Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
Copyright © 2000
Rev. 1.2c, 2005-05-18
WWW.Microsemi .COM
LX8816
Dual Channel 1A Low Dropout Re
ulator
TM
®
APPLICATION INFORMATION
Description
The LX8816 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 one amp 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 2.2uF is required. Generally we recommend a
20uF ceramic or tantalum or 22uF electrolytic capacitor.
Output Capacitor
The regulator requires output capacitors connected between V
1
, V
2
and
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 LX8816 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
ADJ
is
very small and constant when compared with the current through R2, it
represents a small error and can usually be ignored.
LX8816-04
R1
R2
V
IN
ADJ
V
1
FIGURE 2 - BASIC ADJUSTABLE
REGULATOR
V
REF
1
2
1
1
1
RI
R
R
VV
ADJREF
+
⎜
⎜
⎝
⎛
⎟
⎠
⎞
+=
I
ADJ
1µA
V
2
Minimum Load Requirement
The LX8816 has a minimum load is requirement for proper output
regulation. This minimum current is specified at 0mA for the fixed
output and 2ma for the adjustable output regulators.
Temperature Protection
The thermal protection shuts the LX8816 down when the junction
temperature exceeds 140
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 LX8816 includes over current protection, when the output load
current exceeds ty
ically 1.4A 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:
P
D
= (V
IN(MAX)
-V
1
) * I
1
+
(V
IN(MAX)
-V
2
) * I
2
(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:
T
JUNCTION
= T
JUNCTION-TAB RISE _
+ T
TAB-AMB RISE
+
T
AMB
T
TAB
= P
D MAX
* θ
JT ;
T
TAB-AMB
= (P
D REG1
+ P
D REG2
) * θ
PCB
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:
T
J-TAB REG1
= (5V-2.5V) * (210mA) * 4.5°C/W = (0.525) * 4.5°C/W = 2.4°C
T
J-TAB REG2
= (5V-3.3V) * (1.0A) * 4.5°C/W = (1.7) * 4.5°C/W = 7.7°C
T
TAB-AMB RISE
= ( 0.525W + 1.7W ) * 30°C/W = 66.8°C
T
JUNCTION
=
10.1°C + 66.8°C +
5
0°C = 126.9°C
It is important to note that although each output of the regulator will
roduce 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.
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
