User's Guide SNVA306B - November 2007 - Revised April 2013 AN-1758 LP38502SD-ADJ Evaluation Board 1 Introduction The LP38502SD-ADJ is a 1.5A Low-Dropout (LDO) linear regulator in the LLP-8 package which has an adjustable output voltage (set by external resistors). 2 Basic Application Circuit The basic application circuit which 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. The equation to determine output voltage is: VOUT = VADJ x (1 + R1/R2) (1) where VADJ is typically 0.6V. The board has installed values of: R1 = 19.1k R2 = 6.04k which sets the output to 2.5V. 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 0.6V. This is obtained by installing a jumper or low value resistor (less than 10) at R1. The maximum usable output voltage is limited by the maximum input voltage which is 5.5V. Since rated dropout voltage at full current is 0.375V, this means the maximum usable output voltage for full current operation is about 5.1V. 4 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 k, 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. All trademarks are the property of their respective owners. SNVA306B - November 2007 - Revised April 2013 Submit Documentation Feedback AN-1758 LP38502SD-ADJ Evaluation Board Copyright (c) 2007-2013, Texas Instruments Incorporated 1 Power Dissipation www.ti.com However, if the value of R2 is increased above 10 k, the effect of the internal zero gradually diminishes and the phase margin is reduced. At an R2 value of approximately 50 k, the phase margin will be low enough that instability may occur. In such cases, some of the lost phase margin can be regained by placing a capacitor at C2. Although it is sometimes possible to regain adequate phase margin this way, it is recommended that the data sheet guidelines be followed and R2 not exceed 10 k (so C2 is never required). 5 Power Dissipation The power dissipated within the regulator IC is given by: PD = IL (VIN - VOUT) (2) Where: PD is the power dissipated in the IC regulator U1. IL is the load current. VIN is the value of VIN measured at TP2 (not at J1). VOUT is the value of VOUT measured at TP5 (not at J4). The thermal resistance of U1 from junction to ambient can be assumed to be approximately 80C/W for this assembly. That means that the junction temperature will rise about 80C above ambient for each Watt of power dissipated within the IC. The parametric specifications of the IC are for a maximum junction temperature of 125C, and the maximum allowable junction temperature is 150C. This limits maximum usable power dissipation for this package to approximately 1.5W. If power dissipation exceeds this (and the junction temperature approaches 150C) the part may go into thermal shutdown. 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. Top View of the PCB Assembly 2 AN-1758 LP38502SD-ADJ Evaluation Board SNVA306B - November 2007 - Revised April 2013 Submit Documentation Feedback Copyright (c) 2007-2013, Texas Instruments Incorporated Bill of Materials www.ti.com 7 Bill of Materials The initial evaluation boards were built up for a 2.5V regulated output. The components used in the PCB assembly are listed below: Table 1. Bill of Materials Designator Qty. Description Part Number PCB 1 PC board 551600046-001 R1 1 Resistor, 19.1k, 1% Vishay CRCW08051912F R2 1 Resistor, 6.04k, 1% Vishay CRCW08056041F R3 1 Resistor, 51.1k, 1% Vishay CRCW08055112F J1 1 Red Banana Jack Digi-Key J-151-ND J2,J3 2 Black Banana Jack Digi-Key J-152-ND J4 1 Blue Banana Jack Digi-Key J-155-ND C1,C3 2 Cap, CER, 10 F Digi-Key PCC2225CT-ND TP1-TP5 5 Test Points Newark 160-2043-02-01-00 U1 1 IC, LP38502 LP38502SD-ADJ C2 ****NOT INSTALLED FOR THIS ASSEMBLY SNVA306B - November 2007 - Revised April 2013 Submit Documentation Feedback AN-1758 LP38502SD-ADJ Evaluation Board Copyright (c) 2007-2013, Texas Instruments Incorporated 3 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. 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