February 2005

9

MIC3975

MIC3975

Micrel

Applications Information

TheMIC3975isahigh-performancelow-dropoutvoltageregu-

lator suitable for moderate to high-current voltage regulator

applications. Its 500mV dropout voltage at full load and over-

temperature makes it especially valuable in battery-powered

systems and as high-efficiency noise filters in post-regulator

applications.UnlikeolderNPN-passtransistordesigns,where

the minimum dropout voltage is limited by the base-to-emit-

ter 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 Superβeta PNP™ process reduces this

drive requirement to only 2% of the load current.

The MIC3975 regulator is fully protected from damage due to

faultconditions.Linearcurrentlimitingisprovided.Outputcur-

rentduringoverloadconditionsisconstant.Thermalshutdown

disables the device when the die temperature exceeds the

maximum safe operating temperature. Transient protection

allows device (and load) survival even when the input volt-

age 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.

MIC3975x.x

IN

OUT

GND

Figure 1. Capacitor Requirements

Output Capacitor

The MIC3975 requires an output capacitor for stable opera-

tion. As a µCap LDO, the MIC3975 can operate with ceramic

output capacitors as long as the amount of capacitance is

10µF or greater. For values of output capacitance lower than

10µF, the recommended ESR range is 200mΩ to 2Ω. The

minimum value of output capacitance recommended for the

MIC3975 is 4.7µF.

For 10µF or greater the ESR range recommended is less than

1Ω. Ultra-low ESR ceramic capacitors are recommended

for output capacitance of 10µF or greater to help improve

transient response and noise reduction at high frequency.

X7R/X5R dielectric-type ceramic capacitors are recom-

mendedbecauseoftheirtemperatureperformance.X7R-type

capacitors change capacitance by 15% over their operating

temperature range and are the most stable type of ceramic

capacitors. Z5U and Y5V dielectric capacitors change value

by as much as 50% and 60% respectively over their operat-

ing temperature ranges. To use a ceramic chip capacitor with

Y5V dielectric, the value must be much higher than an X7R

ceramic capacitor to ensure the same minimum capacitance

over the equivalent operating temperature range.

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 MIC3975 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

The MIC3975 features an active-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/CMOS compatible

thresholds for simple logic interfacing. EN may be directly tied

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

1.65V Conversion

TheMIC3975hasexcellenttransientresponsetovariationsin

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, 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 or 1.65V,

the NPN based regulators are already operating in dropout,

withtypicaldropoutrequirementsof1.2Vorgreater.Toconvert

downto2.5Vor1.8Vwithoutoperatingindropout,NPN-based

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

The MIC3975 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 MIC3975 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.