LM5085EVAL/NOPB National Semiconductor, LM5085EVAL/NOPB Datasheet

LM5085EVAL/NOPB

Manufacturer Part Number

LM5085EVAL/NOPB

Description

EVAL BOARD FOR LM5085

Manufacturer

National Semiconductor

Datasheet

1.LM5085EVAL.pdf (14 pages)

Specifications of LM5085EVAL/NOPB

Lead Free Status / RoHS Status

Lead free / RoHS Compliant

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LM5085 Evaluation Board

Introduction

The LM5085EVAL evaluation board provides the design en-

gineer with a fully functional buck regulator, employing the

LM5085 PFET switching controller which uses the constant

on-time (COT) operating principle. This evaluation board pro-

vides a 5V output over an input range of 5.5V to 55V. The

circuit delivers load currents to 4.5A, with current limit set at

≊

C5. Its use is described later in this document.

The board’s specification are:

Theory of Operation

Refer to the evaluation board schematic in Figure 7. 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:

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.

The data sheet for the Si7465 PFET indicates a typical turn-

on delay of 8 ns, and a typical turn-off delay of 65 ns, resulting

in an additional 57 ns at the SW node. The SW on-time of this

evaluation board ranges from

≊

to maintain a nearly constant switching frequency.

7.6A. The board is populated with all components except

357 ns at V

= 55V. The on-time varies inversely with V

≊

3479 ns at V

300720

FIGURE 1. Evaluation Board - Top Side

= 5.5V, to

National Semiconductor

Application Note 1878

Dennis Morgan

November 13, 2008

•

During the off-time, the load current is supplied by the inductor

and the output capacitor (C6, C7). When the output voltage

falls sufficiently that the voltage at FB is below the reference

voltage (1.25V), the regulation comparator initiates a new on-

time period. For stable, fixed frequency operation, a minimum

of 25 mV of ripple is required at the FB pin to switch the reg-

ulation comparator. The required ripple is generated by R7

and C10, and supplied to the FB pin via C9.

The current limit threshold is set by the sense resistor (R5),

and R3 at the ADJ pin, and is

sink at the ADJ pin sets a constant voltage across R3. When

the voltage across R5 exceeds the voltage across R3 the

current limit comparator switches to shut off Q1, and the

LM5085 forces a longer-than-normal off-time. The long off-

time is a function of the input voltage (V

the FB pin, and is necessary to allow the inductor current to

decrease at least as much, if not more, than the current in-

crease which occurred during the on-time.

The circuit may be shutdown at any time by grounding the

Enable test point (EN, TP1). Removing the ground connection

allows normal operation to resume.

Input Voltage: 5.5V to 55V, 60V maximum

Output Voltage: 5V

Maximum load current: 4.5A for V

Minimum load current: 0A

Current Limit Threshold:

Measured Efficiency: 97.2% (V

Nominal Switching Frequency: 300 kHz

Size: 3.1 in. x 1.5 in. x 0.78 in

≊

7.6A

≊

7.6A on this board. A current

= 5.5V, I

>8V

) and the voltage at

OUT

www.national.com

= 0.6Amps)

30072001

Related parts for LM5085EVAL/NOPB

LM5085EVAL/NOPB Summary of contents

Page 1

... V ≊ 357 55V. The on-time varies inversely with maintain a nearly constant switching frequency. © 2008 National Semiconductor Corporation National Semiconductor Application Note 1878 Dennis Morgan November 13, 2008 • Input Voltage: 5.5V to 55V, 60V maximum • ...

Page 2

Refer to the LM5085 data sheet for a detailed block diagram, and a complete description of the various functional blocks. Board Layout and Probing The pictorial in Figure 1 shows the placement of the circuit components. The following should be ...

Page 3

A) Sense resistor method – This evaluation board is sup- plied configured for the sense resistor method of current limit detection. Jumpers A-B are in place at both jumper locations (JP1, JP2), which connects the ADJ pin resistor (R3) and ...

Page 4

B) Reduced Ripple Level Configuration: This configuration generates more ripple at V than the above configuration, OUT but uses one less capacitor. If some ripple is acceptable in the application, this configuration is slightly more economical, and simpler. R6 and ...

Page 5

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 V ≥ resistors (R1, R2). Since 25 mVp-p are required at the ...

Page 6

... TKD C2012X7R2A332K On Semi MBRB2060CT Wurth XXL 7447709150 Vishay Si7465DP Vishay CRCW08051002F Vishay CRCW08053401F Vishay CRCW08051911F Vishay CRCW08059092F Vishay WSL2010R0100F Vishay CRCW08050000Z Vishay CRCW08052322F National Semiconductor LM5085MY 6 Package Value 2220 6.8 µF, 100V 1812 4.7 µF, 100V 0805 0.47 µF, 16V 0805 ...

Page 7

Circuit Performance FIGURE 10. Switching Frequency vs. Input Voltage 30072015 FIGURE 8. Efficiency vs Load Current 30072032 FIGURE 9. Efficiency vs Input Voltage 30072017 7 www.national.com ...

Page 8

FIGURE 11. Output Voltage Ripple 30072030 FIGURE 12. Line Regulation 8 ...

Page 9

FIGURE 13. Load Regulation 9 www.national.com ...

Page 10

Typical Waveforms Trace 4 = Inductor Current Trace OUT Trace Node V = 48V 1Amp IN OUT Minimum Ripple Configuration (Option A) Trace 4 = Inductor Current Trace OUT ...

Page 11

Trace 4 = Inductor Current Trace OUT Trace Node V = 48V OUT FIGURE 16. Discontinuous Conduction Mode (Expanded Scale) 30072021 11 www.national.com ...

Page 12

PC Board Layout www.national.com Board Silkscreen Board Top Layer Board Bottom Layer (Viewed from Top) 12 30072022 30072023 30072024 ...

Page 13

13 www.national.com ...

Page 14

... For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers www.national.com/amplifiers Audio www.national.com/audio Clock and Timing www.national.com/timing Data Converters www.national.com/adc Interface www.national.com/interface LVDS www.national.com/lvds Power Management www.national.com/power Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www ...

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