Micrel, Inc.
MIC2950/2951
Application Information
Error Detection Comparator Output
Automotive Applications
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 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 overvolt-age
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 VOUT = 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 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
External Capacitors
A 1.5µF (or greater) capacitor is required between the
MIC2950/MIC2951 output and ground to prevent
oscillations due to instability. Most types of tantalum or
aluminum elec-trolytics 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 recomm-ended 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.
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.
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.
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 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 recomm-ended.
The complete equation for the output voltage is:
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.
⎧
⎫
⎬
⎭
R1
R2
VOUT = VREF × 1+
+ IFB R1
⎨
⎩
where VREF is the nominal 1.235 reference voltage and IFB
is the feedback pin bias current, nominally –20nA. The
minimum recommended load current of 1µA 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 = 100k reduces this
error to 0.17% while increasing the resistor program
current to 12µA.
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.
M9999-021610
February 2010
11
MICREL [ MICREL SEMICONDUCTOR ]
