LP2951-4.8BM Micrel Semiconductor, LP2951-4.8BM Datasheet - Page 9

• Manufacturer Part Number: LP2951-4.8BM
• Description: 100mA Low-Dropout Voltage Regulator
• Manufacturer: Micrel Semiconductor

LP2950/2951
Applications Information
External Capacitors
A 1.0 F (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
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
0.33 F for current below 10mA or 0.1 F 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 3.3 F
(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 1 A is recommended.
A 0.1 F 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 3.3 F 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.
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
February 1999
or less and a resonant frequency
3-43
which V
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 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 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
where V
the feedback pin bias current, nominally 20 nA. The minimum
recommended load current of 1 A forces an upper limit of 1.2
M on the value of R
(a condition often found in CMOS in standby), I
a 2% typical error in V
temperature by trimming R
R
resistor program current to 12 A. Since the LP2951 typically
draws 60 A 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 lead LP2950 and is relatively inefficient, as
increasing the capacitor from 1 F to 220 F only decreases
the noise from 430 V to 160 V rms for a 100kHz bandwidth
at 5V output.
Noise can be reduced fourfold by a bypass capacitor across
R
Pick
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
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.
2
1
, since it reduces the high frequency gain from 4 to unity.
= 100k reduces this error to 0.17% while increasing the
V
OUT
OUT
REF
= V
= 4.75V). Since the LP2951’s dropout voltage is
is the nominal 1.235 reference voltage and I
REF
C
x { 1 + R
BYPASS
2
, if the regulator must work with no load
OUT
1
/R
which may be eliminated at room
1
2 R
. For better accuracy, choosing
2
} + I
1
• 200 Hz
1
FB
R
2
FB
will produce
Micrel
FB
is
3