LM34923

LM34923 Evaluation Board LM34923 Evaluation Board National Semiconductor Application Note 2147 Vijay Choudhary May 16, 2011 Introduction The LM34923 EVAL evaluation board provides the design engineer with a fully functional buck regulator, employing the constant on-time (COT) operating principle. This evaluation board provides a 5V output over an input range of 6V to 75V. The circuit delivers load currents to 500 mA, with current limit set at a nominal 1 Amp. The board’s specification are: • • • • • • • • Input Voltage: 6V to 75V Output Voltage: 5V Maximum load current: 500 mA Minimum load current: 0A Current Limit: 1 Amp (nominal) Measured Efficiency: 94.75% (VIN = 6V, IOUT = 100 mA) Nominal Switching Frequency: 200 kHz Size: 2.6 in. x 1.6 in. 30161530 FIGURE 1. Evaluation Board - Top Side Theory of Operation Refer to the evaluation board schematic in Figure 6. When the circuit is in regulation, the buck switch is on each cycle for a time determined by R1 and VIN according to the equation: The on-time of this evaluation board ranges from ≊4.38 µs at VIN = 6V, to ≊351 ns at VIN = 75V. The on-time varies in- versely with VIN to maintain a nearly constant switching frequency. At the end of each on-time the Minimum Off-Timer ensures the buck switch is off for at least 260 ns. In normal operation, the off-time is much longer. During the off-time, the load current is supplied by the output capacitor (C2). When the output voltage falls sufficiently that the voltage at FB is below 2.5V, the regulation comparator initiates a new on-time period. For stable, fixed frequency operation, a minimum of 25 mV of ripple is required at FB to switch the regulation comparator. Refer to the LM34923 data sheet for a more detailed block diagram, and a complete description of the various functional blocks. AN-2147 © 2011 National Semiconductor Corporation 301615 www.national.com AN-2147 Board Layout and Probing The pictorial in Figure 1 shows the placement of the circuit components. The following should be kept in mind when the board is powered: 1) When operating at high input voltage and high load current, forced air flow may be necessary. 2) The LM34923 may be hot to the touch when operating at high input voltage and high load current. 3) Use CAUTION when probing the circuit at high input voltages to prevent injury, as well as possible damage to the circuit. 4) At maximum load current, the wire size and length used to connect the load becomes important. Ensure there is not a significant drop in the wires between this evaluation board and the load. ommended that the input voltage be increased gradually to 6V, at which time the output voltage should be 5V. If the output voltage is correct with 6V at VIN, then increase the input voltage as desired and proceed with evaluating the circuit. DO NOT EXCEED 75V AT VIN. Output Ripple Control The LM34923 requires a minimum of 25 mVp-p ripple at the FB pin, in phase with the switching waveform at the SW pin, for proper operation. The required ripple can be supplied from ripple at VOUT, through the feedback resistors as described in Option A below. Options B and C provide lower output ripple with one or two additional components. Option A) Lowest Cost Configuration: In this configuration R7 is installed in series with the output capacitance (C2). Since ≥25 mVp-p are required at the FB pin, R7 must be chosen to generate ≥50 mVp-p at VOUT, knowing that the minimum ripple current in this circuit is ≊51 mAp-p at minimum VIN. Using 1Ω for R7, the ripple at VOUT ranges from ≊51 mVp-p to ≊280 mVp-p over the input voltage range. If the application can accept this ripple level, this is the most economical solution. The circuit is shown in Figure 2. See Figure 8. R8, C6, C7, and C8 are not used in this configuration. Board Connection/Start-up The input connections are made to the J1 connector. The load is connected to the J2 (OUT) and J3 (GND) terminals. Ensure the wires are adequately sized for the intended load current. Before start-up a voltmeter should be connected to the input terminals, and to the output terminals. The load current should be monitored with an ammeter or a current probe. It is rec- 30161503 FIGURE 2. Lowest Cost Configuration www.national.com 2 AN-2147 Option B) Reduced Ripple Configuration: This configuration generates less ripple at VOUT than option A above by the addition of one capacitor (C8) across R5, as shown in Figure 3. 30161504 FIGURE 3. Reduced Ripple Configuration Since the output ripple is passed by C8 to the FB pin with little or no attenuation, R7 can be reduced so the minimum ripple at VOUT is ≊25 mVp-p. The minimum value for Cff is calculated from: 1) Calculate the voltage VA: VA = VOUT – (VSW x (1 – (VOUT/VIN))) where VSW is the absolute value of the voltage at the SW pin during the off-time, and VIN is the minimum input voltage. For this circuit, VA calculates to 4.98V. This is the approximate DC voltage at the R8/C6 junction, and is used in the next equation. 2) Calculate the R8 x C6 product: where tON(max) is the maximum on-time (at minimum VIN), and R5//R6 is the parallel equivalent of the feedback resistors. The ripple at VOUT ranges from 28 mVp-p to 159 mVp-p over the input voltage range. See Figure 8. Option C) Minimum Ripple Configuration: To obtain minimum ripple at VOUT, R7 is set to 0Ω, and R8, C6, and C7 are added to generate the required ripple for the FB pin. In this configuration, the output ripple is determined primarily by the characteristics of the output capacitance and the inductor’s ripple current. See Figure 8. The ripple voltage required by the FB pin is generated by R8, and C6 since the SW pin switches from –0.1V to VIN, and the right end of C6 is a virtual ground. The values for R8 and C6 are chosen to generate a 30-100 mVp-p triangle waveform at their junction. That triangle wave is then coupled to the FB pin through C7. The following procedure is used to calculate values for R8, C6 and C7: where tON is the maximum on-time, VIN is the minimum input voltage, and ΔV is the desired ripple amplitude at the R8/C6 junction, 40 mVp-p for this example. R8 and C6 are then chosen from standard value components to satisfy the above product. Typically C6 is 3000 to 10000 pF, and R8 is 10 kΩ to 300 kΩ. C7 is chosen large compared to C6, typically 0.1 µF. The ripple at VOUT is typically less than 10 mVp-p. See Figure 4 and Figure 8. 3 www.national.com AN-2147 30161508 FIGURE 4. Minimum Output Ripple Configuration 30161509 FIGURE 5. Efficiency at 200 kHz www.national.com 4 AN-2147 Under-Voltage Detector The Under Voltage Detector can be used to monitor the input voltage, or any other system voltage as long as the voltage at the UV pin does not exceed its maximum rating. On this evaluation board the input voltage is monitored via resistors R2 and R3. An appropriate pull-up voltage less than 10 volts must be connected to test point TP2-UVO on this evaluation board. R4 is the pull-up resistor for the UVO output. The under-voltage status can then be monitored at the TP3-Status test point. On this evaluation board the UVO output switches low when the input voltage exceeds 12V, and it switches high when the input voltage is less than 11V. If it is desired to change the thresholds, the equations for determining the resistor values are: Where VUVH is the upper threshold at VIN, and VUVL is the lower threshold. The threshold at the UV pin is 2.5V. The UVO output is high when the VCC voltage is below its UVLO threshold, or when the LM34923 is shutdown by grounding the TP1-SD test point, regardless of the voltage at the UV pin. 30161525 FIGURE 6. Complete Evaluation Board Schematic (As Supplied) Monitor The Inductor Current The inductor’s current can be monitored or viewed on a scope with a current probe. Remove R9, and install an appropriate current loop across the two large pads where R9 was located. In this way the inductor’s ripple current and peak current can be accurately determined. Scope Probe Adapters Scope probe adapters are provided on this evaluation board for monitoring the waveform at the SW pin, and at the circuit’s output (VOUT), without using the probe’s ground lead which can pick up noise from the switching waveforms. 5 www.national.com AN-2147 Bill of Materials Item C1 C2 C3 C4 C5 C6 C7 C8 C9 L1 D1 R1 R2 R3 R4 R5 R6 R7 R8 R9 U1 Description Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Unpopulated Ceramic Capacitor Inductor Schottky Rectifier Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Switching Regulator TDK C1608X7R2A102K Coiltronics DR74-820-R or Wurth Electronics 744771182 Diodes Inc DFLS1100 Vishay CRCW0603191KF Vishay CRCW0603200KF Vishay CRCW060359KOF Vishay CRCW0603100KF Vishay CRCW06033KO1F Vishay CRCW06033KO1F Vishay CRCW06030000Z Vishay CRCW060336K5F Vishay CRCW06030000Z National Semiconductor LM34923 MM Power DI123 0603 0603 0603 0603 0603 0603 0603 0603 0603 MSOP-10 0805 1000 pF, 100V 82 uH,1A 100V, 1.0A 191kΩ 200kΩ 59 kΩ 100 kΩ 3.01 kΩ 3.01 kΩ 0Ω jumper 36.5 kΩ 0Ω jumper Mfg., Part Number TDK C3225X7R2A225M TDK C3225X7R1C156M TDK C1608X7R1C105K TDK C1608X7R2A103K TDK C2012X7R2A104M TDK C1608X7R2A332K TDK C2012X7R2A104M Package 1210 1210 0603 0603 0805 0603 0805 Value 2.2 µF, 100V 15 µF, 16V 1 µF, 16V 0.01 µF, 100V 0.1 µF, 100V 3300 pF, 100V 0.1 µF, 100V www.national.com 6 AN-2147 Circuit Performance 30161512 FIGURE 7. Output Voltage Ripple 30161513 FIGURE 8. Switching Frequency vs. Input Voltage 7 www.national.com AN-2147 30161514 FIGURE 9. Current Limit vs. Input Voltage 30161520 FIGURE 10. Line Regulation 30161521 FIGURE 11. Load Regulation www.national.com 8 AN-2147 Typical Waveforms 30161515 Trace 1 = SW Pin Trace 2 = VOUT Trace 4 = Inductor Current Vin = 12V, Iout = 200 mA FIGURE 12. Typical Waveforms 9 www.national.com AN-2147 PC Board Layout 30161517 Board Silkscreen 30161518 Board Top Layer www.national.com 10 AN-2147 30161519 Board Bottom Layer (Viewed from Top) 11 www.national.com LM34923 Evaluation Board For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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