MIC5237 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – 500mA Low-Dropout Regulator Preliminary Information

MIC5237
Applications Information
The MIC5237 is intended for general-purpose use and can be
implemented in a wide variety of applications where 500mA
of output current is needed. It is available in several voltage
options for ease of use. For voltage options that are not
available on the MIC5237, consult the MIC5209 for a 500mA
adjustable LDO regulator, or the MIC5219 for applications
that require only short-duration peak output current.
Input Capacitor
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.
Output Capacitor
An output capacitor is required between OUT and GND to
prevent oscillation. 1µF minimum is recommended for stan-
dard applications. Larger values improve the regulator’s
transient response. The output capacitor value may be in-
creased without limit.
The output capacitor should have an ESR (equivalent series
resistance) of about 5Ω or less and a resonant frequency
above 1MHz. Ultralow-ESR capacitors can cause low-ampli-
tude oscillations and/or underdamped transient response.
Most tantalum or aluminum electrolytic capacitors are ad-
equate; film types will work, but are more expensive. Since
many aluminum electrolytics have electrolytes that freeze at
about –30°C, solid tantalums are recommended for operation
below –25°C.
At lower values of output current, less output capacitance is
needed for output stability. The capacitor can be reduced to
0.47µF for current below 10mA or 0.33µF for currents below
1mA.
For 2.5V applications a 22µF output capacitor is recom-
mended to reduce startup voltage overshoot.
No-Load Stability
The MIC5237 will remain stable and in regulation with no load
(other than the internal voltage divider) unlike many other
voltage regulators. This is especially important in CMOS
RAM keep-alive applications.
Thermal Considerations
Proper thermal design can be accomplished with some basic
design criteria and some simple equations. The following
information is required to implement a regulator design.
V_IN = input voltage
V_OUT = output voltage
I_OUT = output current
T_A = ambient operating temperature
I_GND = ground current
The regulator ground current, I_GND, can be measured or read
from the data sheet. Assuming the worst case scenario is
good design procedure, and the corresponding ground cur-
Micrel
rent number can be obtained from the data sheet. First,
calculate the power dissipation of the device. This example
uses the MIC5237-5.0BT, a 13V input, and 500mA output
current, which results in 20mA of ground current, worst case.
The power dissipation is the sum of two power calculations:
voltage drop × output current and input voltage × ground
current.
[ ] ( ) ^P_D ^{= (V}_IN ^{– V}_OUT ^{) × I}_OUT ^{+ V}_IN ^{× I}_GND
P_D = [(13V – 5V) × 500mA] + (13V × 20mA)
P_D = 4.260W
From this number, the heat sink thermal resistance is deter-
mined using the regulator’s maximum operating junction
temperature (T_J(max)) and the ambient temperature (T_A)
along with the power dissipation number already calculated.
T_J(MAX) = 125°C
θ_JC = junction-to-case thermal resistance
θ_CS = case-to-sink thermal resistance
θ_JA = junction-to-ambient thermal resistance
θ_SA = sink-to-ambient thermal resistance
To determine the heat sink thermal resistance, the junction-
to-case thermal resistance of the device must be used along
with the case-to-heat sink thermal resistance. These num-
bers show the heat-sink thermal resistance required at T_A =
25°C that does not exceed the maximum operating junction
temperature.
θ_JA
=
T_J(max) − T_A
P_D
θ_SA = θ_JA − θ_JC
θ_CS is approximately 1°C/W and θ_JC for the TO-220 is 3°C/W
in this example.
θ_JA
=
125 – 25
4.260W
θ_JA = 23.5°C/W
θ_SA = 23.5°C/W – (3°C/W + 1°C/W)
θ_SA = 19.5°C/W
Therefore, a heat sink with a thermal resistance of 19.5° C/W
will allow the part to operate safely and it will not exceed the
maximum junction temperature of the device. The heat sink
can be reduced by limiting power dissipation, by reducing the
input voltage or output current. Either the TO-220 or TO-263
package can operate reliably at 2W of power dissipation
without a heat sink. Above 2W, a heat sink is recommended.
For a full discussion on voltage regulator thermal effects,
please refer to “Thermal Management” in Micrel’s Designing
with Low-Dropout Voltage Regulators handbook.
MIC5237
6
January 2000