Micrel, Inc.

MIC2940A/2941A

July 2007

7

M9999-071307

Application Information

External Capacitors

A 10µF (or greater) capacitor is required between the

MIC2940A output and ground to prevent oscillations due

to instability. Most types of tantalum or aluminum

electrolytics will be adequate; film types will work, but

are costly and therefore not recommended. Many

aluminum electrolytics have electrolytes that freeze at

about –30°C, so solid tantalums are recommended for

operation below –25°C. The important parameters of the

capacitor are an effective series resistance of about 5Ω

or less and a resonant frequency above 500kHz. The

value of this capacitor may be increased without limit.

At lower values of output current, less output

capacitance is required for output stability. The capacitor

can be reduced to 3.3µF for current below 100mA or

2.2µF for currents below 10mA. Adjusting the MIC2941A

to voltages below 5V runs the error amplifier at lower

gains so that more output capacitance is needed. For

the worst-case situation of a 1.25A load at 1.23V output

(Output shorted to Adjust) a 22µF (or greater) capacitor

should be used.

The MIC2940A will remain stable and in regulation with

load currents ranging from 5mA on up to the full 1.25A

rating. The external resistors of the MIC2941A version

may be scaled to draw this minimum load current.

A 0.22µF capacitor should be placed from the

MIC2940A input to ground if there is more than 10

inches of wire between the input and the AC filter

capacitor or if a battery is used as the input.

Programming the Output Voltage (MIC2941A)

The MIC2941A may be programmed for any output

voltage between its 1.235V reference and its 26V

maximum rating. An external pair of resistors is required,

as shown in Figure 3.

minimum recommended load current of 1µA forces an

must work with no load (a condition often found in

reduces this error to 0.17% while increasing the resistor

program current to 12µA. Since the MIC2941A typically

draws 100µA at no load with SHUTDOWN open-

circuited, this is a negligible addition.

Reducing Output Noise

In reference applications it may be advantageous to

reduce the AC noise present at the output. One method

is to reduce the regulator bandwidth by increasing the

size of the output capacitor. This is relatively inefficient,

as increasing the capacitor from 1µF to 220µF only

100kHz bandwidth at 5V output. Noise can be reduced

by a factor of four with the MIC2941A by adding a

200Hz

R

2

1

C

1

BYPASS

π

≅

or about 0.01µF. When doing this, the output capacitor

must be increased to 22µF to maintain stability. These

changes reduce the output noise from 430µV to

bypass capacitor added, noise no longer scales with

output voltage so that improvements are more dramatic

at higher output voltages.

Automotive Applications

The MIC2940A is ideally suited for automotive

applications for a variety of reasons. It will operate over

a wide range of input voltages with very low dropout

voltages (40mV at light loads), and very low quiescent

currents (240µA typical). These features are necessary

for use in battery powered systems, such as

automobiles. It is a “bulletproof” device with the ability to

survive both reverse battery (negative transients up to

20V below ground), and load dump (positive transients

up to 60V) conditions. A wide operating temperature

range with low temperature coefficients is yet another

reason to use these versatile regulators in automotive

designs.

NOT *

VALID

NOT *

VALID

OUTPUT

VOLTAGE

4.75V

ERROR

INPUT

VOLTAGE

5V

1.3V

Figure 1. ERROR Output Timing