LP2950 / LP2951
CORPORATION
APPLICATION HINTS
EXTERNAL CAPACITORS
The error comparator has an open-collector output which
requires an external pullup resistor. Depending on the system
requirements the resistor may be returned to 5V output or
other supply voltage. In determining the value of this resistor,
note that the output is rated to sink 400µA, this value adds to
battery drain in a low battery condition. Suggested values
range from 100K to 1MΩ. If the output is unused this resistor
is not required.
• The stability of the LP2950/LP2951 requires a 1.0µF or
greater capacitor between output and ground. Oscillation
could occur without this capacitor. Most types of tantalum
or aluminum electrolytic are acceptable. For operations
below -25oC a solid tantalum is recommended since the
many aluminum types have electrolytes that freeze at about
-30oC. The ESR of about 5Ω or less and resonant
frequency about 500kHz are the most important parameters
in the value of the capacitor. The capacitors value may be
increased without limit.
PROGRAMMING THE OUTPUT VOLTAGE OF 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). Also, 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.
• At lower values of output current, less output capacitance is
required for stability. For currents below 10mA the value of
the capacitor can be reduced to 0.33µF and 0.1µF for 1mA.
More output capacitance is needed for the 8-pin version at
voltages below 5V since it runs the error amplifier at lower
gain.
Refer to the equation below for the programming of the output
voltage:
• At worst case 3.3µF or greater must be used for the
condition of 100mA load at 1.23V output.
VOUT = VREF × (1 + R1\R2) + IFBR1
• The LP2950, unlike other low dropout regulators will remain
stable and in regulation with no load in addition to the
internal voltage divider. This feature is especially important
in applications like CMOS RAM keep-alive circuits. When
setting the output voltage of the LP2951 version with
external resistors, a minimum load of 1µA is recommended.
The VREF is 1.235 and IFB is the feedback bias current,
nominally -20nA. The minimum recommended load current of
1µA forces an upper limit of 1.2MΩ on value of R2. If no load
is presented the IFB produces an error of typically 2% in VOUT
which may be eliminated at room temperature by trimming R1.
To improve the accuracy choose the value of R2 = 100k this
reduces the error by 0.17% and increases the resistor
program current by 12µA. Since the LP2951 typically draws
60µA at no load with Pin 2 open-circuited this is a small price
to pay.
• 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
then a 1µA tantalum or aluminum electrolytic capacitor
should be placed from the input to the ground.
REDUCING OUTPUT NOISE
• Instability can occur if there is a stray capacitance to the
LP2951 feedback terminal (pin 7). This could cause more
problems when using a higher value of external resistors to
set the output voltage. This problem can be eliminated by
adding a 100pF capacitor between output and feedback
and increasing the output capacitor to at least 3.3µF.
It may be an advantage to reduce the AC noise present at the
output. One way 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 lead 3 of LP2950, but 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.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
LP2951 output falls out of regulation by more than around 5%.
This occurs at approximately 60mV offset divided by the
1.235 reference voltage. This trip level remains 5% below
normal regardless of the programmed output voltage of the
regulator. Figure 1 shows the timing diagram depicting the
ERROR signal and the regulator output voltage as the
LP2951 input is ramped up and down. The ERROR signal
becomes low at around 1.3V input, and goes high around 5V
input (input voltage at which VOUT = 4.75). Since the LP2951’s
dropout voltage is load dependent, the input voltage trip point
(around 5V) will vary with the load current. The output voltage
trip point (approx. 4.75V) does not vary with load.
Noise could also be reduced fourfold by a bypass capacitor
across R1, since it reduces the high frequency gain from 4 to
unity.
CBYPASS 1/2πR1 × 200Hz
or choose 0.01µF. When doing this, the output capacitor must
be increased to 3.3µF to maintain stability. These changes
reduce the output noise from 430µV to 100µVrms 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 voltages.
CALOGIC CORPORATION, 237 Whitney Place, Fremont, California 94539, Telephone: 510-656-2900, FAX: 510-651-3025
CALOGIC [ CALOGIC, LLC ]
