Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page
Copyright 2000
Rev. 0.3D, 2002-01-08
WWW.Microsemi .COM
LX8817
Dual Channel 2.5A Low Dropout Regulato
LINFINITY DIVISION
APPLICATION INFORMAT
APPLICATION INFORMATAPPLICATION INFORMAT
APPLICATION INFORMATION
IONION
ION
The LX8817 is part of a family of Dual LDO (Low Drop-Out)
linear regulators in Linfinity’s S-PAK power package which offer
maximum power dissipation in a low profile surface mount
technology. The family includes combination fixed and adjustable
versions. Each channel can supply up to 2.5A independently 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 a input
bypass capacitor is of at least 10uF is required. Generally we
recommend a 10uF ceramic or tantalum or 22uF electrolytic
capacitor.
OUTPUT CAPACITOR
The regulator requires output capacitors connected between each
output (V1, V2) to 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 LX8817x develops a 1.25V reference voltage between the
output and the adjust terminal (See Figure 2). By placing a resistor,
R1, 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
ADJ
is very small and constant when compared
with the current through R1, it represents a small error and can
usually be ignored.
LX8817-25
R1
R2
V
IN
ADJ
V
2
FIGURE 2 - BASIC ADJUSTABLE
REGULATOR
V
REF
1
2
1
12 RI
R
R
VV
ADJREF
+
+=
I
ADJ
1µA
V
1
MINIMUM LOAD REQUIREMENT
The LX8817 has a minimum load is requirement for proper
output regulation. This typical current is specified at 0mA for the
fixed output and 1ma for the adjustable output regulators.
TEMPERATURE PROTECTION
The thermal protection shuts the LX8817 down when the junction
temperature exceeds 160
0
C. Each output has independent thermal
shutdown capability. Exposure to absolute maximum rated
conditions for extended periods may affect device reliability, see
Thermal Considerations below.
CURRENT LIMIT PROTECTION
The LX8817 includes over current protection, when the output
load current exceeds typically 3A the circuit forces the regulator
decrease in output.
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
J
of 150°C can impact
reliability . Due to 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:
22IN(MAX)11IN(MAX)D
I)V(VI)V(VP ×−+×−=
(Note: power dissipation resulting from quiescent (ground) current
is negligible)
For the S-PAK package, thermal resistance, θ
TAB-AMB
is 25-
45
0
C/W depending on mounting technique when mounted on a FR4
copper clad PCB. Junction temperature of the integrated circuit can
be calculated using:
AMBRISE AMBTABRISE TABJUNCTIONJUNCTION
TTTT ++=
−−
JTDMAXTAB
θPT ×=
;
PCB2DREG1DREGAMBTAB
θ)P(PT ×+=
−
An example: Given conditions: T
A
= 50°C, V
IN
= 5.0V, V
1
= 2.5V,
I
1
= 210mA, V
2
= 3.3V I
2
= 1A.
Calculated values:
CC/WC/W 2.44.5(0.525)4.52.5V)(5VT
REG1 TABJ
°=°×=°×−=
−
C/WC/WVV 7.74.5(1.7)4.5(1A))3.3(5T
REG2 TABJ
°=°×=°××−=
−
C66.8C/W301.7W)(0.525WT
AMBRISETAB
°=°×+=
−
C124.5C50C66.8C7.7T
JUNCTION
°=°+°+°=
It is important to note that although each output of the regulator
will produce up to 2.5A 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.
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
S
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