LP2950 Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – Series of Adjustable Micropower Voltage Regulators

LP2950/2951
Micrel
Applications Information
which V_OUT = 4.75V). Since the LP2951’s dropout voltage is
load-dependent (see curve in Typical Performance
External Capacitors
Characteristics), the input voltage trip point (about 5V) will
vary with the load current. The output voltage trip point
A 1.0µF (or greater) capacitor is required between the LP2950/ _{(approximately 4.75V) does not vary with load.}
LP2951 output and ground to prevent oscillations due to
instability. Most types of tantalum or aluminum electrolytics ^{The error comparator has an open-collector output which}
will be adequate; film types will work, but are costly and ^{requires an external pull-up resistor. Depending on system}
therefore not recommended. Many aluminum electrolytics ^{requirements, this resistor may be returned to the 5V output}
have electrolytes that freeze at about –30°C, so solid tantalum
capacitors are recommended for operation below –25°C. The
important parameters of the capacitor are an effective series
or some other supply voltage. In determining a value for this
resistor, note that while the output is rated to sink 400µA, this
sink current adds to battery drain in a low battery condition.
resistance of about 5Ω or less and a resonant frequency ^{Suggested values range from 100k to 1MΩ. The resistor is not}
above 500kHz. The value of this capacitor may be increased ^{required if this output is unused.}
without limit.
Programming the Output Voltage (LP2951)
At lower values of output current, less output capacitance is
required for output stability. The capacitor can be reduced to ^{The LP2951 may be pin-strapped for 5V using its internal}
0.33µF for current below 10mA or 0.1µF for currents below ^{voltage divider by tying Pin 1 (output) to Pin 2 (SENSE) and}
1mA. Using the 8-Pin versions at voltages below 5V runs the ^{Pin 7 (FEEDBACK) to Pin 6 (5V TAP). Alternatively, it may be}
error amplifier at lower gains so that more output capacitance ^{programmed for any output voltage between its 1.235V}
is needed. For the worst-case situation of a 100mA load at ^{reference and its 30V maximum rating. An external pair of}
1.23V output (Output shorted to Feedback) a 3.3µF ^{resistors is required, as shown in Figure 2.}
(or greater) capacitor should be used.
The complete equation for the output voltage is
3
The LP2950 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 LP2951 version with external resistors, a minimum load
of 1µA is recommended.
A 0.1µF capacitor should be placed from the LP2950/LP2951
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 LP2951 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.
Error Detection Comparator Output
A logic low output will be produced by the comparator whenever
the LP2951 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 LP2951. 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, or thermal limiting.
Figure 1 is a timing diagram depicting the ERROR signal and
the regulated output voltage as the LP2951 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
V_OUT = V_REF x { 1 + R₁/R2 } + I_FB R₂
where V_REF is the nominal 1.235 reference voltage and I_FB is
the feedback pin bias current, nominally 20 nA. The minimum
recommended load current of 1 µA forces an upper limit of 1.2
MΩ on the value of R₂, 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 . For better accuracy, choosing
1
R₂ = 100kΩ reduces this error to 0.17% while increasing the
resistor program current to 12µA. Since the LP2951 typically
draws 60 µA at no load with Pin 2 open-circuited, this is a small
price to pay.
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 the only method by which noise can be
reduced on the 3 lead LP2950 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.
Noise can be reduced fourfold by a bypass capacitor across
R₁, since it reduces the high frequency gain from 4 to unity.
Pick
CBYPASS ≅
1
2 πR1 • 200 Hz
or about 0.01 µF. When doing this, the output capacitor must
be increased to 3.3 µF to maintain stability. These changes
reduce the output noise from 430µV to 100µV rms for a
100kHz 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.
February 1999
3-43