Micrel, Inc.

MIC35152

October 2009

6

M9999-102309-A

Application Information

The MIC35152 is a high-performance low-dropout

voltage regulator suitable for moderate to high-current

regulator applications. Its 600mV dropout voltage at full

load and over-temperature makes it especially valuable

in battery-powered systems and as high-efficiency noise

filters in post-regulator applications. Unlike older NPN-

pass transistor designs, there the minimum dropout

voltage is limited by the based-to-emitter voltage drop

and collector-to-emitter saturation voltage, dropout

performance of the PNP output of these devices is

A trade-off for the low dropout voltage is a varying base

drive requirement. Micrel’s Super ßeta PNP

reduces this drive requirement to only 2% to 5% of the

load current.

The MIC35152 regulator is fully protected from damage

due to fault conditions. Current limiting is provided. This

limiting is linear; output current during overload

conditions is constant. Thermal shutdown disables the

device when the die temperature exceeds the maximum

safe operating temperature. Transient protection allows

device (and load) survival even when the input voltage

spikes above and below nominal. The output structure of

these regulators allows voltages in excess of the desired

output voltage to be applied without reverse current flow.

Thermal Design

Linear regulators are simple to use. The most

complicated design parameters to consider are thermal

characteristics. Thermal design requires the following

application-specific parameters:

•

•

•

•

•

First, calculate the power dissipation of the regulator

from these numbers and the device parameters from this

datasheet.

Where the ground current is approximated by using

numbers from the “Electrical Characteristics” or “Typical

Characteristics.” Then, the heat sink thermal resistance

is determined with this formula:

2°C/W. The heat sink may be significantly reduced in

applications where the minimum input voltage is known

and is large compared with the dropout voltage. Use a

series input resistor to drop excessive voltage and

distribute the heat between this resistor and the

regulator. The low dropout properties of Micrel Super

ßeta PNP

regulator power dissipation and the associated heat sink

without compromising performance. When this technique

is employed, a capacitor of at least 1.0µF is needed

directly between the input and regulator ground.

Refer to “Application Note 9” for further details and

examples on thermal design and heat sink applications.

With no heat sink in the application, calculate the

junction temperature to determine the maximum power

dissipation that will be allowed before exceeding the

maximum junction temperature of the MIC35152. The

maximum power allowed can be calculated using the

following criteria for the PCB design: 2 oz. copper and

100mm

As an example, given an expected maximum ambient

Equation (1);

Next, calcualte the junction temperature for the expected

power dissipation.

Now determine the maximum power dissipation allowed

that would not exceed the IC’s maximum junction

temperature (125

°C) without the use of a heat sink by

Output Capacitor

The MIC35152 requires an output capacitor for stable

operation. As a µCap LDO, the MIC35152 can operate

with ceramic output capacitors as long as the amount of

capacitance is 47µF or greater. For values of output

capacitance lower than 47µF, the recommended ESR

range is 200mΩ to 2Ω. The minimum value of output

capacitance recommended for the MIC35152 is 10µF.

For 47µF or greater, the ESR range recommended is

less than 1Ω. Ultra-low ESR ceramic capacitors are

recommended for output capacitance of 47µF or greater

to help improve transient response and noise reduction

at high frequency. X7R/X5R dielectric-type ceramic

capacitors

are

recommended

because

of

their

temperature performance. X7R-type capacitors change

capacitance by 15% over their operating temperature

range and are the most stable type of ceramic

capacitors. Z5U and Y5V dielectric capacitors change

value by as much as 50% and 60% respectively over

their operating temperature ranges. To use a ceramic

chip capacitor with Y5V dielectric, the value must be

much higher than an X7R ceramic capacitor to ensure

the same minimum capacitance over the equivalent

operating temperature range.