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User's Guide

SNVA482A–May 2011–Revised April 2013

AN-2147 LM34923 Evaluation Board

1 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% (V

= 6V, I

OUT

= 100 mA)

• Nominal Switching Frequency: 200 kHz

• Size: 2.6 in. x 1.6 in.

Figure 1. Evaluation Board - Top Side

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SNVA482A–May 2011–Revised April 2013 AN-2147 LM34923 Evaluation Board

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Summary of content (12 pages)

PAGE 1
User's Guide SNVA482A – May 2011 – Revised April 2013 AN-2147 LM34923 Evaluation Board 1 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.
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Theory of Operation 2 www.ti.com Theory of Operation Figure 6 shows the evaluation board schematic. When the circuit is in regulation, the buck switch is on each cycle for a time determined by R1 and VIN according to Equation 1: -10 ton = 1.25 x 10 x (R1 + 500:) + 30 ns VIN - 0.5V (1) The on-time of this evaluation board ranges from ≊4.38 µs at VIN = 6V, to ≊351 ns at VIN = 75V. The ontime varies inversely with VIN to maintain a nearly constant switching frequency.
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Output Ripple Control www.ti.com VCC C3 1 PF LM34923 BST 0.01 PF C4 L1 82 PH VOUT 5V SW D1 R7 1: R5 3.01 k: C2 15 PF FB GND R6 3.01 k: RTN Figure 2. Lowest Cost Configuration 5.2 Option B) Reduced Ripple Configuration This configuration generates less ripple at VOUT than Section 5.1 by the addition of one capacitor (C8) across R5, as shown in Figure 3. VCC LM34923 C3 1 PF BST 0.01 PF C4 L1 82 PH VOUT SW 5V D1 C8 0.01 PF R7 0.56: R5 3.01k: C2 15 PF FB GND RTN R6 3.
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Output Ripple Control 5.3 www.ti.com 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.
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Under-Voltage Detector www.ti.com Figure 5. Efficiency at 200 kHz 6 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.
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Monitor the Inductor Current 6V to 75V Input VIN C1 2.2 PF GND C5 0.1 PF R1 191 k: TP1 SD www.ti.com VIN VCC LM34923 BST RT/SD R2 200 k: TP2 UVO R4 100 k: TP3 STATUS C9 1000 pF C3 1 PF 0.01 PF C4 SW R3 59 k: FB UVO L1 82 PH VOUT 5V R9 0: R8 D1 UV SW Scope TP4 36.5 k: 3300 C6 pF C7 0.1 PF RTN C8 R5 3.01 k: R7 0: C2 15 PF VOUT Scope TP5 R6 3.01 k: GND Figure 6.
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Scope Probe Adapters www.ti.com 8.1 Bill of Materials (BOM) Table 1. Bill of Materials (BOM) Item Description Mfg., Part Number Package Value C1 C2 Ceramic Capacitor TDK C3225X7R2A225M 1210 2.2 µF, 100V Ceramic Capacitor TDK C3225X7R1C156M 1210 15 µF, 16V C3 Ceramic Capacitor TDK C1608X7R1C105K 0603 1 µF, 16V C4 Ceramic Capacitor TDK C1608X7R2A103K 0603 0.01 µF, 100V C5 Ceramic Capacitor TDK C2012X7R2A104M 0805 0.
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Circuit Performance 9 8 www.ti.com Circuit Performance Figure 7. Output Voltage Ripple Figure 8. Switching Frequency vs. Input Voltage Figure 9. Current Limit vs. Input Voltage Figure 10.
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Typical Waveforms www.ti.com Figure 11. Load Regulation 10 Typical Waveforms Trace 1 = SW Pin Trace 2 = VOUT Trace 4 = Inductor Current Vin = 12V, Iout = 200 mA Figure 12.
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PC Board Layout 11 www.ti.com PC Board Layout Figure 13. Board Silkscreen Figure 14.
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PC Board Layout www.ti.com Figure 15.
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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.