MIC5237

Micrel

MIC5237

6

January 2000

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

µF 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

µF 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

Ω 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

0.47

µF for current below 10mA or 0.33µF for currents below

1mA.

For 2.5V applications a 22

µF 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

V

I

T

I

The regulator ground current, I

from the data sheet. Assuming the worst case scenario is

good design procedure, and the corresponding ground cur-

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

= (V

– V

)

I

+ V

I

DIN

OUT

OUT

IN

GND

×

[]

×

()

P

= [(13V – 5V)

500mA] + 13V

20mA

D

××

()

P

= 4.260W

D

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.

T

θ

θ

θ

θ

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.

θ

JA

J(max)

A

D

T

T

P

=

−

θθ

θ

SA

JA

JC

=−

θ

in this example.

θ

125 – 25

4.260W

θ

°

θ

3 C/W + 1 C/W

°°

°

()

θ

°

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.