LP2950/51
remains 5% below normal regardless of the programmed output
voltage of the regulator.
APPLICATION HINTS
EXTERNAL CAPACITORS
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 works
fine here. For operations below -25°C solid tantalum is recommended
since the many aluminum types have electrolytes that freeze at about
-30°C. The ESR of about 5Ω or less and resonant frequency above
500kHz are the most important parameters in the value of the
capacitor. The capacitors value may be increased without limit.
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 LP2951Õs 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.
The error comparator has an open-collector output, which requires an
external pull-up 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.
At lower values of output current, less output capacitance is required
for stability. For the 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. At worst case 3.3 µF or
greater must be used for the condition of 100mA load at 1.23V
output.
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. Refer to
the below equation for the programming of the output voltage:
The LP2950/51 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. When setting the output voltage of the
LP2950/51 version with external resistors, a minimum load of 1µA is
recommended
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.
V
OUT = VREF × (1 + R1/R2)+ IFBR1
The VREF is 1.235 and IFB is the feedback bias current, nominally -
20nA. The minimum recommended load current of 1 µA 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
Instability can occur if there is 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 fixed by adding a 100pF capacitor between
output and feedback and increasing the output capacitor to at least 3.3
µF.
4.75V
OUTPUT
VOLTAGE
REDUCING OUTPUT NOISE
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 3 lead LP2950 but is relatively inefficient, as increasing the
capacitor from 1µF to 220 µF only decreases the noise from 430µV
to 160µVRMS for a 100kHz bandwidth at 5V output. Noise could also
be reduced fourfold by a bypass capacitor across R1, since it reduces
the high frequency gain from 4 to unity. Pick
\
_______
ERROR*
+
+
5.0V
INPUT
VOLTAGE
+
1.3V
CBYPASS 1 / 2πR1 × 200 Hz
+
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
output voltages.
* See Application Info.
Figure 1. ERROR Output Timing
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 is around
60 mV offset divided by the 1.235 reference voltage. This trip level
Rev. 8/10/01
LP2950/51 100mA Low Dropout Voltage Regulators
©Copyright 2001 Sipex Corporation
8
EXAR [ EXAR CORPORATION ]
