Texas Instruments LP38798EVM, User Manual, Page: 2 / 11

Introduction
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1 Introduction
The LP38798EVM evaluation board is designed to demonstrate the capabilities of the LP38798-ADJ high
performance LDO voltage regulator. It is intended to provide for flexible circuit configurations and access
to points of interest. The circuit schematic is shown in
Figure 1 .
2 Operating Range
• Minimum Operating Input Voltage: 5.50V
• Maximum Operating Input Voltage: 20.0V
• Output Voltage : 5.00V
• Maximum Operating Output Current: 800mA
• Ambient Temperature Operating Range 0°C to 50°C
• Board Size 1.00 inches x 1.33 inches
While the values listed for the Operating Range are all accurate, some combinations of ambient
temperature (T
A
), device dissipation (P
D
), and the maximum junction temperature (T
J
) of 125°C may limit
the useable range. See the device datasheet for a full discussion of thermal considerations.
Typical evaluation board performance and characteristics curves are shown in the LP38798-ADJ
datasheet (Literature Number SNOSCT6). The PCB layout is shown in
Figure 3 through Figure 7 . Test
points are provided on the evaluation board for signal monitoring and optional Enable control.
3 Evaluation Board Start-Up
Before applying power to the LP38798EVM board, all external connections should be verified. The
external power supply must be turned off before being connected, Confirm proper polarity to the 'VIN' and
'GND' terminals before turning the external power supply on. An appropriate load should be connected
between the 'VOUT' and 'GND' terminals. Under basic evaluation conditions all of the test points can be
left open. The evaluation board will be in the normal operating mode when input power is applied.
Use of the ENABLE function is optional. Pulling the ENABLE pin low, either by connecting to voltage lower
than the typical OFF threshold of 1.00V, or by connecting directly to ground, will disable the output. Do not
connect the ENABLE pin to any voltage above the EN
(CLAMP)
voltage of 5.0V (typical).
4 Setting the Output Voltage
The LP38798EVM board is assembled with R1= 47.5 k Ω and R2= 15.0 k Ω , to set the output voltage to
5.00V.
Resistors R1 and R2 may be replaced, as needed, to achieve the desired output voltage as long as the
value for R2 is no less than 12.9 k
Ω
The following formula is used to determine the typical output voltage:
V
OUT
= ( ( V
FB
x ( 1 + (R1 / R2 ) ) ) + V
OS
) (1)
Alternately, the following formula can be used to determine the appropriate R1 value for a given R2 value
with a V
OUT
≥ 1.20V:
R1 = ( ( ( V
OUT
/ V
FB
) - 1) x R2 ) (2)
The following table suggests some ±1% tolerance values for R1, keeping R2 held at the installed value of
15.0 k
Ω , for a range of output voltages using the typical V
FB
value of 1.200V. This is not a definitive list,
as other combinations R1 and R2 exist that will provide similar, possibly better, performance.
Target V
OUT
R1 R2 Typical V
OUT
1.50V 3.74 k Ω 15.0 k Ω 1.499V
1.80V 7.50 k Ω 15.0 k Ω 1.800V
1.90V 8.66 k
Ω
15.0 k
Ω
1.893V
2.00V 10.0 k Ω 15.0 k Ω 2.000V
2.50V 16.2 k Ω 15.0 k Ω 2.496V
2 LP38798EVM User's Guide SNOA914 – March 2013
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