Application Report SLOA198 – September 2014 DRV8662, DRV2665, and DRV2667 Configuration Guide Brian Burk .................................................................................................... Haptic and Piezo Products ABSTRACT The DRV8662, DRV2665, and DRV2667 are high-voltage, Piezo drivers each with an integrated boost converter and amplifier. The integrated high-voltage solution eliminates design complexities and reduces the overall solution size for driving Piezos. These devices have been designed for simplicity and are easy to configure; however, there are some common design issues that should be avoided. This application note discusses boost converter basics, hysteretic boost operation, and describes the proper steps for configuring the boost converter including calculating the load requirements, selecting the inductor, current limit resistor, input capacitor, and output capacitor. 1 2 3 4 5 6 7 8 9 10 11 Contents Boost Converter Basics ..................................................................................................... 3 DRV8662, DRV2665, and DRV2667 Boost Converter ................................................................. 4 2.1 DRV8662, DRV2665, and DRV2667 Boost Converter Efficiency ............................................ 5 2.2 DRV8662, DRV2665, and DRV2667 Boost Converter Load Regulation .................................... 6 Configuring the Boost Converter........................................................................................... 7 Boost Converter Output Voltage ........................................................................................... 7 Calculating the Load Current ............................................................................................... 8 Selecting an Inductor ........................................................................................................ 8 6.1 Inductance Rating .................................................................................................. 9 6.2 Saturation Current Rating.......................................................................................... 9 6.3 Thermal Current Rating .......................................................................................... 10 6.4 Choosing REXT ...................................................................................................... 11 6.5 What to Avoid: Using Incorrect Inductor Current Ratings .................................................... 11 Calculate the Maximum Boost Current .................................................................................. 13 Output Capacitor Selection ............................................................................................... 13 Input Capacitor Selection.................................................................................................. 14 PCB Layout ................................................................................................................. 16 10.1 What to Avoid: Incorrect Inductor Placement .................................................................. 17 Examples .................................................................................................................... 20 11.1 Example: Based on the DRV8662EVM ........................................................................ 20 11.2 Example: Based on the DRV2667EVM-CT with 25-nF Piezo Module ..................................... 24 List of Figures 1 Efficiency with VBST = 30 V .................................................................................................. 5 2 Efficiency with VBST = 55 V .................................................................................................. 5 3 Efficiency with VBST = 80 V .................................................................................................. 5 4 Efficiency with VBST = 105 V ................................................................................................ 5 5 Load Regulation with VBST = 30 V .......................................................................................... 6 6 Load Regulation with VBST = 55 V .......................................................................................... 6 7 Load Regulation with VBST = 80 V .......................................................................................... 6 8 Load Regulation with VBST = 105 V ........................................................................................ 6 9 Inductance vs DC Current................................................................................................. 10 SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 1 www.ti.com 10 SW Node Parasitic Capacitance ......................................................................................... 17 11 Inductor Charging with Parasitic Capacitance .......................................................................... 18 12 Simplified Version of the DRV8662EVM ................................................................................ 20 13 Actual Load Regulation and Boost Efficiency 14 15 16 17 2 .......................................................................... Boost Efficiency............................................................................................................. Simplified Version of the DRV2667EVM-CT Schematic .............................................................. Boost Voltage vs. Boost Current - DRV2667EVM-CT-80 V .......................................................... Efficiency vs. Boost Current - VBST = 80 V .............................................................................. DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 23 23 24 27 27 SLOA198 – September 2014 Submit Documentation Feedback Boost Converter Basics www.ti.com 1 Boost Converter Basics A boost converter, as the name implies, converts a low voltage power rail and boosts it to a higher voltage. A boost converter consists of three main components: an inductor, a switch (MOSFET), and a diode. The basic boost converter schematic is shown in the following figure. L D VDD VBST SW COUT To convert the low voltage to a high voltage, the boost converter operates in two phases. In the first phase the inductor is charged by closing the switch and forcing current through the inductor to ground. During this period the current through the inductor increases allowing the inductor to store charge. L D VDD VBST IL ISW SW COUT Once the inductor current reaches a maximum threshold, the switch opens and forces the inductor to dump the stored charge through the diode and onto the output capacitor and load. L D VDD VBST IL ID SW COUT By repeating this charge and dump process, the boost converter is able to increase the output voltage. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 3 DRV8662, DRV2665, and DRV2667 Boost Converter 2 www.ti.com DRV8662, DRV2665, and DRV2667 Boost Converter The DRV8662, DRV2665, and DRV2667 use a hysteretic boost converter design to generate the high voltage needed to drive Piezos. This section describes the basic operating principle of the hysteretic boost converter. The hysteretic boost converter uses fairly simple feedback to control the timing and frequency of the switch. The ILIM value in the following figure is the peak current through the inductor every time the switch turns on. Once the peak current is reached, the switch opens. The peak inductor current of the DRV8662, DRV2665, and DRV2667 is set by the resistor on the REXT pin (pin 15). Inductor Current (A) - ISW ILIM REXT = K VREF - RINT ILIM Diode Reverse-Recovery Time (s) 1/fSW The boost converter only switches when the output voltage (VBST) is below the final target value, meaning that it will only switch when it needs to. Unlike a fixed-frequency boost converter design, the hysteretic boost converter design has a continually varying switching frequency and is load-dependent. Note that the DRV8662, DRV2665, and DRV2667 have forced switching at approximately 37 kHz. 4 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Boost Converter www.ti.com 2.1 DRV8662, DRV2665, and DRV2667 Boost Converter Efficiency The boost converter efficiency for the DRV8662, DRV2665, and DRV2667 is shown in Figure 1 through Figure 4. The measurements were taken using the DRV8662EVM. 2.1.1 Boost Efficiency vs Boost Current 100 100 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 90 80 80 70 Efficiency (%) 70 Efficiency (%) VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 90 60 50 40 60 50 40 30 30 20 20 10 10 0 0 0 10 20 30 40 Boost Current (mA) 50 60 68.9 0 Figure 1. Efficiency with VBST = 30 V 20 30 Boost Current (mA) 40 45.6 D001 Figure 2. Efficiency with VBST = 55 V 100 100 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 90 80 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 90 80 70 Efficiency (%) 70 Efficiency (%) 10 D001 60 50 40 60 50 40 30 30 20 20 10 10 0 0 0 5 10 15 Boost Current (mA) 20 0 2 D003 Figure 3. Efficiency with VBST = 80 V SLOA198 – September 2014 Submit Documentation Feedback 25 4 6 8 10 Boost Current (mA) 12 14 16 D004 Figure 4. Efficiency with VBST = 105 V DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 5 DRV8662, DRV2665, and DRV2667 Boost Converter 2.2 www.ti.com DRV8662, DRV2665, and DRV2667 Boost Converter Load Regulation The boost converter load regulation for the DRV8662, DRV2665, and DRV2667 is shown in Figure 5 through Figure 8. The measurements were taken using the DRV8662EVM. Boost Regulation vs Current 35 60 30 50 25 Boost Voltage (V) Boost Voltage (V) 2.2.1 20 15 10 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 5 40 30 20 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 10 0 0 0 10 20 30 40 Boost Current (mA) 50 60 68.9 0 Figure 5. Load Regulation with VBST = 30 V 20 30 40 Boost Current (mA) 50 60 67.6 D006 Figure 6. Load Regulation with VBST = 55 V 120 90 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 80 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 100 Boost Voltage (V) 70 Boost Voltage (V) 10 D005 60 50 40 30 80 60 40 20 20 10 0 0 0 10 20 30 40 Boost Current (mA) 50 60 68.9 Figure 7. Load Regulation with VBST = 80 V 6 0 10 D007 20 30 40 Boost Current (mA) 50 60 67.6 D008 Figure 8. Load Regulation with VBST = 105 V DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Configuring the Boost Converter www.ti.com 3 Configuring the Boost Converter This section describes the basic steps for configuring the boost converter. See the following sections for more details. 1. Select the boost voltage. Based on the rated voltage of the load, set the feedback resistors to the appropriate boost voltage (Section 4). 2. Calculate the load current. Estimate the maximum required load current to support the voltage, capacitance, and frequency of the load (Section 5). 3. Select an inductor. Choose an inductor with an inductance value between 3.3 µH and 22 µH, and a saturation current rating more than 1 A (Section 6). 4. Set the current limit and REXT resistor. Calculate the REXT resistor based on the saturation current of the inductor (Section 6.4). 5. Compare the maximum boost current. Compare the maximum boost current with the required load current calculated in step 2 (Section 7). 6. Choose an input and output capacitor. Input and output capacitors help reduce the effects of ripple and transient load currents on the supply and output voltages (Section 8). 7. Verify performance. Verify the performance of the components selected. 4 Boost Converter Output Voltage The boost converter should be set based on the rated or maximum voltage required by the load. An additional 5 V should be added to provide headroom when using the amplifier. Use the following equation to calculate the boost voltage. VBOOST = VPEAK + 5 V Symbol VBOOST VPEAK (1) Description Boost output voltage Peak amplifier output voltage Value - Unit V V The boost output voltage is programmed by two external resistors shown in the following diagram: VBST DRVxxxx R1 FB R2 The boost feedback resistors can be calculated using equation Equation 2: æ R ö V = V ç1 + 1 ÷ FB ç BOOST ÷ è R2 ø (2) NOTE: Ensure that the sum of R1 and R2 is greater than 400 kΩ to prevent large leakage currents due to high voltages on VBOOST. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 7 Calculating the Load Current 5 www.ti.com Calculating the Load Current The peak load current of the DRV8662, DRV2665, or DRV2667 on the OUT+ and OUT– terminals can be approximated using Equation 3. I(load-peak) = 2π × CLOAD × VBOOST × ƒMAX Symbol CLOAD VBOOST fMAX (3) Description Load capacitance Boost voltage Maximum output frequency Value – – – Unit F V f Use the load parameters capacitance, boost voltage (peak output voltage), and maximum output frequency to estimate the peak current required at the load. This provides a rough current approximation that can be used to estimate the inductor size for the boost design. 6 Selecting an Inductor The inductor plays a critical role in the performance of the DRV8662, DRV2665, and DRV2667, so selecting and testing a suitable inductor is important to ensure the best performance. An inductor can be described with relatively few parameters. The following table shows the typical parameters listed in an inductor datasheet: Part Number Inductance (µH) CIG22E3R3SNE 3.3 DC Resistance (Ω) 0.200 ISATURATION (A) IRMS SRF (MHz) 1.1 1.30 42 DEFINITIONS Inductance – the primary functional parameter of an inductor. DC Resistance (DCR) – the resistance in the inductor due to the wire ISATURATION or Saturation Current – the peak current flowing through the inductor that causes the inductance to drop due to core saturation. IRMS or RMS Current or Thermal Current – the amount of continuous RMS current flowing through the inductor that causes the maximum allowable temperature rise. SRF or Self Resonant Frequency – the frequency at which the inductance of the inductor winding resonates with the capacitance of the inductor winding. To help narrow the number of inductors quickly, begin by looking at these three parameters: spaceer 1. Inductance – the range of recommend inductances is from 3.3 µH to 22 µH. 2. Saturation Current (ISAT, 30% decrease in inductance due to DC current) – saturation current should typically be above 1 A for most applications, but will vary depending on the load. Use the examples at the end of this document as a reference for choosing an inductor. 3. RMS Current or Thermal Limit – the RMS current is less of an issue for haptic applications because of the low duty cycle of operation; however, in other applications where continuous operation is likely, be sure to select an appropriate RMS current rating. spaceer Inductor current ratings are always the biggest source of confusion when selecting an inductor, because there are multiple, non-standardized current ratings to look for. See Section 6.5 for more information on inductor current ratings. 8 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Selecting an Inductor www.ti.com See the following sections for more information on each parameter. NOTE: The inductor will see high voltages (VBST – VDD) during normal operation. Ideal inductors do not have a voltage rating and thus most manufacturers will not publish a voltage rating; however, certain inductor core materials have voltage limitations. Please contact the manufacturer and ensure that the inductor material can operate at high voltages. 6.1 Inductance Rating The inductance sets the maximum switching frequency of the boost converter. The general trade off with inductances between 3.3 µH and 22 µH are: • Larger inductances (10 µH or greater) – Advantage: Cause the boost converter to run at a lower switching frequency meaning less switching losses. – Disadvantage: Larger values typically have higher series resistance and lower saturation currents, requiring physically larger inductors. • Smaller inductances (Less than 10 µH) – Advantage: Typically have higher saturation currents and are a better choice for maximizing output current of the boost converter per inductor area. – Disadvantage: Higher switching frequencies can lead to more losses. Switching losses are not a major concern in most applications, but if thermal dissipation is a concern because of a small PCB or extreme temperatures, then consider using a larger inductance. The approximate switching frequency can be calculated using Equation 4: æ 1 ö 1 ƒ switching = ILIMIT ´ L ç + ÷ è VIN VIN - VBOOST ø Symbol fswitching VIN VBOOST ILIMIT (4) Description DRVxxx switching frequency Minimum VDD voltage applied to the DRVxxxx Maximum boost output voltage Current limit set by the DRVxxxx REXT resistor Value – – – – Unit Hz V V Ap Tip Smaller inductances (3.3 – 4.7 µH) are often preferred in space-constrained applications because of their size, higher saturation current, and ability to deliver more charge to the load. 6.2 Saturation Current Rating Saturation current is the second-most important parameter of an inductor when using a hysteretic boost converter. Inductor saturation current is typically measured as the peak current that causes the inductance to decrease by 30%. This is the maximum operating current of the inductor. In Figure 9, the saturation current for the 3.3-µH inductor is approximately 400 mA, which is the current that causes the inductance to reduce to 2 µH or by about 30%. A graph like Figure 9 can be used to determine and verify the saturation current rating of a specific inductor. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 9 Selecting an Inductor www.ti.com Figure 9. Inductance vs DC Current The inductor saturation current value affects two things in the DRV8662, DRV2665, and DRV2667 boost design: 1. The amount of current that can be delivered to the load; the larger the saturation current, the larger amount of current can be delivered to the load. 2. The value of the current limit resistor (REXT) for the DRV8662, DRV2665, and DRV2667. It should be set equal to or less than the saturation current of the inductor. Section 6.4 describes how to choose the correct current limit resistor. ILIM_DRVxxxx ≤ ISaturation_inductor (5) Remember that the current limit on the DRV8662, DRV2665, and DRV2667 is not a safety mechanism, but a threshold to signal when the boost switch should open. Tip Often the saturation current is listed on the front page of an inductor datasheet; however, it is good practice to verify this value using an “Inductance vs DC Current” graph similar to Figure 9. 6.3 Thermal Current Rating The thermal current rating is typically measured as the RMS current that causes some fixed rise in temperature (typically 20 – 30°C). Do not exceed this RMS current value. Calculate the RMS current through the inductor using Equation 6. This is an approximation based on a triangular RMS waveform. I IRMS = LIMIT 3 (6) Symbol IRMS ILIMIT 10 Description Inductor RMS Current Peak current limit set by the DRVxxxx REXT resistor DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated Value – – Unit ARMS Ap SLOA198 – September 2014 Submit Documentation Feedback Selecting an Inductor www.ti.com 6.4 Choosing REXT The resistor on the REXT pin is found using Equation 7: æ V ö REXT = ç K REF ÷ - RINT è ILIM ø Symbol K VREF RINT ILIMIT (7) Description Value 10500 1.35 60 – Internal reference voltage on REXT Internal resistance Inductor current limit from inductor datasheet Unit V Ω Ap The following graph shows the relationship between REXT and the current limit. 2.5 VDD = 3.6 V Inductor = 3.3 PH Not Loaded Gain = 41 dB ILIM - Inductor Current (A) 2 1.5 1 0.5 0 6 8 10 12 14 16 18 20 REXT (k:) 22 24 D001 D009 Use the REXT resistor to set the current limit for the inductor. This can serve two purposes: 1. Prevent Inductor Saturation – the current limit must be set equal to or lower than the inductor saturation current. If the inductance were to drop below 2.2 µH because of saturation, the boost converter may have difficulty regulating the output voltage. 2. Limit Peak Battery Current – the resistor can help limit the maximum current from the battery; however, a lower current limit decreases the amount of current delivered to the load each switch cycle. Tip The best way to lower peak battery currents is by adding a bulk input capacitor. 6.5 What to Avoid: Using Incorrect Inductor Current Ratings Manufacturers provide multiple current ratings, because boost converters typically operate with a very high peak-to-average ratio. This means there are very large current spikes, but the average current flowing through the inductor is very low. Because these are very different current operating points, the inductor manufacturers provide two current ratings to help make inductor selection easier: one for peak currents and one for RMS currents. The thermal current rating indicates maximum RMS currents, while the saturation current rating indicates maximum peak currents that could cause core saturation. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 11 Selecting an Inductor www.ti.com While inductor saturation current and thermal rating are readily available in most inductor datasheets, they are often obscured by manufacturer operating conditions and nomenclature. In some cases, one manufacturer may refer to one parameter as the rated current and another manufacturer may refer to a completely different parameter as the rated current. The point of this section is to dispel any confusion and clarify what to look for when choosing the saturation current and thermal rating. The following parameters are taken from two different inductor datasheets compared side-by-side. Both are good inductors, but only one has sufficient current ratings for a 1-A boost design. Inductor 1 Murata – LQM2MPN3R3MG0L Inductor 2 TDK – VLS3010 Datasheet Front Page “Rated DC Current” Rated DC Current: 1.2 A Datasheet Front Page “Rated DC Current” Rated DC Current: 1.1-1.3 A Datasheet Graph Data ISAT : 400 mA ITHERMAL : 1.2 A Datasheet Graph Data ISAT : 1.1–1.3 A ITHERMAL : 1.5 A Inductance vs Peak Current Inductance vs Peak Current Inductor 1 (on the left) has a Rated DC Current of 1.2 A and Inductor 2 has a Rated DC Current between 1.1–1.3 A. Initially, both seem to work if our boost configuration requires 1 A; however, they are not equal. If you notice the two parameters underneath “Rated DC Current” labeled ISAT and ITHERMAL are different. You can see that Inductor 1 has a thermal rating of 1.2 A, but only a 400 mA saturation current. Inductor 2 has a rated current of 1.1–1.3 A and a saturation current of 1.1–1.3 A and a thermal rating of 1.5 A. It is apparent from this comparison that “Rated DC Current” does not always refer to the same parameter, so be careful when choosing an inductor based on its current ratings. 12 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Calculate the Maximum Boost Current www.ti.com 7 Calculate the Maximum Boost Current The maximum support boost current is estimated using Equation 8 and Equation 9. First calculate the maximum duty cycle of the hysteretic converter. Equation 8 assumes worst-case duty cycle during maximum current. Vin _ min ´ h D = 1Vboost (8) Symbol D Vin_min ETA (η) Vboost Description Boost Duty Cycle Minimum VDD Boost Efficiency Boost Voltage Value – – 60 – Unit – V % V If the approximate and the actual measurements do not align perfectly, then you can adjust η, which is the efficiency of the boost converter. I æ ö Iboost _ max = ç Ilimit - limit ÷ ´ (1 - D ) 2 è ø Symbol Iboost_max Ilimit D (9) Description Maximum boost current Boost current limit (REXT) Boost duty cycle Value – – – Unit A A – Iboost_max is the maximum boost current the boost can deliver to the amplifier. Compare this current to the current calculated for the load current. The boost current should be higher than the load current. 8 Output Capacitor Selection The output capacitor is important for decreasing output voltage ripple and reducing the effects of load transients on the boost voltage. The boost output voltage can be configured from 20 V up to 105 V, so the boost output capacitor must have a voltage rating equivalent to the boost output voltage or higher. A 250V rated, 100-nF capacitor of X5R or X7R type is recommended for a boost converter voltage of 105 V. The selected capacitor should have a minimum working capacitance of 50 nF. To estimate the absolute minimum capacitance required, use Equation 10. Typically the DRVxxxx devices operate with a switching frequency between 800 kHz to 1 MHz. To include additional margin for the device loop response, it is best to use one-sixth of the switching frequency (ƒ). DI C= 2 ´ fSW ´ VDROOP (10) Symbol DELTA I VDROOP fSW Description Boost transient current Maximum boost output voltage droop Boost switching frequency C Output capacitor SLOA198 – September 2014 Submit Documentation Feedback Value 0 – 0.070 – Typically 800kHz-1MHz – Unit mA V Hz F DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 13 Input Capacitor Selection www.ti.com Tip A guideline for ceramic capacitors: the de-rated capacitance is approximately equal to the rated capacitance multiplied by one minus the applied voltage over the rated voltage. Cde-rated = Crated (1 – Vapplied/Vrated) For example, when 50 V is applied to a 100-V rated capacitor, the capacitance will decrease by about 50%. Most capacitor vendors provide a capacitance versus voltage curve for reference. 9 Input Capacitor Selection The input capacitor provides current to the DRVxxxx when there are large current transients during startup and heavy load periods. Charge Required from Bulk Capacitor ITransient Current Supplied by Host t tr When a bulk input capacitor is included, the following diagram shows that the battery actually sees a filtered version of ILIM. IAVG L1 RBAT ISW SW CBULK VBAT When the boost converter enters a heavy load condition or during a startup sequence, the switching frequency reaches a maximum value set by the slope of the charge/discharge curve and the ILIM value. Inductor Current (A) - ISW ILIM IAVG Time (s) 14 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Input Capacitor Selection www.ti.com Calculating the exact IAVG value is rather difficult, but as long as the bulk decoupling capacitance in the system is sufficiently high, the average current drawn from the battery will be less than one-half of ILIM as a general guideline. To estimate the bulk input capacitor value, use Equation 11. CBULK = Symbol CBULK ITR LTrace VDROOP 1.21´ ITR 2 ´ L Trace VDROOP2 (11) Description Minimum VDD bulk capacitance required Input transient current (maximum is ILIMIT) Input trace inductance (estimate 50 nH, if unknown) Maximum boost output voltage droop (Ex.: 3.6 V – 0.1 V = 3.5 V, 0.1 V is the allowable droop) Value – – – – Unit F A H V This equation was derived from the RLC circuit formed by the trace resistance and inductance combined with the bulk capacitance and capacitor ESR. RTRACE LTRACE ESR VDD + VIN CBULK CAP Keep in mind that it is difficult to calculate the exact input current required for the DRVxxxx, so a minimum capacitance between 22 µF and 47 µF is recommend. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 15 PCB Layout 10 www.ti.com PCB Layout Use the following guidelines for PCB layout. CVREG GND IN- IN+ SCL SDA VREG CPUMP CVDD REXT PUMP REXT VDD OUT- FB OUT+ GND PVDD GND VBST R1 R2 VBST NC SW SW GND GND CVBST L1 CBULK VDD GND 1. Place the feedback resistors, R1 and R2, close to the FB pin. If the resistors are placed far from the FB pin, then noise can enter the FB trace and causes instability or unwanted ripple. 2. Protect the feedback trace (Red trace). Isolate the feedback trace on a different layer and shield from the SW node using a ground plane. 3. Minimize the size of SW node trace. The trace between the SW pins and the inductor terminal should be as physically small as possible. A large switch node can add parasitic capacitance and slow the switching frequency of the boost converter, preventing it from delivering the required current. A small SW node also helps prevent radiated emissions. 4. Place the bulk input capacitor near the inductor. The bulk input capacitor provides the current during quick transient current spikes. Closer means less resistance between the bulk capacitor and inductor. 5. Place the output capacitor near the VBST pin. 16 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback PCB Layout www.ti.com 10.1 What to Avoid: Incorrect Inductor Placement In space-constrained applications, PCB real estate often trumps correct component placement. While layout guidelines were provided in the previous section, this section specifically covers inductor placement in more detail. The inductor should be placed as close to the SW node as possible; this helps reduce parasitic resistances, inductances, and most importantly, parasitic capacitances. L D VDD VBST CParasitic Q COUT Figure 10. SW Node Parasitic Capacitance What happens if the SW node has too much parasitic capacitance? Two things: 1. The switching frequency decreases due to a higher RC constant, resulting in less current delivery to the load 2. The SW node stores charge in the parasitic capacitor, resulting in less current delivery to the load The switching frequency changes as a result of the charging and discharging of the parasitic capacitor each cycle. Figure 11 shows how the current charge cycle of the inductor changes when parasitic capacitance is present. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 17 PCB Layout www.ti.com Difference in Switching Period ILIM Current Without Parasitic Capacitance Current ILIM With Parasitic Capacitance Time Figure 11. Inductor Charging with Parasitic Capacitance Figure 11 shows that the period of the inductor charge cycle increases with parasitic capacitance. This longer charge cycle results in a slower boost-switching frequency. In addition to a slower switching frequency, the parasitic capacitance on the SW node consumes charge. If, for example, the DRVxxxx boost design normally supports 10-mA current and there is parasitic capacitance present, a significant portion – sometimes up to 1 mA of the current – can be consumed by the parasitic capacitance. This means that 10% of the current intended for the load is being consumed by the parasitic capacitance. 18 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback PCB Layout www.ti.com 1A 9mA L D VDD VBST CParasitic Q 1mA COUT Both of these issues can have drastic effects on the output waveform. To identify switch node parasitic capacitance, look for continuous switching on the SW pin. During normal operation, the switching turns on and off depending on the current required; however, with parasitic capacitance the SW node often continuously switches to recover for lost switch cycles and current. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 19 Examples 11 www.ti.com Examples This section provides two examples based on the DRV8662EVM and DRV2667EVM-CT and a 25-nF Piezo actuator. 11.1 Example: Based on the DRV8662EVM The DRV8662EVM is the evaluation module for the DRV8662 analog high-voltage Piezo haptic driver. The board includes a MSP430 to control the DRV8662. Figure 12 illustrates a simplified version of the DRV8662EVM. L1 4.7uH / 1.8A 3.0 V to 5.5 V 110uF DRV8662 VDD SW VREG VBST C1 0.1uF C5 0.1uF / 250V PVDD FB GAIN0 GAIN R1 768k R2 13k GAIN1 Analog Input IN+ OUT+ IN- OUT- VPUMP C2 0.1uF REXT Piezo Actuator 25nF R5 13k GND Figure 12. Simplified Version of the DRV8662EVM The following table contains the primary components of the design. The board was configured for a SEMCO Piezo actuator. Reference Designator Value Manufacturer Part # Manufacturer L1 4.7 µH / 1.8 ASAT LPS4018-472MLB Coilcraft Piezo Actuator 25 nF at 150 V, 200 Hz PHAT423535XX SEMCO The full design documents including the schematic, layout, and BOM can be found in the DRV2667EVMCT User’s Guide (SLOU323). 11.1.1 Configure the Boost Voltage The values in this section were calculated using the DRV Design Equations excel file available on www.ti.com. The SEMCO actuator has a rated voltage of 150 Vpp, which is defined as the voltage required for 100% vibration. The following values were entered into the design equation spreadsheet: 20 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Examples www.ti.com Actuator Properties Actuator Model: SEMCO 42mm Cactuator = 25 nF Vactuator = 150 Vpp Next, the settings section was completed to reflect a typical board environment. Vboost was set to 80 V using Equation 12. The additional 5 V provides headroom for the amplifier. V Vboost = actuator + 5V (12) 2 Settings Vin = 3.6 V Vboost = 80 V ƒin_min = 100 Hz Vin_min = 3 V Vforward = 0.7 V ƒin_max = 300 Hz Vin_max = 5 V µboost = 70% (efficiency) Iripple_% = 20% To configure the boost voltage, R1 and R2 must be set so that the voltage divider equals 1.32 V. The values are calculated using the Feedback Resistors section of the design equations excel file. R1 was selected to be 768 kΩ so that the total resistance of the resistor divider is large, preventing high leakage currents. R2 is calculated automatically and then R2_actual is the closest standard resistor value. Finally, the boost voltage is back-calculated to show the expected boost voltage. Feedback Resistors Vfeedback = 132 V R1 ´ Vfeedback R2 = Vboost - Vfeedback VBST R1 = 768 kΩ R2 = 12.885 kΩ R2_actual = 13 kΩ DRVxxxx R1 FB R2 æ R1 Vboost _ actual = Vfeedback ç 1 + ç R2 _ actual è 11.1.2 ö ÷ ÷ ø Vboost_actual = 79.302 V Configure the Inductor Current The Piezo load requires approximately 1.88 mA of average current based on the following equation: Piezo Actuator Current Requirements (Approximate) Iout_max = 2π × Cactuator × Vboost × ƒin_max Iout_max = 1.884956 mA Maximum required amplifier current (estimate) æ Vboost ö Ibat _ max = 2p ´ ƒin _ max ´ Cactuator ´ Vboost ç ÷ è Vin ´ mboost ø Ibat_max = 139.626 mA Maximum VBAT current This means that the inductor current limit (REXT) must be sufficiently large to support 1.88 mA. The DRV8662EVM inductor is already fixed, but from here forward you may choose a different inductor that fits your load requirements. The maximum boost current with 80 Vp boost voltage is 20.3 mA based on the 4.7 µH / 1.8 A Coilcraft inductor selected using the following equations: Boost Converter Current Capacity (Approximate) Vin _ min ´ h D = 1Vboost SLOA198 – September 2014 Submit Documentation Feedback D = 0.978 Estimated duty cycle at maximum frequency DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 21 Examples www.ti.com Boost Converter Current Capacity (Approximate) I æ ö Vboost _ max = ç Ilimit - limit ÷ ´ (1 - D ) 2 è ø Iboost_max = 20.289 mA Maximum boost output current estimate 350 Vboost (V) 25 50 75 100 300 Iboost_max (mA) 250 200 150 100 50 IOUT (mA) 64.92 32.46 21.64 16.23 0 0 5 10 15 20 25 30 35 40 45 50 55 60 VBoost (V) 65 70 75 80 85 90 95 100 105 110 D013 The inductor on the DRV8662 is the LPS4018-472MLB from Coilcraft. The saturation current is 1.8 A and the current limit due to continuous current is 1.8 A. Enter these values into the Boost Current Limit Resistor section of the design equations excel file. Boost Current Limit Resistor Inductor Model Coilcraft LPS4018-472MLB L = 4.7 µH Isat = 1.8 A Imin = min (ISAT, Ithermal) Imin = 1.8 A Ilimit = Imin Ilimit = 1.8 A Ithermal = 1.8 A æ V Rext = ç k ´ ref Ilimit è Rext_actual = 7.87 kΩ Ilimit _ actual ö ÷ - Rint ø Vref = k´ Iext _ actual + Rint Rext = 7.815 kΩ Ilimit_actual = 1.788 A The resulting current-limit resistor is calculated as 7.815 kΩ in Rext. The nearest standard resistor value is 7.87 kΩ shown in the Rext+actual box. The current limit was then back-calculated to show the expected current limit, which is less than the saturation current. 22 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Examples www.ti.com 11.1.3 Boost Performance Results The actual load regulation and boost efficiency for the DRV8662EVM are shown in Figure 13. The load regulation is a measure of the boost voltage versus the boost output current. In Figure 13, the voltage regulation begins to drop above approximately 31 mA of boost output current. 90 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 80 70 Boost Voltage (V) 60 50 40 30 20 10 0 0 10 20 30 40 Boost Current (mA) 50 60 69.6 D014 D014 Figure 13. Actual Load Regulation and Boost Efficiency The boost efficiency is shown in Figure 14. The efficiency data was removed after the boost voltage decreased by more than 5 V. 100 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 90 80 Efficiency (%) 70 60 50 40 30 20 10 0 0 5 10 15 Boost Current (mA) 20 25 D015 D015 Figure 14. Boost Efficiency SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 23 Examples www.ti.com 11.2 Example: Based on the DRV2667EVM-CT with 25-nF Piezo Module The DRV2667EVM-CT is the evaluation module for the DRV2667 high-voltage piezo haptic driver with digital front-end. The kit includes all the components to evaluate and test the DRV2667 including a Piezo haptics module manufactured by Samsung Electric and Mechanical. Figure 15 is a simplified version of the DRV2667EVM-CT schematic. L1 3.3uH / 1.1A 110uF DRV2667 VDD SW VREG VBST C1 0.1uF C5 0.1uF / 250V PVDD FB SDA I2C R1 768k R2 13k SCL Analog Input IN+ OUT+ IN- OUT- VPUMP C2 0.1uF REXT Piezo Actuator 25nF R5 13k GND Figure 15. Simplified Version of the DRV2667EVM-CT Schematic The following table contains the primary components of the design: Reference Designator Value Manufacturer Part # Manufacturer L1 3.3 µH / 1.1 ASAT VLS3010ET-3R3M TDK Piezo Actuator 25 nF at 150 V, 200 Hz PHAT423535XX SEMCO The full design documents including the schematic, layout, and BOM, can be found in the DRV2667EVMCT User’s Guide (SLOU323). 11.2.1 Configure the Boost Voltage The values in this section were calculated using the DRV2667 Design Equations excel file available on www.ti.com. The SEMCO actuator has a rated voltage of 150 Vpp, which is defined as the voltage required for 100% vibration. The following values were entered into the design equation spreadsheet: Actuator Properties Actuator Model: SEMCO 42mm Cactuator = 25 nF 24 Vactuator = 150 Vpp DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Examples www.ti.com Next, the settings section was completed to reflect a typical board environment. Vboost was set to 80 V using Equation 13. The additional 5 V provides headroom for the amplifier. V Vboost = actuator + 5V (13) 2 Settings Vin = 3.6 V Vboost = 80 V ƒin_min = 100 Hz Vin_min = 3 V Vforward = 0.7 V ƒin_max = 300 Hz Vin_max = 5 V µboost= 70% (efficiency) Iripple_% = 20% To configure the boost voltage, R1 and R2 must be set so that the voltage divider equals 1.32 V. The values are calculated using the Feedback Resistors section of the design equations excel file. R1 was selected to be 768 kΩ so that the total resistance of the resistor divider is large, preventing high leakage currents. R2 is calculated automatically and then R2_actual is the closest standard resistor value. Finally, the boost voltage is back-calculated to show the expected boost voltage. Feedback Resistors Vfeedback = 1.32 V R1 ´ Vfeedback R2 = Vboost - Vfeedback R1 = 768 kΩ R2 = 12.885 kΩ R2_actual = 13 kΩ VBST DRVxxxx R1 FB R2 æ R1 Vboost _ actual = Vfeedback ç 1 + ç R2 _ actual è 11.2.2 ö ÷ ÷ ø Vboost_actual = 79.302 V Configure the Inductor Current The Piezo load requires approximately 1.88 mA of average current based on the following equation: Piezo Actuator Current Requirements (Approximate) Iout_max = 2π × Cactuator × Vboost × ƒin_max Iout_max = 1.884956 mA Maximum required amplifier current (estimate) æ Vboost ö Ibat _ max = 2p ´ ƒin _ max ´ Cactuator ´ Vboost ç ÷ è Vin ´ mboost ø Ibat_max = 139.626 mA Maximum VBAT current This means that the inductor current limit (REXT) needs to be sufficiently large to support 1.88 mA. SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 25 Examples www.ti.com The DRV2667EVM-CT inductor is already fixed, but from here forward you may choose a different inductor that fits your load requirements. The maximum boost current with 80 Vp boost voltage is 12.4 mA, based on the 3.3-µH /1.1-A TDK inductor selected using the following equations: Boost Converter Current Capacity (Approximate) Vin _ min ´ h D = 1Vboost D = 0.978 Estimated duty cycle at maximum frequency I æ ö Iboost _ max = ç Ilim it - limit ÷ ´ (1 - D ) 2 è ø Iboost_max = 12.400 mA Maximum boost output current estimate 250 Vboost (V) 25 50 75 100 Iboost_max (mA) 200 150 100 IOUT (mA) 39.68 19.84 13.23 9.92 50 0 0 5 10 15 20 25 30 35 40 45 50 55 60 VBoost (V) 65 70 75 80 85 90 95 100 105 110 D016 D016 The inductor on the DRV2667EVM-CT is the VLS3010ET-3R3M from TDK. The saturation current is 1.1 A and the current limit due to continuous current is 1.5 A. Enter these values into the Boost Current Limit Resistor section of the design equations excel file. Boost Current Limit Resistor Inductor model VLS3010ET-3R3M L = 3.3 µH Isat = 1.1 A Imin = min (ISAT, Ithermal) Imin = 1.1 A Ilimit = Imin Ilimit = 1.1 A æ V ö Rext = ç k ´ ref ÷ - Rint Ilimit ø è Vref Ilimit _ actual = k ´ Iext _ actual + Rint Rext = 12.83 kΩ Ithermal = 1.5 A Rext_actual = 13 kΩ Ilimit_actual = 1.085 A The resulting current limit resistor is calculated as 12.83 kΩ in Rext. The nearest standard resistor value is 13 kΩ shown in the Rext+actual box. The current limit was then back-calculated to show the expected current limit, which is less than the saturation current. 26 DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated SLOA198 – September 2014 Submit Documentation Feedback Examples www.ti.com 11.2.3 Boost Performance Results The actual load regulation and boost efficiency for the DRV2667EVM-CT are shown in Figure 16. The load regulation is a measure of the boost voltage versus the boost output current. In Figure 16, the voltage regulation begins to drop above approximately 10 mA of boost output current. Comparing the voltage regulation of the DRV2667EVM-CT to the DRV8662EVM, the DRV2667EVM-CT boost has much less current capacity because of the smaller inductor and current limit resistor settings. 90 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 80 70 Boost Voltage (V) 60 50 40 30 20 10 0 0 10 20 30 40 Boost Current (mA) 50 60 69.6 D017 D017 Figure 16. Boost Voltage vs. Boost Current - DRV2667EVM-CT-80 V The boost efficiency is shown in Figure 17. The efficiency data was removed after the boost voltage decreased by more than 5 V. 100 VDD = 3.6 V VDD = 4.7 V VDD = 5.5 V 90 80 Efficiency (%) 70 60 50 40 30 20 10 0 0 1 2 3 4 5 Boost Current (mA) 6 7 8 9 10 D018 D018 Figure 17. Efficiency vs. Boost Current - VBST = 80 V SLOA198 – September 2014 Submit Documentation Feedback DRV8662, DRV2665, and DRV2667 Configuration Guide Copyright © 2014, Texas Instruments Incorporated 27 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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