LP2950_08 Datasheet, PDF (9/17 Pages) Texas Instruments – ADJUSTABLE MICROPOWER VOLTAGE REGULATORS WITH SHUTDOWN

Micrel, Inc.
Application Information
External Capacitors
A 1.0µF (or greater) capacitor is required between the
LP2950/LP2951 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 tantalum capacitors 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 0.33µF for current below 10mA or
0.1µF for currents below 1mA. 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 100mA load at 1.23V output
(Output shorted to Feedback) a 3.3µF (or greater)
capacitor should be used.
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 recomm-
ended.
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.
LP2950/2951
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 which V_OUT = 4.75V).
Since the LP2951’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 (approximately
4.75V) 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 5V output 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. Suggested
values range from 100k to 1MΩ. The resistor is not
required if this output is unused.
Programming the Output Voltage (LP2951)
The LP2951 may be pin-strapped for 5V 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
⎧
× _⎨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 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), I_FB will produce a 2% typical error in
V_OUT which may be eliminated at room temperature by
trimming R₁. For better accuracy, choosing R₂ = 100kΩ
reduces this error to 0.17% while increasing the resistor
program current to 12µA. Since the LP2951 typically
draws 60µA 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-pin LP2950 and is
relatively inefficient, as increasing the capacitor from 1µF
to 220µF only decreases the noise from 430µV to
160µVrms for a 100kHz bandwidth at 5V output.
September 2008
9
M9999-091208