Application Hints

EXTERNAL CAPACITORS

A 1.0µF (or greater) capacitor is required between the output

and ground for stability at output voltages of 5V or more. At

lower output voltages, more capacitance is required (2.2µF

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 alumi-

num electrolytics 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

Ω or less and a resonant frequency above

500kHz. The value of this capacitor may be increased with-

out limit.

Ceramic capacitors whose value is greater than 1000pF

should not be connected directly from the LP2951 output to

ground. Ceramic capacitors typically have ESR values in the

range of 5 to 10m

Ω, a value below the lower limit for stable

operation (see curve Output Capacitor ESR Range).

The reason for the lower ESR limit is that the loop compen-

sation of the part relies on the ESR of the output capacitor to

provide the zero that gives added phase lead. The ESR of

ceramic capacitors is so low that this phase lead does not

occur, significantly reducing phase margin. A ceramic output

capacitor can be used if a series resistance is added (rec-

ommended value of resistance about 0.1

Ω to 2Ω).

At lower values of output current, less output capacitance is

required for stability. The capacitor can be reduced to 0.33µF

for currents below 10mA or 0.1µF for currents below 1mA.

Using the adjustable versions at voltages below 5V runs the

error amplifier at lower gains so that more output capaci-

tance is needed. For the worst-case situation of a 100mA

load at 1.23V output (Output shorted to Feedback) a 3.3µF

(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 versions with external resistors, a minimum load

of 1µA is recommended.

A 1µF tantalum, ceramic 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 filter 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

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 3.3µF 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 60mV 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

either to low input voltage, current limiting, or thermal limit-

ing.

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

requires an external pullup resistor. This resistor may be

returned to the output or some other supply voltage depend-

ing on system requirements. In determining a value for this

resistor, note that while the output is rated to sink 400µA, this

sink current adds to battery drain in a low battery condition.

Suggested values range from 100k to 1 M

Ω. 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 out-

put voltage using its internal voltage divider by tying the

output and sense pins together, and also tying the feedback

and V

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

the feedback pin bias current, nominally −20nA. The mini-

mum recommended load current of 1µA forces an upper limit

of 1.2 M

Ω on the value of R

no load (a condition often found in CMOS in standby). I

produce a 2% typical error in V

at room temperature by trimming R

choosing R

creasing the resistor program current to 12µA. Since the

LP2951 typically draws 60µA at no load with Pin 2 open-

circuited, this is a small price to pay.

00854620

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 (10k

Ω

suggested), to ensure a low-level logic signal during any fault condition,

while still allowing a valid high logic level during normal operation.

FIGURE 1. ERROR Output Timing

www.national.com

15