LM22670INVEVAL National Semiconductor, LM22670INVEVAL Datasheet

LM22670INVEVAL

Manufacturer Part Number

LM22670INVEVAL

Description

BOARD EVALUATION FOR LM22670INV

Manufacturer

National Semiconductor

Series

SIMPLE SWITCHER®r

Datasheets

1.LM22670TJE-ADJNOPB.pdf (20 pages)

2.LM22670INVEVAL.pdf (10 pages)

Specifications of LM22670INVEVAL

Main Purpose

DC/DC, Step Down

Outputs And Type

1, Non-Isolated

Voltage - Output

-5V

Current - Output

1.5A

Voltage - Input

5 ~ 35V

Regulator Topology

Buck

Frequency - Switching

500kHz

Board Type

Fully Populated

Utilized Ic / Part

LM22670

Lead Free Status / RoHS Status

Not applicable / Not applicable

Power - Output

Other names

*LM22670INVEVAL

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LM22670 Evaluation Board

Inverting Topology and

Application Notes

Introduction

The LM22670 inverting evaluation board is designed to

demonstrate the capabilities of the LM22670 switching regu-

lator in a polarity-inverting topology. The LM22670 inverting

evaluation board schematic shown in Figure 1 is configured

to provide an output of minus 5V (-5V) up to 1.5A load current

with an input voltage range of 6V to 35V. The typical operating

frequency is 500 kHz.

The evaluation board is designed to operate at ambient tem-

peratures up to 50°C. Typical evaluation board performance

and characteristics curves are shown in Figure 5 through Fig-

ure 7. Figure 8 shows the PCB layout.

To aid in the design and evaluation of DC/DC polarity-invert-

ing converter solutions based on the LM22670 switching

regulator, the evaluation board can be re-configured for dif-

ferent output voltages.

The evaluation board is designed to highlight applications

with a small solution size. This implies that there will be a

trade-off between size and the area of heat dissipation avail-

able. If this evaluation board is operated continuously at a full

1.5A load, it will get hot. For more negative output voltages

than the pre-adjusted -5V, the total output power as well as

the total power conversion losses will increase.

Test points are provided to enable easy connection and mon-

itoring of critical signals.

For more information about device function and electrical

characteristics, refer to the LM22670 datasheet. The evalua-

tion board can be reconfigured for a different load current and

output voltage. For design limitations, see the IC Device Rat-

ings section.

The performance of the evaluation board is as follows:

Input Range: 6V to 35V, 12V nominal

Output Voltage: -5V

Output Current Range: 0A to 1.5A

Frequency of Operation: 500 kHz

Board Size: 1.5 X 1.65 inches

Package: PSOP-8

300752

National Semiconductor

Application Note 1888

Frederik Dostal

July 9, 2009

Evaluation Board Startup

Before applying power to the LM22670 polarity-inverting eval-

uation board, all external connections should be verified. The

external power supply input must be turned off and connected

with proper polarity to the VIN and GND posts. A load resistor

or electronic load should be connected between the VOUT

and GND posts as desired. Both the VIN and VOUT connec-

tions should use the closest GND posts respective to VIN or

VOUT. The output voltage can be monitored with a multi-me-

ter or oscilloscope at the VOUT post. Once all connections to

the evaluation board have been verified, input power can be

applied. A load resistor or electronic load does not need to be

connected during startup. If the EN test point is left floating,

the output voltage will ramp up when an input voltage is ap-

plied. Make sure that the external power supply (input voltage

power source) is capable of providing enough current so that

the adjusted output voltage can be obtained. Keep in mind

that the startup current will be greater than the steady state

current.

Principle of Operation

The polarity-inverting converter, shown in Figure 1, uses the

basic principle of energy storage in the inductor, L1, during

on-time and transfers the energy through the diode, D1, to the

output during off-time. When the switch turns on, the diode is

reverse biased and the inductor current will ramp up linearly.

When the switch turns off, the inductor will reverse its polarity

in order to maintain the peak switch current. At that time, the

diode, D1, will be forward biased and the energy stored in the

inductor will be transferred to the load as well as the output

capacitor, C4.

Since the switch node is negative with respect to ground, the

output voltage across the output capacitors (C4 and C5) will

become negative.

This type of polarity-inverting converter can step-up and step-

down the magnitude of the input voltage, which makes this

circuit a buck-boost converter. However, the output voltage is

always negative in reference to ground.

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LM22670INVEVAL Summary of contents

Page 1

... Input Range 35V, 12V nominal Output Voltage: -5V Output Current Range 1.5A Frequency of Operation: 500 kHz Board Size: 1.5 X 1.65 inches Package: PSOP-8 © 2009 National Semiconductor Corporation National Semiconductor Application Note 1888 Frederik Dostal July 9, 2009 Evaluation Board Startup Before applying power to the LM22670 polarity-inverting eval- uation board, all external connections should be verified ...

Page 2

FIGURE 1. Evaluation Board Schematic Inverting Topology Design Considerations Figure 1 shows the typical configuration of a polarity-inverting converter using the LM22670 switching regulator. This invert- ing topology design can be implemented with any member of the LM2267X SIMPLE SWITCHER ...

Page 3

Inductor Selection Duty-cycle is calculated as: where V is the D1 diode voltage drop and V D drop across the LM22670 internal power N-FET. The R of the FET is specified in the LM22670 datasheet to cal- (ON) culate V ...

Page 4

Input Capacitor Selection The input capacitor needs to be selected based on its low ESR value and the high RMS current rating capable of sup- porting high current changes on the input of the application. Low ESR bypass capacitors located ...

Page 5

... To in- crease the phase margin for applications using low input voltages, select a higher capacitance value for C6 and C7. It TABLE 1. LM22670INVEVAL Bill of Materials for VOUT = -5V, Designed for 1.5A Output Current Ref # C1, C8 2.2 µF 50V ceramic C2 22 µ ...

Page 6

Performance Characteristics Unless otherwise specified 12V FIGURE 5. Start-Up Waveforms (Load Resistor = 4Ω) FIGURE 6. Efficiency vs I www.national.com = 25° -5V. A OUT 30075219 FIGURE 7. Overall Loop Gain and Phase 30075217 ...

Page 7

PCB Layout Diagram 7 30075213 www.national.com ...

Page 8

... FIGURE 8. LM22670INVEVAL PCB Layout 8 30075214 ...

Page 9

9 www.national.com ...

Page 10

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