July 1999

7

MIC29310/29312

MIC29310/29312

Micrel

Applications Information

The MIC29310 and MIC29312 are high performance low-

dropout voltage regulators suitable for all moderate to high-

current voltage regulator applications. Their 600mV of drop-

out 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 tran-

sistor designs, where the minimum dropout voltage is limited

by the base-emitter voltage drop and collector-emitter satu-

ration voltage, dropout performance of the PNP output of

these devices is limited merely by the low V

voltage.

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

requirement. But Micrel’s Super ßeta PNP™ process re-

duces 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 tem-

perature. Transient protection allows device (and load) sur-

vival even when the input voltage spike 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. 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:

• Maximum ambient temperature, T

A

• Output Current, I

OUT

• Output Voltage, V

OUT

• Input Voltage, V

IN

First, we calculate the power dissipation of the regulator from

these numbers and the device parameters from this datasheet.

P

Where the ground current is approximated by 2% of I

Then the heat sink thermal resistance is determined with this

formula:

T

θ

P

D

Where T

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 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

0.1

µF is needed directly between the input and regulator

ground.

Please refer to Application Note 9 for further details and

examples 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 10

µF 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 ca-

pacitors 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 0.1

µF 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

supplies 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