Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsDev KitsPCB design board

User's Guide

SNVA384B–February 2009–Revised April 2013

AN-1933 LM25085A Evaluation Board

1 Introduction

The LM25085AEVAL evaluation board provides the design engineer with a fully functional buck regulator,

employing the LM25085A PFET switching controller which uses the constant on-time (COT) operating

principle. This evaluation board provides a 1V output over an input range of 4.5V to 24V. The circuit

delivers load currents to 5A, with current limit set at ≊8.2A. The board is populated with all components

except C5 and C7.

The board’s specification are:

• Input Voltage: 4.5V to 24V

• Output Voltage: 1V

• Maximum load current: 5A

• Minimum load current: 0A

• Current Limit Threshold: ≊8.2A

• Measured Efficiency: 77.5% (V

= 4.5V, I

OUT

= 1Amp, typical efficiency for converter providing a 1V

output)

• Nominal Switching Frequency: 200 kHz

• Size: 3.1 in. x 1.5 in.

Figure 1. Evaluation Board - Top Side

All trademarks are the property of their respective owners.

SNVA384B–February 2009–Revised April 2013 AN-1933 LM25085A Evaluation Board

Submit Documentation Feedback

Summary of content (12 pages)

PAGE 1
User's Guide SNVA384B – February 2009 – Revised April 2013 AN-1933 LM25085A Evaluation Board 1 Introduction The LM25085AEVAL evaluation board provides the design engineer with a fully functional buck regulator, employing the LM25085A PFET switching controller which uses the constant on-time (COT) operating principle. This evaluation board provides a 1V output over an input range of 4.5V to 24V. The circuit delivers load currents to 5A, with current limit set at ≊8.2A.
PAGE 2
Theory of Operation 2 www.ti.com Theory of Operation Refer to the evaluation board schematic in Figure 6. When the circuit is in regulation, the on-time at the PGATE output pin is determined by R4 and the voltage at VIN according to the equation: -7 tON = 1.45 x 10 x (R4 + 1.4) + 50 ns VIN ± 1.56V + R4/3167 (1) where R4 is in kohms. The on-time at the SW node (junction of Q1, L1 and D1) is longer than the above calculated on-time due to the difference of the turn-on and turn-off delay of Q1.
PAGE 3
Current Limit www.ti.com 5 Current Limit The LM25085A peak current limit detection operates by sensing the voltage across either the RDS(ON) of Q1, or a sense resistor (R5), during the on-time and comparing it to the voltage across R3 at the ADJ pin. The current limit threshold is reached when the sensed voltage exceeds the voltage across R3. When current limit is reached Q1 is immediately switched off.
PAGE 4
Output Ripple Control www.ti.com and the load current at current limit calculates to 5.35A at 4.5V, and 5.24A at 24V. To change the current limit threshold the value for R5 should be chosen to achieve 50 mV to 100 mV across it at current limit, staying within the practical limitations of power dissipation and physical size of the resistor. A larger value for R5 reduces the effects of the current limit comparator offset, but at the expense of higher power dissipation.
PAGE 5
Output Ripple Control www.ti.com LM25085A PGATE Q1 L1 6.8 PH Pads for wire loop VOUT 1V D1 GND FB R1 1.1k C10 3300 pF R7 48.7k C9 0.01 PF R6 0: C6 68 PF R2 10k GND Figure 3. Minimum Ripple Using R7, C9, C10 B) Reduced Ripple Level Configuration: This configuration generates more ripple at VOUT than the above configuration, but uses one less capacitor. If some ripple is acceptable in the application, this configuration is slightly more economical, and simpler.
PAGE 6
Monitor The Inductor Current www.ti.com C) Lowest Cost Configuration: This configuration is the same as option B above, but with C5 removed. The ripple at the FB pin is attenuated from that at VOUT by the feedback resistors (R1, R2). Since ≥25 mVp-p are required at the FB pin, R6 is chosen to generate ≥28 mV at VOUT. Since the minimum ripple current in this circuit is 622 mAp-p the minimum value for R6 calculates to 45 mΩ.
PAGE 7
Bill of Materials www.ti.com 9 Bill of Materials Table 1. Bill of Materials Item Description Mfg., Part Number Package Value C1, C2, C11 Ceramic Capacitor Taiyo Yuden GMK325BJ106KN 1210 10 µF, 35V C3 Ceramic Capacitor TDK C2012X7R1C474K 0805 0.47 µF, 16V C4 Ceramic Capacitor TDK C2012X7R2A102K 0805 1000 pF, 100V Unpopulated 0805 Ceramic Capacitor TDK C3225X5R0J686M 1210 C5 C6 C7 68 µF, 6.3V Unpopulated C8 Ceramic Capacitor TDK C2012X7R2A104K 0805 0.
PAGE 8
Circuit Performance 10 www.ti.com Circuit Performance (Efficiencies in this range are typical for a buck converter producing a 1.0V output) Figure 7. Efficiency vs Load Current Figure 8. Efficiency vs Input Voltage Figure 9. Switching Frequency vs.
PAGE 9
Circuit Performance www.ti.com Figure 10. Output Voltage Ripple Figure 11. Line Regulation Figure 12.
PAGE 10
Typical Waveforms 11 www.ti.com Typical Waveforms Trace 4 = Inductor Current Trace 3 = VOUT Trace 1 = SW Node VIN = 12V, IOUT = 1Amp Minimum Ripple Configuration (Option A) Figure 13. Continuous Conduction Mode Trace 4 = Inductor Current Trace 3 = VOUT Trace 1 = SW Node VIN = 12V, IOUT = 0 Figure 14. Discontinuous Conduction Mode Trace 4 = Inductor Current Trace 3 = VOUT Trace 1 = SW Node VIN = 12V, IOUT = 0 Figure 15.
PAGE 11
PC Board Layout www.ti.com 12 PC Board Layout Figure 16. Board Silkscreen Figure 17. Board Top Layer Figure 18.
PAGE 12
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.