Micrel, Inc.

MIC39300/39301/39302

October 2009

10

M9999-102309-A

Application Information

The MIC39300/1/2 are high-performance, low-dropout

voltage regulators suitable for moderate to high-current

voltage regulator applications. Its 550mV dropout

voltage at full load makes it especially valuable in

battery-powered systems and as a high-efficiency noise

filter in post-regulator applications. Unlike older NPN-

pass transistor designs, where the minimum dropout

voltage is limited by the base-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 βeta PNP

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

load current.

The MIC39300/1/2 regulators are 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 four application-

specific parameters:

Calculate the power dissipation of the regulator from

these numbers and the device parameters from this

datasheet, where the ground current is taken from the

data sheet.

The heat sink thermal resistance is determined by:

()

CS

JC

D

A

(max)

J

SA

P

T

T

θ

+

θ

−

−

=

θ

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

βeta 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 specification.

Figure 1. Capacitor Requirements

Output Capacitor

The MIC39300/1/2 requires an output capacitor to

maintain stability and improve transient response.

Proper capacitor selection is important to ensure proper

operation. The MIC39300/1/2 output capacitor selection

is dependent upon the ESR (equivalent series

resistance) of the output capacitor to maintain stability.

When the output capacitor is 47μF or greater, the output

capacitor should have less than 1Ω of ESR. This will

improve transient response as well as promote stability.

Ultralow ESR capacitors, such as ceramic chip

capacitors may promote instability. These very low ESR

levels may cause an oscillation and/or underdamped

transient response. A low-ESR solid tantalum capacitor

works extremely well and provides good transient

response and stability over temperature. Aluminum

electrolytics can also be used, as long as the ESR of the

capacitor is < 1Ω.

The value of the output capacitor can be increased

without limit. Higher capacitance values help to improve

transient response and ripple rejection and reduce

output noise.

Input Capacitor

An input capacitor of 1μF or greater is recommended

when the device is more than 4 inches away from the

bulk ac supply capacitance, or when the supply is a

battery. Small, surface mount, ceramic chip capacitors

can be used for the bypassing. Larger values will help to

improve ripple rejection by bypassing the input to the

regulator, further improving the integrity of the output

voltage.