LX8818-01CDT Microsemi Corporation, LX8818-01CDT Datasheet - Page 7

• Manufacturer Part Number: LX8818-01CDT
• Description: Low Drop Out Regulator - Positive Adjustable, Package : (none)
• Manufacturer: Microsemi Corporation

Copyright  2000
Rev. 1.0, 2002-10-01
Description
The LX8818 is part of a family of LDO (Low Drop-Out) linear
regulators in the TO-252 or MO229 packages which offer maximum
power dissipation in a low profile surface mount technology. The
family includes either fixed or adjustable output versions. The output
can supply up to one amp with a regulator design optimized for system
efficiency by consuming minimal ground current and directing
quiescent current to the load.
Input Capacitor
To improve load transient response and noise rejection an input bypass
capacitor of at least 10uF is required. Generally we recommend a
10uF ceramic or tantalum or 22uF electrolytic capacitor.
Output Capacitor
The regulator requires an output capacitor connected between V
GND to stabilize the internal control loop. Many types of capacitors
are available, with different capacitance values tolerances, temperature
coefficients and equivalent series resistance. We recommend a
minimum of 4.7uF. To ensure good transient response from the power
supply system under rapidly changing current load conditions,
designers generally use additional output capacitors connected in
parallel. Such an arrangement serves to minimize the effects of the
parasitic resistance (ESR) and inductance (ESL) that are present in all
capacitors. The regulator has been tested stable with capacitor ESR’s
in the range of 0.05 to 2 ohms. We have found it best to use the same
type of capacitor for both input and output bypass.
Adjustable Output Voltage
The LX8818 develops a 1.25V reference voltage between the adjust
terminal and GND (See Figure 2). By placing a resistor, R2, between
these two terminals, a constant current is caused to flow through R1
and down through R2 to set the overall output voltage.. Because I
very small and constant when compared with the current through R2, it
represents a small error and can usually be ignored.
Minimum Load Requirement
The LX8818 has a minimum load requirement for proper output
regulation. This minimum current is specified at 0mA for the fixed
output and 5ma for the adjustable output.
V
out
FIGURE 2 - BASIC ADJUSTABLE
V
I N T E G R A T E D
=
IN
V
REF




LX8818-0x
1
REGULATOR
+
R
R
1
2
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
1µA




I
ADJ
+
I
ADJ
ADJ
R
1
P R O D U C T S
V
V
REF
out
A P P L I C A T I O N I N F O R M A T I O N
R1
R2
Linfinity Microelectronics Division
Microsemi
OUT
ADJ
to
is
Temperature Protection
The thermal protection shuts the LX8818 down when the junction
temperature exceeds 140
conditions for extended periods may affect device reliability (see
Thermal Considerations below).
Current Limit Protection
The LX8818 includes over current protection. When the output load
current exceeds typically 1.4A, the circuit forces the regulator output
to decrease.
Thermal Considerations
Thermal shutdown protects the integrated circuit from thermal
overload caused from a rise in junction temperature during power
dissipation. This means of protection is intended for fault protection
only and not as a means of current or power limiting during normal
application usage. Proper thermal evaluation should be done to ensure
that the junction temperature dose not exceed it’s maximum rating.
Operating at the maximum T
variation in individual device electrical characteristics and thermal
resistance , the built in thermal overload protection may be activated at
power levels slightly above or below the rated dissipation. Also peak
output power should be considered for each individual output.
Power dissipation for regulator can be calculated using the following
equation:
P
(Note: power dissipation resulting from quiescent (ground) current is negligible)
For the D-PAK package, thermal resistance, θ
depending on mounting technique when mounted on a FR4 copper
clad PCB. Junction temperature of the integrated circuit can be
calculated using:
T
T
An example: Given conditions: T
I
Calculated values:
T
T
T
It is important to note that although each output of the regulator will
produce up to 1A in current, the individual or total power dissipation
may limit the useful total current draw. The junction temperature
should be calculated for each individual output as well as the
combined outputs to insure the maximum junction temperature in not
exceeded.
out
D
JUNCTION
TAB
J-TAB REG
TAB-AMB RISE
JUNCTION
= 200mA.
= (5V-3.3V) * (200mA) * 6°C/W = (440mW) * 6°C/W = 2.6°C
= (V
= P
1A Low Dropout Regulator
IN(MAX)
D MAX
= T
=
P
2.6°C + 14.1°C +
RODUCTION DATA SHEET
JUNCTION-TAB RISE _
-V
= ( 440mW ) * 32°C/W = 14.1°C
* θ
out
JT ;
) * I
T
0
out
C. Exposure to absolute maximum rated
TAB-AMB
J
of 150°C can impact reliability . Due to
5
0°C = 66.7°C
A
+ T
= P
= 50°C, V
TAB-AMB RISE
D REG
* θ
TAB-AMB
IN
= 5.0V, V
PCB
+ T
LX8818
is 25-45
AMB
out
= 3.3V,
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