MIC2940 Micrel Semiconductor, MIC2940 Datasheet - Page 7

• Manufacturer Part Number: MIC2940
• Description: 1.25A Low-Dropout Voltage Regulator Preliminary Information
• Manufacturer: Micrel Semiconductor

MIC2940A/2941A
Applications Information
External Capacitors
A 10 F (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 5 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 F for current below 100mA or 2.2 F 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 F (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 F 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
where V
the Adjust pin bias current, nominally 20nA. The minimum
recommended load current of 1 A forces an upper limit of
1.2M on the value of R
load (a condition often found in CMOS in standby), I
produce a –2% typical error in V
at room temperature by trimming R
choosing
increasing the resistor program current to 12 A. Since the
MIC2941A typically draws 100 A at no load with SHUTDOWN
open-circuited, this is a negligible addition.
June 1999
V
OUT
REF
= V
is the nominal 1.235 reference voltage and I
R
REF
2
= 100k
x { 1 + R
2
reduces this error to 0.17% while
, if the regulator must work with no
1
/R
2
} –
OUT
|
I
FB
which may be eliminated
1
|
. For better accuracy,
R
1
FB
FB
will
is
7
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 F to 220 F only decreases the noise from
430 V to 160 V
Noise can be reduced by a factor of four with the MIC2941A
by adding a bypass capacitor across R
high frequency gain from 4 to unity. Pick
or about 0.01 F. When doing this, the output capacitor must
be increased to 22 F to maintain stability. These changes
reduce the output noise from 430 V to 100 V 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 A 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.
* SEE APPLICATIONS INFORMATION
VOLTAGE
VOLTAGE
OUTPUT
ERROR
INPUT
Figure 1. ERROR Output Timing
C
VALID
RMS
BYPASS
NOT
*
for a 100kHz bandwidth at 5V output.
4.75V
1.3V
2 R
5V
1
• 200 Hz
1
1
, since it reduces the
VALID
MIC2940A/2941A
NOT
*
Micrel