LP2951 datasheet(18/30 Pages) MICROCHIP

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LP2951

DS20005736B-page 18

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2017 - 2022 Microchip Technology Inc. and its subsidiaries

4.0

APPLICATION INFORMATION

4.1

External Capacitors

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

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 5Ω or less and a resonant

frequency 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 output stability. The

capacitor can be reduced to 0.33 µF for current below

10 mA or 0.1 µF for currents below 1 mA. 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 100 mA load at 1.23V output (Output shorted to

Feedback) a 3.3 µF (or greater) capacitor should be

used.

When the 5V Tap pin and Feedback pin are connected

together for 5V output voltage, the LP2951 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 with external resistors, a

minimum load of 1 µA is recommended.

A 0.1 µF capacitor should be placed from the 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 100 pF capacitor between

Output and Feedback and increasing the output

capacitor to at least 3.3 µF will remedy this.

4.2

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 Functional

Block Diagram). 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 4-1 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 which VOUT =

4.75V). Because the LP2951’s dropout voltage is

load-dependent (see curve in Typical Performance

Curves), 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 µA, this sink current adds to

battery drain in a low battery condition. Suggested

values range from 100 kΩ to 1 MΩ. The resistor is not

required if this output is unused.

4.3

Programming the Output Voltage

The LP2951 may be pin-strapped for 5V output voltage

using its internal voltage divider, by tying Pin 1 (VOUT)

and Pin 2 (SENSE) together, as well as tying Pin 7

(Feedback) and Pin 6 (5V TAP) together. 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 4-2.

The complete equation for the output voltage is:

EQUATION 4-1:

The minimum recommended load current of 1 µA

forces an upper limit of 1.2 MΩ on the value of R2, if the

regulator must work with no load (a condition often

found in CMOS in standby), IFB will produce a 2%

typical error in VOUT which may be eliminated at room

temperature by trimming R1. For better accuracy,

choosing R2 = 100 kΩ reduces this error to 0.17%

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.

Where:

VREF

= the nominal 1.235 reference voltage

IFB

the feedback pin bias current,

nominally 20 nA

VOUT VREF

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IFBR2

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