MIC5237 Micrel Semiconductor, MIC5237 Datasheet - Page 6

• Manufacturer Part Number: MIC5237
• Description: 500mA Low-Dropout Regulator Preliminary Information
• Manufacturer: Micrel Semiconductor

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 1 F 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. 1 F 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 5
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
0.47 F for current below 10mA or 0.33 F for currents below
1mA.
For 2.5V applications a 22 F 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.
The regulator ground current, I
from the data sheet. Assuming the worst case scenario is
good design procedure, and the corresponding ground cur-
MIC5237
V
V
I
T
I
OUT
GND
A
IN
OUT
= ambient operating temperature
= input voltage
= output current
= ground current
= output voltage
or less and a resonant frequency
GND
, can be measured or read
6
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
current.
From this number, the heat sink thermal resistance is deter-
mined using the regulator’s maximum operating junction
temperature (T
along with the power dissipation number already calculated.
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
25 C that does not exceed the maximum operating junction
temperature.
in this example.
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.
CS
is approximately 1 C/W and
T
P = (V
P = [(13V – 5V)
P = 4.260W
JC
CS
JA
SA
D
D
D
J(MAX)
JA
SA
JA
JA
SA
SA
= junction-to-ambient thermal resistance
= junction-to-case thermal resistance
= sink-to-ambient thermal resistance
= case-to-sink thermal resistance
=
= 23.5 C/W
= 23.5 C/W – 3 C/W + 1 C/W
= 19.5 C/W
= 125 C
125 – 25
T
IN
JA
4.260W
J(max)
J(max)
output current and input voltage
– V
P
D
) and the ambient temperature (T
OUT
JC
T
)
A
500mA] + 13V
I
OUT
JC
for the TO-220 is 3 C/W
+ V
IN
January 2000
I
GND
20mA
ground
Micrel
A
A
=
)