LX8211A Microsemi Corporation, LX8211A Datasheet - Page 7

• Manufacturer Part Number: LX8211A
• Description: 150ma Low Dropout Regulator
• Manufacturer: Microsemi Corporation

Copyright © 2006
Rev. 1.0, 2006-04-07
Description
To improve load transient response and noise rejection a bypass
capacitor is recommended, but it is not required for stability. There are
no requirements for the ESR (Equivalent Series Resistor) on the input
capacitor, but tolerance and temperature coefficient must be
considered when selecting the capacitor to ensure the capacitor will be
around 1µF over the entire operating temperature range. For the
LX8211A connect a ceramic type capacitor (1µF) between V
ground.
To maintain a constant output voltage a feedback loop is necessary.
The feedback loop causes a well known a phase shift, the amount of
phase shift determines the loop stability. Therefore the LX8211A, like
any low dropout regulator, requires an output capacitor with low ESR
(Equivalent Series Resistor) connected between V
stabilize the internal control loop. For the LX8211A a ceramic,
tantalum, or electrolytic capacitor with the minimum recommended
capacitance value at 10µF with ESR between 5mΩ and 1Ω will satisfy
the stability for whole operating range.
The LX8211A-00 adjustable regulator’s output voltage can be
externally set by connecting the ADJ pin to an external resistor divider
(See Figure 1). The output voltage can be calculated using:
The Enable pin allows the LX8211A to be turned on and off. The
Enable pin is compatible with standard TTL-CMOS levels. A logic
zero (0.4V) on the Enable pin shuts the LX8211A off and reduces the
supply current to less than 1µA. Pulling the Enable input high (2.0V)
causes normal operation to resume. If the Enable feature is not used,
this pin can be connected to V
The LX8211A is a family of LDO (Low Dropout) linear regulators in
a small SOT-23 Package, which provides a compact and cost effective
solution. The family includes several fixed output voltages, along with
an adjustable version. The internal PNP power device provides low
dropout regulation with fast line and load transient response. It also
includes internal current limit and thermal shutdown circuitry. This
section contains information about the selection of external capacitors,
thermal, and layout consideration.
Input Capacitor
Output Capacitor
Adjustable Output Voltage
Enable
V
OUT
=
1.25
⎡
⎢
⎣
TM
1
+
R2
R1
⎤
⎥
⎦
+
[
I
ADJ
•
R2
]
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
IN
.
A P P L I C A T I O N I N F O R M A T I O N
OUT
Integrated Products Division
and GND to
®
Microsemi
IN
and
The LX8211A is stable even at zero load; no minimum load is required
for operation, which is especially important in keep-alive applications.
However line regulation is improved with a minimum load of 100µA.
Thermal protection shuts the LX8211A down when the junction
temperature exceeds approximately 150°C; there is no appreciable
thermal hysteresis.
The LX8211A includes over current protection, when the output load
current exceeds typically 400mA the circuit forces the regulator to
decrease in output.
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 does not exceed its 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.
Power dissipation for regulator can be calculated using the following
equation:
P
(Note: power dissipation resulting from quiescent current is negligible)
For the SOT23 package, thermal resistance, θ
mounted on a FR4 copper clad PCB. Junction temperature of the
integrated circuit can be calculated using:
T
An example:
Given conditions: T
Total Power dissipation
Junction Temperature
The layout must be done with low impedance paths for V
Ground by applying sufficient wide traces to avoid voltage drops and
pick up noise. Adding an area of PCB copper to the GND pin (pin2)
will reduce the overall
substrate. This will lower the device junction temperature improving
the overall output accuracy. The input / output capacitors must be
placed as close as possible to the voltage regulator output pin. To allow
the same voltage reference for all circuits, use a star configuration from
the output capacitor to the different loads.
Minimum Load Requirement
Temperature Protection
Short Circuit Current Protection
Thermal Consideration
Calculated values:
Layout Consideration:
D
J
= T
= (V
150mA Low Dropout Regulator
A
IN(MAX)
+ T
P
RISE
RODUCTION
-V
, substituting:
OUT(MIN)
A
= 60°C, V
θ
JA
P
T
D(total)
) * I
J
since it is thermally connected to the device
= 60°C +(0.15W *220°C/W) = 93°C
D
J
of 150°C can impact reliability. Due to
T
ATA
OUT(MAX)
IN
= (4.2-2.7) V * 0.1A =0.15W
A
= 4.2V, V
+ (P
S
HEET
D
* θ
OUT
JA
)
= 2.7V, I
.
JA
LX8211A
is 220°C/W when
OUT
IN
= 100mA.
, V
Page 7
OUT
, and