D =

FB

VIN

LM1771

HG

LG

GND

EN

EN

LM1771

www.ti.com

SNVS446D – JUNE 2006 – REVISED JANUARY 2016

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers must

validate and test their design implementation to confirm system functionality.

8.1 Application Information

Design Guide details the design procedure to follow for the typical application. Because of the nature of constant

ON-time controller, a certain degree of iteration might be necessary on the sizing of some key components to

achieve optimal response, such as the inductor L1.

8.2 Typical Applications

8.2.1 LM1771 Typical Application

Figure 19. Example Circuit Schematic

8.2.1.1 Design Requirements

The main parameters needed to successfully design an application based on the LM1171 are typical of buck

DC–DC converters. The input and output voltage must be known as well as the worst-case input voltage

operating conditions. The maximum output current helps the designer size the inductor appropriately. In addition,

the designer needs to be aware of the requirement on the output capacitor to achieve stable operation. See

Output Capacitor for details on the output capacitor requirements.

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Design Guide

As with any DC–DC converter, numerous trade-offs are possible to optimize the design for efficiency, size, or

performance. These are taken into account and highlighted throughout this discussion.

Equation 7 calculates for any buck converter is duty-cycle. Ignoring conduction losses associated with the FETs

and parasitic resistances it can be approximated by:

(7)

A more accurate calculation for duty-cycle can be used that takes into account the voltage drops across the

FETs. Equation 8 determines the slight load dependency on switch frequency if needed. Otherwise the simplified

equation works well for component calculation.

Copyright © 2006–2016, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM1771