Micrel, Inc.
MIC5219
Thermal Considerations
Applications Information
The MIC5219 is designed to provide 200mA of continuous
current in two very small profile packages. Maximum power
dissipationcanbecalculatedbasedontheoutputcurrentand
the voltage drop across the part. To determine the maximum
powerdissipationofthepackage,usethethermalresistance,
junction-to-ambient, of the device and the following basic
equation.
TheMIC5219isdesignedfor150mAto200mAoutputcurrent
applicationswhereahighcurrentspike(500mA)isneededfor
short, start-up conditions. Basic application of the device will
be discussed initially followed by a more detailed discussion
of higher current applications.
Enable/Shutdown
Forcing EN (enable/shutdown) high (>2V) enables the
regulator. EN is compatible with CMOS logic. If the enable/
shutdown feature is not required, connect EN to IN (supply
input). See Figure 5.
TJ (max ) − TA
(
)
PD (max ) =
θJA
T (max) is the maximum junction temperature of the die,
Input Capacitor
J
125°C, and T is the ambient operating temperature. θ
A
JA
A 1µF capacitor should be placed from IN to GND if there is
more than 10 inches of wire between the input and the AC
filter capacitor or if a battery is used as the input.
is layout dependent; Table 1 shows examples of thermal
resistance, junction-to-ambient, for the MIC5219.
Package
θ
JA Recommended
Minimum Footprint 2oz. Copper
θJA 1" Squareꢀ θJC
Output Capacitor
An output capacitor is required between OUT and GND to
prevent oscillation. The minimum size of the output capacitor
is dependent upon whether a reference bypass capacitor is
MM8® (MM)
160°C/W
220°C/W
90°C/W
70°C/W
170°C/W
—
30°C/W
130°C/W
—
SOT-23-5 (M5)
2×2 MLF® (ML)
used. 1µF minimum is recommended when C
is not used
BYP
2×2 Thin
(see Figure 5). 2.2µF minimum is recommended when C
BYP
MLF® (MT)
90°C/W
—
—
is 470pF (see Figure 6). For applications <3V, the output
capacitor should be increased to 22µF minimum to reduce
start-up overshoot. Larger values improve the regulator’s
transient response. The output capacitor value may be in-
creased without limit.
Table 1. MIC5219 Thermal Resistance
The actual power dissipation of the regulator circuit can be
determined using one simple equation.
P = (V – V
) I
+ V I
IN GND
D
IN
OUT OUT
The output capacitor should have an ESR (equivalent series
resistance) of about 1Ω or less and a resonant frequency
above 1MHz. Ultra-low-ESR capacitors could cause oscilla-
tion and/or underdamped transient response. Most tantalum
or aluminum electrolytic capacitors are adequate; film types
will work, but are more expensive. Many aluminum electro-
lytics have electrolytes that freeze at about –30°C, so solid
tantalums are recommended for operation below –25°C.
Substituting P (max) for P and solving for the operating
D
D
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit. For
example, if we are operating the MIC5219-3.3BM5 at room
temperature, with a minimum footprint layout, we can deter-
mine the maximum input voltage for a set output current.
125 °C − 25°C
(
)
At lower values of output current, less output capacitance is
needed for stability. The capacitor can be reduced to 0.47µF
for current below 10mA, or 0.33µF for currents below 1mA.
PD(max ) =
220°C / W
P (max) = 455mW
D
No-Load Stability
Thethermalresistance,junction-to-ambient,fortheminimum
footprintis220°C/W,takenfromTable1.Themaximumpower
dissipation number cannot be exceeded for proper opera-
tion of the device. Using the output voltage of 3.3V, and an
output current of 150mA, we can determine the maximum
input voltage. Ground current, maximum of 3mA for 150mA
of output current, can be taken from the “Electrical Charac-
teristics” section of the data sheet.
TheMIC5219willremainstableandinregulationwithnoload
(other than the internal voltage divider) unlike many other
voltage regulators. This is especially important in CMOS
RAM keep-alive applications.
Reference Bypass Capacitor
BYP is connected to the internal voltage reference. A 470pF
capacitor (C
) connected from BYP to GND quiets this
BYP
455mW = (V – 3.3V) × 150mA + V × 3mA
455mW = (150mA) × V + 3mA × V – 495mW
950mW = 153mA × V
reference, providing a significant reduction in output noise
(ultra-low-noise performance). C reduces the regulator
IN
IN
BYP
IN
IN
phase margin; when using C , output capacitors of 2.2µF
BYP
IN
or greater are generally required to maintain stability.
V
= 6.2V
MAX
IN
The start-up speed of the MIC5219 is inversely proportional
to the size of the reference bypass capacitor. Applications
requiring a slow ramp-up of output voltage should consider
Therefore, a 3.3V application at 150mA of output current
can accept a maximum input voltage of 6.2V in a SOT-23-5
package. For a full discussion of heat sinking and thermal
effects on voltage regulators, refer to the “Regulator Ther-
mals”sectionofMicrel’sDesigningwithLow-DropoutVoltage
Regulators handbook.
larger values of C . Likewise, if rapid turn-on is necessary,
BYP
consider omitting C
.
BYP
June 2009
9
M0371-061809
MICREL [ MICREL SEMICONDUCTOR ]
