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User's Guide

SNVA321A–January 2008–Revised April 2013

AN-1782 LP38502TJ-ADJ Evaluation Board

1 Introduction

The LP38502TJ-ADJ is a 1.5A low-dropout (LDO) linear regulator in the thin TO-263 package that has an

adjustable output voltage (set by external resistors).

2 Basic Application Circuit

The basic application circuit that is built up on the evaluation board is shown in Figure 1.

Figure 1. Basic Application Circuit

3 Changing the Output Voltage

The resistors R1 and R2 set the output voltage. Equation 1 determines output voltage:

OUT

= V

ADJ

x (1 + R1/R2) (1)

where V

ADJ

is typically 0.6 V.

The board has installed values of:

R1 = 19.1kΩ

R2 = 6.04kΩ

Which sets the output to 2.5 V. R1 can be adjusted to change the nominal output voltage to other values.

The minimum output voltage which can be set is the ADJ pin voltage, which is approximately 0.6 V. This is

obtained by installing a jumper or low value resistor (less than 10 Ω) at R1. The maximum usable output

voltage is limited by the maximum input voltage which is 5.5 V. Since rated dropout voltage at full current

is 0.375 V, this means the maximum usable output voltage for full current operation is about 5.1 V.

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SNVA321A–January 2008–Revised April 2013 AN-1782 LP38502TJ-ADJ Evaluation Board

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Summary of content (5 pages)

PAGE 1
User's Guide SNVA321A – January 2008 – Revised April 2013 AN-1782 LP38502TJ-ADJ Evaluation Board 1 Introduction The LP38502TJ-ADJ is a 1.5A low-dropout (LDO) linear regulator in the thin TO-263 package that has an adjustable output voltage (set by external resistors). 2 Basic Application Circuit The basic application circuit that is built up on the evaluation board is shown in Figure 1. Figure 1. Basic Application Circuit 3 Changing the Output Voltage The resistors R1 and R2 set the output voltage.
PAGE 2
Feedforward Capacitor C2 4 www.ti.com Feedforward Capacitor C2 The PCB layout includes a location for C2, which is a feedforward capacitor connected across R1. If the data sheet guidelines are followed, and R2 does not exceed 10 kΩ, C2 is not required and has no effect on performance. The internal compensation is such that an internal zero provides more than adequate phase margin so external compensation is never needed.
PAGE 3
PCB Component Layout www.ti.com 6 PCB Component Layout The components listed in the basic application circuit can be identified using the silkscreen on the top layer of the PCB (see Figure 2): Figure 2.
PAGE 4
Bill of Materials (BOM) 7 www.ti.com Bill of Materials (BOM) The initial evaluation boards were built up for a 2.5 V regulated output. The components used in the PCB assembly are listed Table 1. Table 1. Bill of Materials (BOM) Qty. Designator Description 1 PCB PC board 551600099-001 1 R1 Resistor, 19.1 kΩ, 1% Vishay CRCW08051912F 1 R2 Resistor, 6.04 kΩ, 1% Vishay CRCW08056041F 1 R3 Resistor, 51.
PAGE 5
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.