 is turned on, and the high-side switch Q1 is turned off. The GNDA of the high-side is pulled down to ground, and the capacitor C1 is charged. When Q1 is turned on, and Q3 is turned off, the GNDA is pulled up to the dc supply voltage. The diode D1 is reversed biased, and the C1 voltage forces the VDDA voltage of the ADuM7234 to approximately 24 V. The capacitor C1, therefore, maintains a voltage of approximately 12 V between the VDDA and GNDA terminals of the ADuM7234. In this manner, the gate drive voltage to the high-side MOSFET Q1 is always referenced to the floating source voltage of Q1. Bootstrap Start-Up Resistors (R3, R4) Voltage Spikes on the Source of the High-Side MOSFET When Q1 and Q4 are on, the load current flows from Q1 through the load to Q4 and to ground. When Q1 and Q4 are turned off, the current still flows in the same direction through the free-wheeling diodes D6 and D7, producing a negative spike on the source of Q1. This can damage some gate drivers employing other topologies but has no effect on the ADuM7234, which supports low-side switching to negative voltages. Bootstrap Capacitors (C1, C2) The bootstrap capacitor is charged every time the low-side driver is on, and it is discharged only when the high-side switch is turned on. Therefore, the first parameter to consider in selecting the value of the bootstrap capacitor is the maximum allowable voltage drop when the high-side switch is turned on and the capacitor is used as the dc supply for the high-side of the gate driver ADuM7234. When the high-side switch is turned on, the dc supply current of ADuM7234 is typically 22 mA. Assume that the on-time of high-side switch is 10 ms (50 Hz, 50% duty cycle). Using the equation C = I × ΔT/ΔV, the capacitor should be larger than 220 µF if the allowable voltage drop ΔV = 1 V, I = 22 mA, and ΔT = 10 ms. In this design, a value of 330 µF was chosen. The resistor R5 discharges the The resistor R3 starts up the bootstrap circuit. Immediately after power-on, dc voltages are not established, and the MOSFETs are off. Under these conditions C1 is charged through the path R1, R3, D1, VS, described by the following equation: v C ( t ) = (VS − VD )(1 − e − t / τ ) , (1) where vC(t) is the capacitor voltage, VS is the supply voltage, VD is the diode voltage drop, and τ is the time constant, τ = (R1 + R3) C1. The circuit values are R1 = 10 Ω, C1 = 330 µF, VD = 0.5 V, and VS = 12 V. From the equation, it takes one time constant (158 ms) to charge the capacitor to 67% of its final value for R3 = 470 Ω. A larger value resistor will increase the charging time of the capacitor. However, when the high-side MOSFET Q1 is turned on, there will be 12 V across resistor R1, so if the value of the resistor is too low, it will dissipate considerable power. For R3 = 470 Ω, the power dissipation in the resistor at 12 V is 306 mW. Overvoltage Protection for the Bootstrap Capacitors (Z1, Z2) As mentioned above, with inductive load, when the high side MOSFET turns off, the current will flow through the freewheeling diodes. Due to the resonance between the inductance and parasitic capacitance, the charging energy to the bootstrap capacitor may be higher than the dissipated energy of the ADuM7234, and the voltage on the capacitor may rise to an overvoltage condition. The 13 V Zener diode clamps the voltage on the capacitor, thereby avoiding the overvoltage situation. Gate Drive Resistors (R7, R8, R9, R10) The gate resistors (R7, R8, R9, R10) are selected based on the desired switching time, tSW. The switching time is the time required to charge Cgd and Cgs and to charge the switching MOSFETs to the required charge, Qgd and Qgs. Rev. 0 | Page 3 of 6 CN-0196 Circuit Note FROM DC SOURCE OR BATTERY VDD +12V +5V_1 U4 4 2 VDD2 VDD1 1 4 GND2 VDD1 3 VI 2 1 GND2 5 VO 1 2 ADCMP350 3 GND 4 OUT R12 33kΩ 1% 2 0805 VIN 1 R13 2kΩ 1% 2 0805 1 GND UNDERVOLTAGE 2 ADuM3100 +12V +5V_1 U5 1 VCC U6 R15 470Ω TO +12V FIGURE 1 1 2 L1 22µh + C25 + C26 + C27 + C28 + C22 4700µF 4700µF 4700µF 4700µF 0.1µF 25V 25V 25V 25V 16V 2 IN 4 EN 5 GND GND 6 OUT 3 GND 8 1 GND GND 7 ADP1720 09644-002 +5V_2 Figure 2. Power Rail Filtering and Undervoltage Lockout Protection for ADuM7234 The following equation describes the gate drive current, Ig: Ig = Q gd + Q gs t SW = VDD − Vgs ( th ) R g + R DRV (2) where VDD is the supply voltage and RDRV is the equivalent resistance of the gate driver ADuM7234, Vgs(th)is the threshold The ADuM7234 undervoltage lockout is prevented by disabling the input to the ADuM7234 when the power supply voltage is less than 10 V using the circuit shown in Figure 2. The circuit is disabled by applying a logic high to the DISABLE pin of the ADuM7234. From the FDP5800 MOSFET data sheet, Qgd = 18 nC; Qgs = The open-drain, active low ADCMP350 comparator is used to monitor the dc supply voltage. The ratio of the resistor divider (R12, R13) is chosen such that when the supply voltage is 10.5 V, the divider output is 0.6 V, which is equal to the 0.6 V on-chip reference of the comparator. When the supply voltage drops below 10.5 V, the output of the comparator goes high. Because the input and output side of the ADuM7234 are galvanically isolated, the DISABLE signal from the output side must be transferred to the input side through an isolator. The ADuM3100 is a digital isolator based on the iCoupler technology. The ADuM3100 is compatible with 3.3 V and 5 V operation. The 12 V filtered supply drives the ADP1720 linear regulator, which supplies the 5 V (+5V_1) for the right-hand isolated side of ADuM3100, as shown in Figure 2. 23 nC; Vgs(th) = 1 V. The Load and the PWM Signal If the desired switching time, tSW , is 100 ns, Equation 2 is solved for Rg, yielding Rg ~ 22 Ω. In the actual design, 15 Ω If we use an inductor as the load, the current flowing through the inductor will change linearly if a constant voltage is applied. The voltage, U, is 12 V, and if we ignore the voltage drop across the MOSFETs due to the on–resistance, the following equation is true: voltage, Rg is the external gate drive resistor, Qgd and Qgs are the required MOSFET charge, and tSW is the required switching time. The equivalent resistance of the ADuM7234 gate driver is calculated from: R DRV = VDDA I OA (SC) (3) According to the ADuM7234 data sheet, for VDDA = 15 V and the output short circuit pulsed current IOA(SC) = 4 A. Therefore, RDRV is about 4 Ω from Equation 3. resistors were chosen to allow for some margin. Power Rail Filtering and Undervoltage Protection Because of the high peak load current, the dc source voltage (VDD) must be properly filtered to prevent the ADuM7234 from going into undervoltage lockout and also to prevent possible damage to the supply. The filter chosen consists of four parallel 4700 µF, 25 V capacitors in series with a 22 µH power inductor, as shown in Figure 2. The specified maximum rms ripple current of the capacitors at 100 kHz is 3.68 A. Since four of these capacitors are in parallel, the largest allowable rms ripple is 14.72 A. Therefore, IPEAK = 2√2 × IRMS = 41.63 A. The filtered +12 V also drives the circuit shown in Figure 1. U=L di dt (4) With a 50 kHz, 8% duty cycle PWM signal and a 4 µH Coilcraft power inductor (SER2014-402) as the load, the load current waveform is as shown in Figure 3. A current probe is used to measure the inductor current. For a 12 V supply voltage and a 4 µH inductor, Equation 4 predicts a slope of 3 A/µs. The actual slope was measured to be 2.8 A/µs, and the reduction is due to the voltage dropped across the on resistance of the MOSFETs. Rev. 0 | Page 4 of 6 Circuit Note CN-0196 20µs 1.6µs PWM0 PWM1 +4.4A SLOPE = 2.8A/µs LOAD CURRENT 0A 09644-003 –4.4A Figure 3. Load Current as a Function of the PWM Pulses with a 4 µH Load Note that there is a small amount of ringing on the waveform immediately after the current turns off. This is due to the resonance between the inductive load and the parasitic capacitance of the free-wheeling diodes and the MOSFETs. • EVAL-CN0196-EB1Z circuit evaluation board • EVAL-ADuC7061MKZ evaluation board • DC supply or battery: +12 V, 10 A It is important that the maximum rated current of the inductor is not exceeded in the circuit. If this occurs, the inductor goes into saturation, and the current will increase rapidly, which can damage the circuit and the power supply. The Coilcraft SER2014-402 inductor used as a load in the circuit has a rated saturation current of 25 A. COMMON VARIATIONS The circuit can easily be expanded to a 3-phase control application with some additional components. The circuit can also be used in applications requiring higher supply voltages, but care must be taken to ensure the ratings of the MOSFETS and filter capacitors are not exceeded. Equipment Needed • PC with a USB port and Windows XP, Windows Vista (32-bit), or Windows 7 (32-bit) • Load, such as Coilcraft SER2014-402 power inductor • Oscilloscope with current probe Getting Started Load the evaluation software by placing the CN0196 evaluation software disc in the CD drive of the PC. Locate the drive that contains the evaluation software disc and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software. Setup and Test Download the firmware code to EVAL-ADuC7061-MKZ, install CN-0196 evaluation software, and connect the controlling signal from EVAL-ADuC7061-MKZ and EVAL-CN0196-EB1Z according to jumper configuration in the Readme file. Rev. 0 | Page 5 of 6 CN-0196 Circuit Note 12V DC SUPPLY OR BATTERY +12V GND EVAL-ADUC7061-MKZ PC MOTOR OR INDUCTOR EVAL-CN0196-EB1Z 2.5V LOGIC SIGNALS USB CURRENT PROBE 09644-004 OSCILLOSCOPE Figure 4. Functional Block Diagram of Test Setup Data Sheets and Evaluation Boards Connect the jumper LK1, apply +12 V power to CN2, launch the software, and connect the USB cable from the PC to the USB mini-connector on the EVAL-ADuC7061-MKZ board. Using an inductor as the load, run the software and use a current probe to measure the current of the inductor. CN-0196 Circuit Evaluation Board (EVAL-CN0196-EB1Z) ADuM7234 Data Sheet ADuM7234 Evaluation Board Information and details regarding to how to use the evaluation software for proper PWM signals can be found in the CN0196 Evaluation Software Readme file. ADuC7061 Evaluation Board ADG787 Data Sheet ADuM3100 Data Sheet LEARN MORE ADP1720 Data Sheet CN-0196 Design Support Package: www.analog.com/CN0196-DesignSupport ADCMP350 Data Sheet Ardizzoni, John. “A Practical Guide to High-Speed PrintedCircuit-Board Layout.” Analog Dialogue. 39-09, September 2005. MT-101 Tutorial, Decoupling Techniques, Analog Devices. ADuC7061 Data Sheet REVISION HISTORY 9/11—Revision 0: Initial Version (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09644-0-9/11(0) Rev. 0 | Page 6 of 6