National Semiconductor Application Note 2016 Dennis Morgan May 27, 2010 Introduction The board’s specification are: • Input Voltage: 6V to 24V • Output Voltage: 3.3V • Maximum load current: 600 mA • Minimum load current: 0A • Current Limit: 780 mA to 815 mA • Measured Efficiency: 88.7% (VIN = 6V, IOUT = 300 mA) • Nominal Switching Frequency: 1.5 MHz • Size: 2.6 in. x 1.6 in. x 0.5 in The LM34919BEVAL 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 3.3V output over an input range of 6V to 24V. The circuit delivers load currents to 600 mA, with current limit set at a nominal 800 mA. The board is populated with all components except R5, C9 and C10. These components provide options for managing the output ripple as described later in this document. LM34919B Evaluation Board LM34919B Evaluation Board 30110020 FIGURE 1. Evaluation Board - Top Side Theory of Operation Refer to the evaluation board schematic in Figure 1, which contains a simplified block diagram of the LM34919B. 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: © 2010 National Semiconductor Corporation 301100 www.national.com AN-2016 The on-time of this evaluation board ranges from ≊424 ns at VIN =6V, to ≊129 ns at VIN = 24V. 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 88 ns. In normal operation, the off-time is much longer. During the off-time, the load current is supplied by the output capacitor (C7, C8). 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. The current limit threshold is ≊780 mA at Vin = 6V, and ≊812 mA at Vin = 24V. The variation is due to the change in ripple current amplitude as Vin varies. Refer to the LM34919B data sheet for a more detailed block dia- AN-2016 gram, and a complete description of the various functional blocks. be monitored with an ammeter or a current probe. It is recommended that the input voltage be increased gradually to 6V, at which time the output voltage should be 3.3V. 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 40V AT VIN. 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) The LM34919B, and diode D1 may be hot to the touch when operating at high input voltage and high load current. 2) Use CAUTION when probing the circuit at high input voltages to prevent injury, as well as possible damage to the circuit. 3) At maximum load current (0.6A), 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. Output Ripple Control The LM34919B 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 Options A and B below, or the ripple can be generated separately (using R5, C9, and C10) in order to keep the ripple at VOUT at a minimum (Option C). Option A) Lowest Cost Configuration: This evaluation board is supplied with R4 installed in series with the output capacitance (C7, C8). R4 is chosen to generate ≥25 mVp-p at VOUT, knowing that the minimum ripple current in this circuit is ≊140 mAp-p at minimum VIN. Using 0.27Ω for R4, the ripple at VOUT ranges from ≊37 mVp-p to ≊88 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. 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 30110003 FIGURE 2. Lowest Cost Configuration Option B) Intermediate Ripple Configuration: This configuration generates less ripple at VOUT than option A above by www.national.com the addition of one capacitor (Cff) across R2, as shown in Figure 3. 2 AN-2016 30110004 FIGURE 3. Intermediate Ripple Configuration Since the output ripple is passed by Cff to the FB pin with little or no attenuation, R4 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 (typically 1V), and VIN is the minimum input voltage. For this circuit, VA calculates to 2.84V. This is the approximate DC voltage at the R5/C10 junction, and is used in the next equation. 2) Calculate the R5 x C10 product: where tON(max) is the maximum on-time (at minimum VIN), and R2//R3 is the parallel equivalent of the feedback resistors. See Figure 8. Option C) Minimum Ripple Configuration: To obtain minimum ripple at VOUT, R4 is set to 0Ω, and R5, C9, and C10 are added to generate the required ripple for the FB pin. In this configuration, the output ripple is determined primarily by the ESR of the output capacitance and the inductor’s ripple current. The ripple voltage required by the FB pin is generated by R5, C10, and C9 since the SW pin switches from -1V to VIN, and the right end of C10 is a virtual ground. The values for R5 and C10 are chosen to generate a 50-100 mVp-p triangle waveform at their junction. That triangle wave is then coupled to the FB pin through C9. The following procedure is used to calculate values for R5, C10 and C9: where tON is the maximum on-time (≊424 ns), VIN is the minimum input voltage, and ΔV is the desired ripple amplitude at the R5/C10 junction, 50 mVp-p for this example. R5 and C10 are then chosen from standard value components to satisfy the above product. Typically C10 is 3000 to 5000 pF, and R5 is 10kΩ to 300 kΩ. C9 is chosen large compared to C10, typically 0.1 µF. See Figure 4 and Figure 8. 3 www.national.com AN-2016 30110008 FIGURE 4. Minimum Output Ripple Configuration output (VOUT), without using the probe’s ground lead which can pick up noise from the switching waveforms.. Monitor The Inductor Current The inductor’s current can be monitored or viewed on a scope with a current probe. Remove R6, and install an appropriate current loop across the two large pads where R6 was located. In this way the inductor’s ripple current and peak current can be accurately determined. Minimum Load Current The LM34919B requires a minimum load current of ≊1 mA to ensure the boost capacitor (C5) is recharged sufficiently during each off-time. In this evaluation board, the minimum load current is provided by the feedback resistors allowing the board’s minimum load current at VOUT to be specified at zero. 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 30110009 FIGURE 5. Complete Evaluation Board Schematic www.national.com 4 Item Description Mfg., Part Number Package Value C1 Ceramic Capacitor TDK C3216X7R1H105M 1206 1.0 µF, 50V C2 Ceramic Capacitor TDK C3216X7R1H105M 1206 1.0 µF, 50V C3 Ceramic Capacitor TDK C1608X7R1H104K 0603 0.1 µF, 50V C4 Ceramic Capacitor TDK C1608X7R1H104K 0603 0.1 µF, 50V C5 Ceramic Capacitor TDK C1608X7R1H223K 0603 0.022 µF, 50V C6 Ceramic Capacitor TDK C1608X7R1H223K 0603 0.022 µF, 50V C7, C8 Ceramic Capacitor TDK C3216X7R1C106K 1206 10 µF, 16V C9 Ceramic Capacitor Unpopulated 0603 C10 Ceramic Capacitor Unpopulated 0603 D1 Schottky Diode Zetex ZLLS2000 SOT23-6 40V, 2.2A L1 Power Inductor Bussman DR74-8R2–R 7.6 mm x 7.6 mm 8.2 µH, 2.5A R1 Resistor Vishay CRCW060328KOFK 0603 28 kΩ R2 Resistor Vishay CRCW0603787RFK 0603 787 Ω R3 Resistor Vishay CRCW06032K49FK 0603 2.49 kΩ R4 Resistor Panasonic ERJ3RQFR27 0603 0.27Ω R5 Resistor Unpopulated 0603 R6 Resistor Vishay CRCW08050000Z 0805 U1 Switching Regulator National Semiconductor LM34919BTL 10 Bump µSMD 5 0Ω Jumper www.national.com AN-2016 Bill of Materials AN-2016 Circuit Performance 30110010 FIGURE 6. Efficiency vs Load Current 30110011 FIGURE 7. Efficiency vs Input Voltage www.national.com 6 AN-2016 30110012 FIGURE 8. Output Voltage Ripple 30110013 FIGURE 9. Switching Frequency vs. Input Voltage 30110014 FIGURE 10. Load Current Limit vs Input Voltage 7 www.national.com AN-2016 Typical Waveforms 30110015 Trace 2= VOUT Trace 4= inductor Current Trace 1= SW Pin Vin = 24V, Iout = 400 mA FIGURE 11. Continuous Conduction Mode 30110016 Trace 2= VOUT Trace 4= inductor Current Trace 1= SW Pin Vin = 24V, Iout = 20 mA FIGURE 12. Discontinuous Conduction Mode www.national.com 8 AN-2016 PC Board Layout 30110017 Board Silkscreen 30110018 Board Top Layer 9 www.national.com AN-2016 30110019 Board Bottom Layer (Viewed from Top) www.national.com 10 AN-2016 11 www.national.com LM34919B Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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