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LX8821 Datasheet(PDF) 5 Page - Microsemi Corporation

Part No. LX8821
Description  DUAL CHANNEL 2.5A LOW DROPOUT REGULATOR
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Maker  MICROSEMI [Microsemi Corporation]
Homepage  http://www.microsemi.com
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LX8821 Datasheet(HTML) 5 Page - Microsemi Corporation

   
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PRODUCTION DATA SHEET
Microsemi
Microsemi Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
Copyright
© 2005
Rev. 1.1, 2005-03-09
LX8821
DUAL CHANNEL 2.5A LOW DROPOUT REGULATOR
TM
®
APPLICATION INFORMATION
The LX8821 is part of a family of Dual LDO (Low Drop-Out)
linear regulators in Microsemi’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 an input
bypass capacitor is of at least 10µF is required. Generally we
recommend a 10µF ceramic or tantalum or 22µF 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 10µF. 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 LX8821 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 IADJ is very small and constant when compared with the
current through R2, it represents a small error and can usually be
ignored.
R1
R2
V
IN
ADJ
V
2
V
REF
1
2
1
1
2
R
I
R
R
V
V
ADJ
REF
+
⎜⎜
+
=
I
ADJ
1µA
V
1
Part
LX8821
Figure 2 –
Basic Adjustable Regulator
MINIMUM LOAD REQUIREMENT
The LX8821 has a minimum load 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 LX8821 down when the junction
temperature exceeds 160°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 LX8821 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 TJ 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:
2
2
IN(MAX)
1
1
IN(MAX)
D
I
)
V
(V
I
)
V
(V
P
×
+
×
=
(Note: power dissipation resulting from quiescent (ground) current
is negligible)
Thermal resistance,
θTAB-AMB ranges from 25-450C/W depending
on mounting technique using a FR4 copper clad PCB. Junction
temperature of the integrated circuit can be calculated using:
AMB
AMB
TAB
TAB
JUNCTION
JUNCTION
T
T
T
T
+
+
=
JT
DMAX
TAB
θ
P
T
×
=
;
PCB
2
DREG
1
DREG
AMB
TAB
θ
)
P
(P
T
×
+
=
An example: Given conditions: TA = 50°C, VIN= 5.0V, V1= 1.5V,
I1= 210mA, V2= 3.3V I2= 1A.
Calculated values:
C
3.3
C/W
4.5
(0.735)
C/W
4.5
(0.21A)
1.5V)
(5V
T
REG1
TAB
J
°
=
°
×
=
°
×
×
=
C
C/W
C/W
V
V
7.7
4.5
(1.7)
4.5
(1A)
)
3.3
(5
T
REG2
TAB
J
°
=
°
×
=
°
×
×
=
C
73.1
C/W
30
1.7W)
(0.735W
T
AMB
TAB
°
=
°
×
+
=
C
130.8
C
50
C
73.1
C
7.7
T
MAX
-
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 is not exceeded.


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