MIC2950/2951
Micrel
Applications Information
Automotive Applications
The MIC2950/2951 are ideally suited for automotive
applications for a variety of reasons. They will operate over a
wide range of input voltages, have very low dropout voltages
(40mV at light loads), and very low quiescent currents. These
features are necessary for use in battery powered systems,
such as automobiles. They are also “bulletproof” devices; 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.
offset of about 60mV divided by the 1.235V reference voltage.
(Refer to the block diagram on Page 1). This trip level remains
“5% below normal” regardless of the programmed output
voltage of the MIC2951. For example, the error flag trip level
is typically 4.75V for a 5V output or 11.4V for a 12V output. The
out of regulation condition may be due either to low input
voltage, current limiting, thermal limiting, or overvoltage on
input (over
≅
40V).
Figure 1 is a timing diagram depicting the ERROR signal and
the regulated output voltage as the MIC2951 input is ramped
up and down. The ERROR signal becomes valid (low) at
about 1.3V input. It goes high at about 5V input (the input
voltage at which V
OUT
= 4.75—for 5.0V applications). Since
the MIC2951’s dropout voltage is load-dependent (see curve
in Typical Performance Characteristics), the input voltage trip
point (about 5V) will vary with the load current. The output
voltage trip point does not vary with load.
The error comparator has an open-collector output which
requires an external pull-up resistor. Depending on system
requirements, this resistor may be returned to the output or
some other supply voltage. In determining a value for this
resistor, note that while the output is rated to sink 200µA, this
sink current adds to battery drain in a low battery condition.
Suggested values range from 100k to 1MΩ. The resistor is not
required if this output is unused.
External Capacitors
A 1.5
µF
(or greater) capacitor is required between the
MIC2950/MIC2951 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
0.5µF for current below 10mA or 0.15µF for currents below 1
mA. Using the 8-pin versions at voltages below 5V runs the
error amplifier at lower gains so that more output capacitance
is needed. For the worst-case situation of a 150mA load at
1.23V output (Output shorted to Feedback) a 5µF (or greater)
capacitor should be used.
The MIC2950 will remain stable and in regulation with no load
in addition to the internal voltage divider, unlike many other
voltage regulators. This is especially important in CMOS RAM
keep-alive applications. When setting the output voltage of
the MIC2951 version with external resistors, a minimum load
of 1µA is recommended.
A 0.1µF capacitor should be placed from the MIC2950/
MIC2951 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.
Stray capacitance to the MIC2951 Feedback terminal (pin 7)
can cause instability. This may especially be a problem when
using high value external resistors to set the output voltage.
Adding a 100pF capacitor between Output and Feedback and
increasing the output capacitor to at least 3.3µF will remedy
this.
Programming the Output Voltage (MIC2951)
The MIC2951 may be pin-strapped for 5V (or 3.3V or 4.85V)
using its internal voltage divider by tying Pin 1 (output) to Pin
2 (sense) and Pin 7 (feedback) to Pin 6 (5V Tap). Alternatively,
it may be programmed for any output voltage between its
1.235V reference and its 30V maximum rating. An external
pair of resistors is required, as shown in Figure 2.
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 feedback 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
= 100k reduces this error to 0.17% while
increasing the resistor program current to 12
µA.
Reducing Output Noise
In some 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 the only method by which noise can be
reduced on the 3 lead MIC2950 and is relatively inefficient, as
increasing the capacitor from 1µF to 220µF only decreases
the noise from 430µV to 160µV rms for a 100kHz bandwidth
at 5V output.
February 1999
Error Detection Comparator Output
A logic low output will be produced by the comparator whenever
the MIC2951 output falls out of regulation by more than
approximately 5%. This figure is the comparator’s built-in
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MICREL [ MICREL SEMICONDUCTOR ]
