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MIC5237-3.3BU

500mA Low-Dropout Regulator

Micrel 

Micrel, Inc.

Application 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

standard applications. Larger values improve the

regulator’s transient response. The output capacitor value

may be increased without limit.

The output capacitor should have an ESR (equivalent

series resistance) of about 5Ω or less and a resonant

frequency above 1MHz. Ultra low-ESR capacitors can

cause low-amplitude oscillations and/or under-damped

transient response. Most tantalum or aluminum electrolytic

capacitors are adequate; 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

CMOSRAM 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.

VIN = input voltage

VOUT = output voltage

IOUT = output current

TA = ambient operating temperature

IGND = ground current

The regulator ground current, IGND, can be measured or

September 2007

MIC5237

read from the data sheet. Assuming the worst case

scenario is good design procedure, and the corresponding

ground current 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.

PD = [(VIN – VOUT) × IOUT] + (VIN × IGND)

PD = [(13V – 5V) × 500mA] + (13V × 20mA)

PD = 4.260W

From this number, the heat sink thermal resistance is

determined using the regulator’s maximum operating

junction temperature (TJ(max)) and the ambient temperature

(TA) along with the power dissipation number already

calculated.

TJ(MAX) = 125°C

θJC = junction-to-case thermal resistance

θCS = case-to-sink thermal resistance

θJA = junction-to-ambient thermal resistance

θSA = 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

numbers show the heat-sink thermal resistance required at

TA =25°C that does not exceed the maximum operating

junction temperature.

θ JA

=

TJ(max) - TA

PD

θSA = θJA - θJC

θCS is approximately 1°C/W and θJC for the TO-220 is

3°C/W in this example.

θ JA

=

125 - 25

4.260W

θJA = 23.5°C/W

θSA = 23.5°C/W – (3°C/W + 1°C/W)

θSA = 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.

7

M9999-090607-C

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