November 2006

7

MIC29310/29312

MIC29310/29312

Micrel

Applications Information

The MIC29310 and MIC29312 are high performance low-drop-

out voltage regulators suitable for all moderate to high-current

voltage regulator applications.Their 600mV of dropout voltage

at full load make them especially valuable in battery powered

systems and as high efficiency noise filters in “post-regulator”

applications. Unlike older NPN-pass transistor designs, where

the minimum dropout voltage is limited by the base-emitter

voltage drop and collector-emitter saturation voltage, dropout

performance of the PNP output of these devices is limited

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

drive requirement. But Micrel’s Super ßeta PNP™ process

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

load current.

MIC29310/312 regulators are fully protected from damage

due to fault conditions. Current limiting is provided. This

limiting is linear; output current under 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 spike above and below

nominal. The output structure of these regulators allows volt-

ages in excess of the desired output voltage to be applied

without reverse current flow. The MIC29312 version offers a

logic level ON/OFF control: when disabled, the devices draw

nearly zero current.

An additional feature of this regulator family is a common

pinout: a design’s current requirement may change up or down

yet use the same board layout, as all of Micrel’s high-current

Super ßeta PNP™ regulators have identical pinouts.

IN

OUT

GND

Figure 3. The MIC29310 regulator requires only two

capacitors for operation.

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, we calculate the power dissipation of the regulator

from these numbers and the device parameters from this

datasheet.

Then the heat sink thermal resistance is determined with

this formula:

θ

The heat sink may be significantly reduced in applications

where the minimum input voltage is known and is large com-

pared 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 regulators allow very significant reductions

in regulator power dissipation and the associated heat sink

without compromising performance. When this technique is

employed, a capacitor of at least 0.1µF is needed directly

between the input and regulator ground.

Please refer to Application Note 9 for further details and ex-

amples on thermal design and heat sink specification.

Capacitor Requirements

For stability and minimum output noise, a capacitor on the

regulator output is necessary. The value of this capacitor

is dependent upon the output current; lower currents allow

smaller capacitors. MIC29310/2 regulators are stable with a

minimum capacitor value of 10µF at full load.

This capacitor need not be an expensive low ESR type:

aluminum electrolytics are adequate. In fact, extremely low

ESR capacitors may contribute to instability. Tantalum capaci-

tors are recommended for systems where fast load transient

response is important.

Where the regulator is powered from a source with a high

AC impedance, a 0.1µF capacitor connected between Input

and GND is recommended. This capacitor should have good

characteristics to above 250kHz.

Transient Response and 5V to 3.3V Conversion

The MIC29310/2 have excellent response to variations in

input voltage and load current. By virtue of their low dropout

voltage, these devices do not saturate into dropout as readily

as similar NPN-based designs. A 3.3V output Micrel LDO will

maintain full speed and performance with an input supply as

low as 4.2V, and will still provide some regulation with sup-

plies down to 3.8V, unlike NPN devices that require 5.1V

or more for good performance and become nothing more

than a resistor under 4.6V of input. Micrel’s PNP regulators

provide superior performance in “5V to 3.3V” conversion

applications than NPN regulators, especially when all toler-

ances are considered.

Minimum Load Current

The MIC29310/2 regulators are specified between finite loads.

If the output current is too small, leakage currents dominate