August 2005

9

M9999-082505-B

MIC39100/39101/39102

Micrel

Applications Information

The MIC39100/1/2 is a high-performance low-dropout volt-

age regulator suitable for moderate to high-current voltage

regulator applications. Its 630mV dropout voltage at full load

and overtemperature makes it especially valuable in bat-

tery-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-to-emitter voltage drop and collector-to-emitter satura-

tion voltage, dropout performance of the PNP output of these

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

requirement.Micrel’s Superβeta PNP™ process reduces this

drive requirement to only 2% of the load current.

The MIC39100/1/2 regulator is fully protected from damage

due to fault conditions. Linear current limiting is provided.

Output current during overload conditions is constant. Ther-

mal 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.

MIC39100-x.x

IN

OUT

GND

Figure 1. Capacitor Requirements

Output Capacitor

The MIC39100/1/2 requires an output capacitor to maintain

stability and improve transient response. Proper capaci-

tor selection is important to ensure proper operation. The

MIC39100/1/2 output capacitor selection is dependent upon

the ESR (equivalent series resistance) of the output capacitor

to maintain stability. When the output capacitor is 10µF or

greater, the output capacitor should have an ESR less than

2Ω. This will improve transient response as well as promote

stability.Ultra-low-ESRcapacitors(<100mΩ),suchasceramic

chip capacitors, may promote instability. These very low ESR

levelsmaycauseanoscillationand/orunderdampedtransient

response.Alow-ESRsolidtantalumcapacitorworksextremely

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 <2Ω.

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 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.

Error Flag

The MIC39101 features an error flag (FLG), which monitors

the output voltage and signals an error condition when this

voltage drops 5% below its expected value. The error flag is

an open-collector output that pulls low under fault conditions

and may sink up to 10mA. Low output voltage signifies a

number of possible problems, including an overcurrent fault

(the device is in current limit) or low input voltage. The flag

output is inoperative during overtemperature conditions. A

for proper operation. For information regarding the minimum

and maximum values of pull-up resistance, refer to the graph

in the typical characteristics section of the data sheet.

Enable Input

TheMIC39101andMIC39102versionsfeatureanactive-high

enable input (EN) that allows on-off control of the regulator.

Current drain reduces to “zero” when the device is shutdown,

with only microamperes of leakage current. The EN input has

TTL/CMOScompatiblethresholdsforsimplelogicinterfacing.

supply voltage

Transient Response and 3.3V to 2.5V or 2.5V to 1.8V

Conversion

The MIC39100/1/2 has excellent transient response to

variations 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 10µF output

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

values help to improve performance even further.

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

rate into dropout as readily as similar NPN-based designs.

When converting from 3.3V to 2.5V or 2.5V to 1.8V, the NPN

based regulators are already operating in dropout, with typi-

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

to 2.5V or 1.8V without operating in dropout, NPN-based

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

The MIC39100 regulator will provide excellent performance

with an input as low as 3.0V or 2.5V respectively. This gives

the PNP based regulators a distinct advantage over older,

NPN based linear regulators.

Minimum Load Current

The MIC39100/1/2 regulator is specified between finite loads.

If the output current is too small, leakage currents dominate

and the output voltage rises. A 10mA minimum load current

is necessary for proper regulation.