LP2951ACM-3.3 National Semiconductor, LP2951ACM-3.3 Datasheet - Page 10

• Manufacturer Part Number: LP2951ACM-3.3
• Description: Linear Voltage Regulator IC
• Manufacturer: National Semiconductor

Specifications of LP2951ACM-3.3

• Dropout Voltage Vdo: 380mV
• No. Of Pins: 8
• Output Current: 0.1A
• Peak Reflow Compatible (260 C): No
• Output Voltage: 3.3V
• Output Voltage Max: 3.3V
• Supply Voltage Max: 30V
• Leaded Process Compatible: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Hints
EXTERNAL CAPACITORS
A 1.0 µF (or greater) capacitor is required between the out-
put and ground for stability at output voltages of 5V or more.
At lower output voltages, more capacitance is required
(2.2 µF or more is recommended for 3V and 3.3V versions).
Without this capacitor the part will oscillate. Most types of
tantalum or aluminum electrolytics work fine here; even film
types work but are not recommended for reasons of cost.
Many aluminum electrolytics have electrolytes that freeze at
about −30˚C, so solid tantalums are recommended for op-
eration below −25˚C. The important parameters of the ca-
pacitor are an ESR of about 5
quency above 500 kHz. 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 µF for currents below 10 mA or 0.1 µF for currents be-
low 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
100 mA load at 1.23V output (Output shorted to Feedback) a
3.3 µF (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 volt-
age divider. This is especially important in CMOS RAM
keep-alive applications. When setting the output voltage of
the LP2951 versions with external resistors, a minimum load
of 1 µA is recommended.
A 1 µF tantalum or aluminum electrolytic 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 fil-
ter capacitor or if a battery is used as the input.
Stray capacitance to the LP2951 Feedback terminal can
cause instability. This may especially be a problem when us-
ing high value 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 µF 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 approxi-
mately 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 pro-
grammed 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 ei-
ther to low input voltage, current limiting, or thermal limiting.
Figure 1 below gives a timing diagram depicting the ERROR
signal and the regulated 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 V
Since the LP2951’s dropout voltage is load-dependent (see
curve in typical performance characteristics), the input volt-
age 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 re-
quires an external pullup resistor. This resistor may be re-
turned to the output or some other supply voltage depending
on system requirements. In determining a value for this re-
or less and a resonant fre-
OUT
= 4.75V).
10
sistor, note that while the output is rated to sink 400 µA, this
sink current adds to battery drain in a low battery condition.
Suggested values range from 100k to 1 M . The resistor is
not required if this output is unused.
*
error flag voltage rises to its pull-up voltage. Using V
voltage (see Figure 2 ), rather than an external 5V source, will keep the
error flag voltage under 1.2V (typ.) in this condition. The user may wish to
divide down the error flag voltage using equal-value resistors (10 k
suggested), to ensure a low-level logic signal during any fault condition,
while still allowing a valid high logic level during normal operation.
PROGRAMMING THE OUTPUT VOLTAGE (LP2951)
The LP2951 may be pin-strapped for the nominal fixed out-
put voltage using its internal voltage divider by tying the out-
put and sense pins together, and also tying the feedback and
V
any output voltage between its 1.235V reference and its 30V
maximum rating. As seen in Figure 2 , an external pair of re-
sistors is required.
The complete equation for the output voltage is
where V
the feedback pin bias current, nominally −20 nA. The mini-
mum recommended load current of 1 µA forces an upper
limit of 1.2 M
with no load (a condition often found in CMOS in standby).
I
eliminated at room temperature by trimming R
accuracy, choosing R
while increasing the resistor program current to 12 µA. Since
the LP2951 typically draws 60 µA at no load with Pin 2
open-circuited, this is a small price to pay.
FB
When V
TAP
will produce a 2% typical error in V
pins together. Alternatively, it may be programmed for
IN
REF
1.3V, the error flag pin becomes a high impedance, and the
FIGURE 1. ERROR Output Timing
is the nominal 1.235 reference voltage and I
on the value of R
2
= 100k reduces this error to 0.17%
2
, if the regulator must work
OUT
OUT
DS008546-20
as the pull-up
which may be
1
. For better
FB
is