MIC29752BWT datasheet(14/20 Pages) MICREL

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MIC29150/29300/29500/29750

Micrel

MIC29150/29300/29500/29750

March 2000

Applications Information

The MIC29150/29300/29500/29750 are high performance

low-dropout voltage regulators suitable for all moderate to

high-current voltage regulator applications. Their 300mV to

400mV 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

CE saturation 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 1% of the load current.

The MIC29150–29750 family of regulators is 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 de-

vice when the die temperature exceeds the 125

°C maximum

safe operating temperature. Transient protection allows de-

vice (and load) survival even when the input voltage spikes

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

about 35V to 40V, the overvoltage sensor temporarily dis-

ables 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.

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, T

• Output Current, I

OUT

• Output Voltage, V

OUT

• Input Voltage, V

First, we calculate the power dissipation of the regulator from

these numbers and the device parameters from this datasheet.

1.01V

OUT

=−

()

Where the ground current is approximated by 1% of I

OUT.

Then the heat sink thermal resistance is determined with this

formula:

θθ

J MAX

−

−+

()

Where T

J MAX ≤ 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

0.1

µF 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.

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. MIC29150—29750 regulators are stable

with the following minimum capacitor values at full load:

Device

Full Load Capacitor

MIC29150 .............................. 10

µF

MIC29300 .............................. 10

µF

MIC29500 .............................. 10

µF

MIC29750 .............................. 22

µF

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.

Minimum Load Current

The MIC29150–29750 regulators are specified between fi-

nite loads. If the output current is too small, leakage currents

dominate and the output voltage rises. The following mini-

mum load current swamps any expected leakage current

across the operating temperature range:

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