MIC2941AWT datasheet(13/24 Pages) MICROCHIP

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DS20006000B-page 13

MIC2940A/41A

4.0

APPLICATION INFORMATION

4.1

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, 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 500 kHz.

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

100 mA or 2.2 μF 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 5 mA 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.

4.2

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 the MIC2941A Adjustable

Regulator Typical Application Circuit.

EQUATION 4-1:

The minimum recommended load current of 1 μA

forces an upper limit of 1.2 MΩ on the value of R2, if the

regulator must work with no load (a condition often

found in CMOS in standby), IFB will produce a –2%

typical error in VOUT which may be eliminated at room

temperature by trimming R1. For better accuracy,

choosing R2 = 100 kΩ reduces this error to 0.17%

while increasing the resistor program current to 12 μA.

Because the MIC2941A typically draws 100 μA at no

load with SHUTDOWN open-circuited, this is a

negligible addition.

4.3

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 160 μVRMS for a

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

by a factor of four with the MIC2941A by adding a

bypass capacitor across R1.

EQUATION 4-2:

Pick a bypass capacitor of 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 μVRMS 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.

4.4

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

currents (240 μA 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.

FIGURE 4-1:

ERROR Output Timing.

VOUT VREF 1 R1

-------











IFB

–



Where:

VREF = The nominal 1.235V reference voltage.

IFB = The adjust pin bias current (nom. 20 nA).

CBYPASS

2 R1



200Hz



------------------------------------------

NOT *

VALID

NOT *

VALID

OUTPUT

VOLTAGE

4.75V

ERROR

INPUT

VOLTAGE

1.3V

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