MIC2940A/2941A
Micrel
Reducing Output Noise
Applications Information
External Capacitors
In reference applications it may be advantageous to reduce
the AC noise present at the output. One method is to reduce
A 10µF (or greater) capacitor is required between the the regulator bandwidth by increasing the size of the output
MIC2940A output and ground to prevent oscillations due to capacitor. This is relatively inefficient, as increasing the
instability. Most types of tantalum or aluminum electrolytics capacitor from 1 µF to 220 µF only decreases the noise from
will be adequate; film types will work, but are costly and 430µV to 160µVRMS for a 100kHz bandwidth at 5V output.
therefore not recommended. Many aluminum electrolytics Noise can be reduced by a factor of four with the MIC2941A
haveelectrolytesthatfreezeatabout–30°C,sosolidtantalums by adding a bypass capacitor across R1, since it reduces the
are recommended for operation below –25°C. The important high frequency gain from 4 to unity. Pick
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.
1
CBYPASS
2
π
R • 200 Hz
1
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 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 22µF (or greater)
capacitor should be used.
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
kHzbandwidthat5Voutput.Withthebypasscapacitoradded,
noisenolongerscaleswithoutputvoltagesothatimprovements
are more dramatic at higher output voltages.
Automotive Applications
TheMIC2940Aisideallysuitedforautomotiveapplicationsfor
a variety of reasons. It will operate over a wide range of input
voltages with very low dropout voltages (40mV at light loads),
andverylowquiescentcurrents(240µAtypical).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.
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
betweenits1.235Vreferenceandits26Vmaximumrating.An
external pair of resistors is required, as shown in Figure 3.
The complete equation for the output voltage is
VOUT = VREF x { 1 + R1/R2 } – |IFB| R1
4.75V
OUTPUT
where VREF is the nominal 1.235 reference voltage and IFB is
the Adjust pin bias current, nominally 20nA. The minimum
recommended load current of 1 µA forces an upper limit of
1.2MΩ 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 = 100kΩ reduces this error to 0.17% while
increasing the resistor program current to 12 µA. Since the
MIC2941Atypicallydraws100µAatnoloadwithSHUTDOWN
open-circuited, this is a negligible addition.
VOLTAGE
NOT
NOT
*
*
ERROR
VALID
VALID
INPUT
VOLTAGE
5V
1.3V
* SEE APPLICATIONS INFORMATION
Figure 1. ERROR Output Timing
June 1999
7
MIC2940A/2941A