MIC2940A/2941A

Micrel

June 1999

7

MIC2940A/2941A

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 1

µF to 220 µF only decreases the noise from

430

µV to 160µV

Noise can be reduced by a factor of four with the MIC2941A

by adding a bypass capacitor across R

1

, since it reduces the

high frequency gain from 4 to unity. Pick

or about 0.01

µF. When doing this, the output capacitor must

be increased to 22

µF to maintain stability. These changes

reduce the output noise from 430

µV to 100 µV rms for a 100

kHz bandwidth at 5V output. With the 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 (240

µAtypical).Thesefeatures

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.

Applications Information

External Capacitors

A 10

µF (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,sosolidtantalums

are recommended for operation below –25

°C. The important

parameters of the capacitor are an effective series resistance

of about 5

Ω 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

3.3

µF for current below 100mA or 2.2µF for currents below 10

mA. 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 22

µF (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 0.22

µF 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.

The complete equation for the output voltage is

V

REF

x { 1 + R

–

R

1

where V

REF

is the nominal 1.235 reference voltage and I

FB

is

the Adjust pin bias current, nominally 20nA. The minimum

recommended load current of 1

µA forces an upper limit of

1.2M

Ω on the value of R

2

, if the regulator must work with no

load (a condition often found in CMOS in standby), I

FB

will

produce a –2% typical error in V

OUT

which may be eliminated

at room temperature by trimming R

choosing

R

2

= 100k

Ω reduces this error to 0.17% while

increasing the resistor program current to 12

µA. Since the

MIC2941A typically draws 100

µAatnoloadwithSHUTDOWN

open-circuited, this is a negligible addition.

* SEE APPLICATIONS INFORMATION

*

*

OUTPUT

VOLTAGE

INPUT

VOLTAGE

ERROR

NOT

VALID

NOT

VALID

5V

1.3V

4.75V

Figure 1. ERROR Output Timing

≅

1

π