LP2950 LP2951
APPLICATIONS INFORMATION
Introduction
The LP2950/LP2951 regulators are designed with
internal current limiting and thermal shutdown making them
user–friendly. Typical application circuits for the LP2950 and
LP2951 are shown in Figures 17 through 25.
These regulators are not internally compensated and thus
require a 1.0 µF (or greater) capacitance between the
LP2950/LP2951 output terminal and ground for stability.
Most types of aluminum, tantalum or multilayer ceramic will
perform adequately. Solid tantalums or appropriate
multilayer ceramic capacitors are recommended for
operation below 25°C.
When operated in the shutdown mode, the error
comparator output will go high if it has been pulled up to an
external supply. To avoid this invalid response, the error
comparator output should be pulled up to V
Figure 15).
(see
out
Figure 15. ERROR Output Timing
5.0 V
4.70 V
4.75 V
Output
Voltage
At lower values of output current, less output capacitance
is required for output stability. The capacitor can be reduced
to 0.33 µF for currents less than 10 mA, or 0.1 µF for currents
below 1.0 mA. Using the 8–pin versions at voltages less than
5.0 V operates the error amplifier at lower values of gain, so
that more output capacitance is needed for stability. For the
worst case operating condition of a 100 mA load at 1.23 V
output (Output Pin 1 connected to the feedback Pin 7) a
minimum capacitance of 3.3 µF is recommended.
Pull–Up
to Ext
Not
Valid
Not
Valid
ERROR
Pull–Up
to V
out
4.75 V + V
dropout
4.70 V + V
dropout
The LP2950 will remain stable and in regulation when
operated with no output load. When setting the output voltage
of the LP2951 with external resistors, the resistance values
should be chosen to draw a minimum of 1.0 µA.
A bypass capacitor is recommended across the
LP2950/LP2951 input to ground if more than 4 inches of
wire connects the input to either a battery or power supply
filter capacitor.
Input capacitance at the LP2951 Feedback Pin 7 can
create a pole, causing instability if high value external
resistors are used to set the output voltage. Adding a 100 pF
capacitor between the Output Pin 1 and the Feedback Pin 7
and increasing the output filter capacitor to at least 3.3 µF will
stabilize the feedback loop.
Input
Voltage
1.3 V
1.3 V
Programming the Output Voltage (LP2951)
The LP2951CX may be pin–strapped for 5.0 V using its
internal voltage divider by tying Pin 1 (output) to Pin 2 (sense)
and Pin 7 (feedback) to Pin 6 (5.0 V tap). Alternatively, it may
be programmed for any output voltage between its 1.235
reference voltage and its 30 V maximum rating. An external
pair of resistors is required, as shown in Figure 16.
Figure 16. Adjustable Regulator
V
in
Error Detection Comparator
The comparator switches to a positive logic low whenever
the LP2951 output voltage falls more than approximately
5.0% out of regulation. This value is the comparator’s
designed–in offset voltage of 60 mV divided by the 1.235 V
internal reference. As shown in the representative block
diagram. This trip level remains 5.0% below normal
regardless of the value of regulated output voltage. For
example, the error flag trip level is 4.75 V for a normal 5.0 V
regulated output, or 9.50 V for a 10 V output voltage.
Figure 1 is a timing diagram which shows the ERROR
signal and the regulated output voltage as the input voltage to
the LP2951 is ramped up and down. The ERROR signal
becomes valid (low) at about 1.3 V input. It goes high when
100 k
5
8
V
in
1
Error
Output
V
out
Error
V
out
1.23 to 30 V
2
NC
NC
SNS
R1
Shutdown
Input
3
6
0.01 µF
SD
Gnd
V
T
O
3.3 µF
FB
4
7
R
2
the input reaches about 5.0 V (V
exceeds about 4.75 V).
out
Since the LP2951’s dropout voltage is dependent upon the
load current (refer to the curve in the Typical Performance
Characteristics), the input voltage trip point will vary with load
current. The output voltage trip point does not vary with load.
The error comparator output is an open collector which
requires an external pull–up resistor. This resistor may be
returned to the output or some other voltage within the
system. The resistance value should be chosen to be
consistent with the 400 µA sink capability of the error
comparator. A value between 100 k and 1.0 MΩ is suggested.
No pull–up resistance is required if this output is unused.
The complete equation for the output voltage is:
(
)
V
1
R1 R2
I
R1
out
ref
FB
where V is the nominal 1.235 V reference voltage and I
ref FB
is the feedback pin bias current, nominally –20 nA. The
minimum recommended load current of 1.0 µA forces an
upper limit of 1.2 MΩ on the value of R2, if the regulator must
work with no load. I
which may be eliminated at room temperature by adjusting
R1. For better accuracy, choosing R2 = 100 k reduces this
will produce a 2% typical error in V
FB
out
8
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