LX8211
TM
®
150mA CMOS Low Dropout Regulator
P
RODUCTION
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ATA
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HEET
APPLICATION INFORMATION
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.C
OM
Description
The LX8211 is a family of LDO (Low Drop-Out) 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 PMOS power device provides low
dropout regulation with fast line and load transient response. It also
includes internal current limit and thermal shutdown circuitry. In this
section you will find information about selection of external
capacitors, thermal and layout consideration.
Minimum Load Requirement
The LX8211 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 100uA.
Temperature Protection
The thermal protection shuts the LX8211 down when the junction
temperature exceeds approximately 150
0
C, there is no appreciable
thermal hysteresis.
Input Capacitor
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 1uF over the entire operating temperature range. For LX8211
connect ceramic type capacitor (1uF) between V
IN
and ground.
Short Circuit Current Protection
The LX8211 includes over current protection, when the output load
current exceeds typically 400mA the circuit forces the regulator
decrease in output.
Thermal Consideration
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.
Power dissipation for regulator can be calculated using the following
equation:
Output Capacitor
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, LX8211 like any
low-dropout regulator, requires an output capacitor with low ESR
(Equivalent Series Resistor) connected between VOUT and GND to
stabilize the internal control loop. For the LX8211 a ceramic, tantalum
or electrolytic capacitor with the minimum recommended capacitance
value at 1.0 uF with ESR between 5mΩ and 1Ω will satisfy the
stability for whole operating range.
Bypass Capacitor (Optional) / Adjustable Output
Voltage
The LX8211 includes the extra Bypass pin to help further reduce the
already low output noise in the regulator. If the output noise
requirement meets the specification, omit the bypass capacitor, that’s
because the capacitor impacts the start up time which is inversely
proportional to the size of bypass capacitor. The bypass capacitor
reduces the regulator phase margin, therefore it requires a larger output
capacitor to maintain stability.
The LX8211-00 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:
R1
⎞
⎛
V
OUT
=
V
ADJ
⎜
1
+
⎟
⎝
R2
⎠
P
D
= (V
IN(MAX)
-V
OUT(MIN)
) * I
OUT(MAX)
(Note: power dissipation resulting from quiescent current is negligible)
For the SOT23 package, thermal resistance,
θ
JA
is 220
0
C/W when
mounted on a FR4 copper clad PCB. Junction temperature of the
integrated circuit can be calculated using:
T
J
= T
A
+ T
RISE
, substituting:
T
A
+ (P
D
*
θ
JA
)
.
An example: Given conditions: T
A
= 60°C, V
IN
= 4.2V, V
OUT
= 2.7V,
I
OUT
= 100mA.
Calculated values:
Total Power dissipation
P
D(total)
= (4.2-2.7) V * 0.1A =0.15W
Junction Temperature
T
J
= 60°C +(0.15W *220°C/W) = 93°C
A
PPLICATION
A
PPLICATION
Layout Consideration:
The layout must be done with low impedance paths for V
IN
, V
OUT
and
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
θ
JA
since it is thermally connected to the device
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.
Enable
The Enable pin allows the LX8211 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 LX8211 off and reduces the supply
current to less than 1uA. 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
IN
.
Copyright
©
2000
Rev. 1.0d, 2007-08-29
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
MICROSEMI [ MICROSEMI CORPORATION ]
