LP2950AC Advanced Monolithic Systems, LP2950AC Datasheet - Page 9

• Manufacturer Part Number: LP2950AC
• Description: 100mA LOW DROPOUT VOLTAGE REGULATOR
• Manufacturer: Advanced Monolithic Systems

APPLICATION HINTS
External Capacitors
A 1.0 F or greater capacitor is required between output and
ground for stability at output voltages of 5V or more. At lower
output voltages, more capacitance is required (2.2 or more is
recommended for 3V and 3.3V versions). Without this capacitor
the part will oscillate. Most types of tantalum or aluminum
electrolytic works fine here; even film types work but are not
recommended for reasons of cost. Many aluminum types 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 ESR of about 5
resonant frequency above 500 kHz parameters in the value of the
capacitor. The value of this capacitor may be increased without
limit.
At lower values of output current, less output capacitance is
required for stability. The capacitor can be reduced to 0.33 F for
currents below 10 mA or 0.1 F for currents below 1 mA. Using
the adjustable 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 3.3 F (or greater)
capacitor should be used.
Unlike many other regulators, the LP2950, will remain stable and
in regulation with no load in addition to the internal voltage
divider. 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 1 A is
recommended.
A 1 F tantalum or aluminum electrolytic capacitor should be
placed from the LP2950/LP2951 input to the 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 can cause
instability.
higher value of external resistors to set the output voltage. Adding
a 100 pF capacitor between Output and Feedback and increasing
the output capacitor to at least 3.3 F will fix this problem.
Error Detection Comparator Output
The comparator produces a logic low output whenever the
LP2951 output falls out of regulation by more than approximately
5%. This figure is the comparator’s built-in offset of about 60 mV
divided by the 1.235 reference voltage. (Refer to the block
diagram in the front of the datasheet.) This trip level remains
“5% below normal” regardless of the programmed output voltage
of the 2951. 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.
the regulator output voltage as the LP2951 input is ramped up
and down. For 5V versions 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 ).
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Figure 1 gives a timing diagram depicting the ERROR signal and
This may especially be a problem when using a
or less and
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 (approx. 4.75V) does not vary with load.
The error comparator has an open-collector output which requires
an external pullup resistor. This resistor may be returned to the
output or some other supply voltage depending on system
requirements. In determining a value for this resistor, note that
the output is rated to sink 400 A, this sink current adds to
battery drain in a low battery condition. Suggested values range
from 100K to 1M . The resistor is not required if this output is
unused.
Programming the Output Voltage (LP2951)
The LP2951 may be pin-strapped for the nominal fixed output
voltage using its internal voltage divider by tying the output and
sense pins together, and also tying the feedback and V
together. Alternatively, it may be programmed for any output
voltage between its 1.235V reference and its 30V maximum
rating. As seen in Figure 2, an external pair of resistors is
required.
The complete equation for the output voltage is:
where V
feedback pin bias current, nominally -20 nA. The minimum
recommended load current of 1 A forces an upper limit of 1.2
M
condition often found in CMOS in standby) I
typical error in V
temperature by trimming R
100k reduces this error to 0.17% while increasing the resistor
program current by 12 A. Since the LP2951 typically draws 60
pay.
Reducing Output Noise
In reference applications it may be an 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 way that noise can be reduced on the 3
lead LP2950 but is relatively inefficient, as increasing the
capacitor from 1 F to 220 F only decreases the noise from 430
Noise could also be reduced fourfold by a bypass capacitor across
R
or about 0.01 F. When doing this, the output capacitor must be
increased to 3.3 F to maintain stability. These changes reduce
the output noise from 430 V to 100 V rms for a 100 kHz
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.
A at no load with Pin 2 open-circuited, this is a small price to
V to 160 V rms for a 100 kHz bandwidth at 5V output.
1
, since it reduces the high frequency gain from 4 to unity. Pick
on value of R
C
BYPASS
V
REF
out
= V
is the nominal 1.235 reference voltage and I
1 / 2 R
REF
2
, if the regulator must work with no load (a
OUT
(1 + R
1
LP2950/LP2951
which may be eliminated at room
200 Hz
1
. For better accuracy, choosing R2 =
1
/ R
2
)+ I
FB
R
1
FB
will produce a 2%
FB
TAP
is the
pins