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User's Guide

SNVA380C–March 2009–Revised April 2013

AN-1925 LM5008A Evaluation Board

1 Introduction

The LM5008AEVAL 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 8V to 75V. The circuit delivers load currents to 300 mA, with current limit set at a

nominal 440 mA.

The board’s specification are:

• Input Voltage: 8V to 75V

• Output Voltage: 5V

• Maximum load current: 300 mA

• Minimum load current: 0A

• Current Limit: 440 mA (nominal)

• Measured Efficiency: 92.4% (V

= 8V, I

OUT

= 100 mA)

• Nominal Switching Frequency: 130 kHz

• Size: 2.6 in. x 1.6 in.

Figure 1. Evaluation Board - Top Side

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SNVA380C–March 2009–Revised April 2013 AN-1925 LM5008A Evaluation Board

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Summary of content (12 pages)

PAGE 1
User's Guide SNVA380C – March 2009 – Revised April 2013 AN-1925 LM5008A Evaluation Board 1 Introduction The LM5008AEVAL 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 8V to 75V. The circuit delivers load currents to 300 mA, with current limit set at a nominal 440 mA.
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Theory of Operation 2 www.ti.com Theory of Operation Refer to the evaluation board schematic in Figure 5. 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: tON = 1.385 x 10 -10 x R1 VIN (1) The on-time of this evaluation board ranges from ≊4.85 µs at VIN = 8V, to ≊517 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 8V to 75V Input VIN VCC VIN C1 1 PF C5 0.1 PF C3 0.47 PF LM5008A GND R1 280k BST SW RT/SD SHUTDOWN (TP1SD) C4 0.1 PF L1 220 PH VOUT 5V D1 RCL R6 0: R3 3.01k R5 0.82: FB R2 715k R4 3.01k C2 22 PF RTN GND Figure 2. Lowest Cost Configuration Option B) Intermediate Ripple Configuration: This configuration generates less ripple at VOUT than option A above by the addition of one capacitor (Cff) across R3, as shown in Figure 3.
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Output Ripple Control www.ti.com The ripple voltage required by the FB pin is generated by RA, and CA since the SW pin switches from -1V to VIN, and the right end of CA is a virtual ground. The values for RA and CA are chosen to generate a 50100 mVp-p triangle waveform at their junction. That triangle wave is then coupled to the FB pin through CB.
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Current Limit Off-Time www.ti.com 6 Current Limit Off-Time When current limit is detected the on-time period is immediately terminated, and the off-time forced by the LM5008A must be greater than the maximum normal off-time, which occurs at maximum input voltage. The longer-than-normal off-time is necessary to allow the inductor current to decrease at least as much, if not more, than the current increase which occurred during the on-time leading to the current limit detection.
PAGE 6
Scope Probe Adapters www.ti.com Table 1. Bill of Materials 6 Item Description Mfg., Part Number Package Value C1 C2 Ceramic Capacitor TDK C3216X7R2A105M or Murata GRM31CR72A105KA01L 1206 1 µF, 100V Ceramic Capacitor TDK C3225X7R1C226M or Murata GRM32ER71C226KE18L 1210 22 µF, 16V C3 Ceramic Capacitor TDK C1608X7R1C474M or TDK C1608X7R1C474K 0603 0.47 µF, 16V C4 Ceramic Capacitor TDK C1608X7R1H103M 0603 0.
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Circuit Performance www.ti.com 9 Circuit Performance Figure 6. Efficiency vs Load Current Figure 7. Efficiency vs Input Voltage Figure 8.
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Circuit Performance www.ti.com Figure 9. Switching Frequency vs. Input Voltage Figure 10. Current Limit vs Input Voltage Figure 11.
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Typical Waveforms www.ti.com Figure 12. Load Regulation 10 Typical Waveforms Trace 1 = SW Pin Trace 3 = VOUT Trace 4 = Inductor Current Vin = 12V, Iout = 200 mA Figure 13. Continuous Conduction Mode Trace 1 = SW Pin Trace 3 = VOUT Trace 4 = Inductor Current Vin = 12V, Iout = 0 mA Figure 14.
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Typical Waveforms www.ti.com Trace 1 = SW Pin Trace 3 = VOUT Trace 4 = Inductor Current Vin = 12V, Iout = 0 mA Figure 15.
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PC Board Layout www.ti.com 11 PC Board Layout Figure 16. Board Silkscreen Figure 17. Board Top Layer Figure 18.
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