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:
VOUT = VADJ x (1 + R1/R2) (1)
where VADJ 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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Feedforward Capacitor C2
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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 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.
However, if the value of R2 is increased above 10 kΩ, the effect of the internal zero gradually diminishes
and the phase margin is reduced. At an R2 value of approximately 50 kΩ, 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 kΩ(so C2 is never
required).
5 Power Dissipation
The power dissipated within the regulator IC is given by:
PD= IL(VIN - VOUT) (2)
Where:
PDis the power dissipated in the IC regulator U1.
ILis 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 30°C/W for
this assembly. That means that the junction temperature will rise about 30°C above ambient for each Watt
of power dissipated within the IC.
The parametric specifications of the IC are for a maximum junction temperature of 125°C, and the
maximum allowable junction temperature is 150°C. This limits maximum usable power dissipation for this
package to approximately 3.3W. If power dissipation exceeds this (and the junction temperature
approaches 150°C) the part may go into thermal shutdown.
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PCB Component Layout
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
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Bill of Materials (BOM)
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7 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 Part Number
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, 51.1 kΩ, 1% Vishay CRCW08055112F
1 J1 Red Banana Jack Digi-Key J-151-ND
2 J2,J3 Black Banana Jack Digi-Key J-152-ND
1 J4 Blue Banana Jack Digi-Key J-155-ND
2 C1,C3 Cap, CER, 10 µF Digi-Key PCC2225CT-ND
5 TP1-TP5 Test Points Newark 160-2043-02-01-00
1 U1 IC, LP38502 LP38502
C2 ****NOT INSTALLED FOR THIS ASSEMBLY
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