MIC2940A/2941A
Micrel
Applications 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 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.
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.
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
430µV to 160µV
RMS
for a 100kHz bandwidth at 5V output.
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
C
BYPASS
≅
1
2
π
R
1
• 200 Hz
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 (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.
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
OUT
= V
REF
x { 1 + R
1
/R
2
} –
|
I
FB
|
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
1.2MΩ 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
1
. For better accuracy,
choosing R
2
= 100kΩ reduces this error to 0.17% while
increasing the resistor program current to 12
µA.
Since the
MIC2941A typically draws 100
µA
at no load with SHUTDOWN
open-circuited, this is a negligible addition.
4.75V
OUTPUT
VOLTAGE
ERROR
NOT
*
VALID
NOT
*
VALID
INPUT
VOLTAGE
1.3V
5V
* SEE APPLICATIONS INFORMATION
Figure 1. ERROR Output Timing
June 1999
7
MIC2940A/2941A
MICREL [ MICREL SEMICONDUCTOR ]
