June 1999

5

MIC39500/39501

MIC39500/39501

Micrel

Applications Information

The MIC39500/1 is a high-performance low-dropout voltage

regulator suitable for moderate to high-current voltage regu-

lator applications. Its 500mV 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 mini-

mum dropout voltage is limited by the base-to-emitter voltage

drop and collector-to-emitter saturation voltage, dropout per-

formance of the PNP output of these devices is limited only

by the low V

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

requirement. Micrel’s Super

βeta PNP™ process reduces

this drive requirement to only 2% to 5% of the load current.

The MIC39500/1 regulator is 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 tem-

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

vival 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.

MIC39500-x.x

IN

OUT

GND

Figure 1. Capacitor Requirements

Thermal Design

Linear regulators are simple to use. The most complicated

design parameters to consider are thermal characteristics.

Thermal design requires four application-specific param-

eters:

• Maximum ambient temperature (T

• Output Current (I

• Output Voltage (V

• Input Voltage (V

• Ground Current (I

Calculate the power dissipation of the regulator from these

numbers and the device parameters from this datasheet,

where the ground current is taken from data sheet.

P

The heat sink thermal resistance is determined by:

θθ

θ

SA

J(max)

A

D

JC

CS

TT

P

=

−

−+

()

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

βeta PNP regulators allow signifi-

cant reductions in regulator power dissipation and the asso-

ciated heat sink without compromising performance. When

this technique is employed, a capacitor of at least 1

µF 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.

Output capacitor

The MIC39500/1 requires an output capacitor to maintain

stability and improve transient response. Proper capacitor

selection is important to ensure proper operation. The

MIC39500/1 output capacitor selection is dependent upon

the ESR (equivalent series resistance) of the output capacitor

to maintain stability. When the output capacitor is 47

µF or

greater, the output capacitor should have less than 1

Ω of

ESR. This will improve transient response as well as promote

stability. Ultra-low-ESR capacitors, such as ceramic chip

capacitors may promote instability. These very low ESR

levels may cause an oscillation and/or underdamped tran-

sient 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 < 1

Ω.

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 1

µF 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 bypassing.

Larger values will help to improve ripple rejection by bypass-

ing the input to the regulator, further improving the integrity of

the output voltage.

Transient Response

The MIC39500/1 has excellent transient response to varia-

tions in input voltage and load current. The device has been

designed to respond quickly to load current variations and

input voltage variations. Large output capacitors are not

required to obtain this performance. A standard 47

µF output

capacitor, preferably tantalum, is all that is required. Larger

values improve performance even further.

3.3V to 2.5V Conversion

By virtue of its low-dropout voltage, this device does not

saturate into dropout as readily as similar NPN-based de-

signs. When converting from 3.3V to 2.5V, the NPN-based

regulators are already operating in dropout, with typical

dropout requirements of 1.2V or greater. To convert down to

2.5V without operating in dropout, NPN-based regulators

require an input voltage of 3.7V at the very least. The

MIC39500/1 regulator provides excellent performance with