MIC29152WD datasheet(17/23 Pages) MICREL

17 / 23 page

Micrel, Inc.

MIC29150/29300/29500/29750

January 2012

M9999-013112-B

Application Information

The

MIC29150/29300/29500/29750

are

high

performance low-dropout voltage regulators suitable for

all

moderate

high-current

voltage

regulator

applications. Their 350mV to 425mV typical 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 merely by

the low VCE saturation voltage.

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

driver requirement. But Micrel’s Super ßeta PNP

process reduces this drive requirement to merely 1% of

the load current.

The MIC29150/29300/29500/29750 family of 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

125°C

maximum

safe

operating

temperature. Line transient protection allows device (and

load) survival even when the input voltage spikes

between –20V and +60V. When the input voltage

exceeds approximately 32V, the over voltage sensor

disables the regulator. The output structure of these

regulators allows voltages in excess of the desired

output voltage to be applied without reverse current flow.

MIC29xx1 and MIC29xx2 versions offer 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 these

regulators have identical pinouts.

MIC29XXX

OUT

VOUT

GND

VIN

Figure 3. Linear regulators require 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, TA

• Output Current, IOUT

• Output Voltage, VOUT

• Input Voltage, VIN

First, we calculate the power dissipation of the regulator

from these numbers and the device parameters from this

datasheet.

(

)

OUT

1.01

−

Where the ground current is approximated by 1% of IOUT.

Then the heat sink thermal resistance is determined with

this formula:

()

JMAX

−

Where TJMAX ≤ 125°C and θCS is between 0 and 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

® 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 and Application Hint

17 for further details and examples on thermal design

and heat sink specification.

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 MIC29152. The

maximum power allowed can be calculated using the

thermal resistance (θJA) of the D-Pak adhering to the

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

100mm

2 copper area for the MIC29152.

For example, given an expected maximum ambient

temperature (TA) of 75°C with VIN = 3.3V, VOUT = 2.5V,

and IOUT = 1.5A, first calculate the expected PD using

Equation (1);

PD=(3.3V–2.5V)1.5A–(3.3V)(0.016A)=1.1472W

Next, calcualte the junction temperature for the expected

power dissipation.

TJ=(θJA×PD)+TA=(56°C/W×1.1472W)+75°C=139.24°C

Now determine the maximum power dissipation allowed

that would not exceed the IC’s maximum junction

temperature (125

°C) without the useof a heat sink by

PD(MAX)=(TJ(MAX)–TA)/θJA=(125°C–75°C)/(56°C/W) =0.893W

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