LP2950, LP2951, NCV2951

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9

APPLICATIONS INFORMATION

Introduction

The LP2950/LP2951 regulators are designed with

internal current limiting and thermal shutdown making them

user−friendly. Typical application circuits for the LP2950

and LP2951 are shown in Figures 20 through 28.

These regulators are not internally compensated and thus

require a 1.0

mF (or greater) capacitance between the

LP2950/LP2951 output terminal and ground for stability.

Most types of aluminum, tantalum or multilayer ceramic

will perform adequately. Solid tantalums or appropriate

multilayer ceramic capacitors are recommended for

operation below 25C.

At lower values of output current, less output capacitance

is required for output stability. The capacitor can be reduced

to 0.33

mF for currents less than 10 mA, or 0.1 mF for currents

below 1.0 mA. Using the 8 pin versions at voltages less than

5.0 V operates the error amplifier at lower values of gain, so

that more output capacitance is needed for stability. For the

worst case operating condition of a 100 mA load at 1.23 V

output (output Pin 1 connected to the feedback Pin 7) a

minimum capacitance of 3.3

mF is recommended.

The LP2950 will remain stable and in regulation when

operated with no output load. When setting the output

voltage of the LP2951 with external resistors, the resistance

values should be chosen to draw a minimum of 1.0

mA.

A bypass capacitor is recommended across the

LP2950/LP2951 input to ground if more than 4 inches of

wire connects the input to either a battery or power supply

filter capacitor.

Input capacitance at the LP2951 Feedback Pin 7 can

create a pole, causing instability if high value external

resistors are used to set the output voltage. Adding a 100 pF

capacitor between the Output Pin 1 and the Feedback Pin 7

and increasing the output filter capacitor to at least 3.3

mF

will stabilize the feedback loop.

Error Detection Comparator

The comparator switches to a positive logic low whenever

the LP2951 output voltage falls more than approximately

5.0% out of regulation. This value is the comparator’s

designed−in offset voltage of 60 mV divided by the 1.235 V

internal reference. As shown in the representative block

diagram. This trip level remains 5.0% below normal

regardless of the value of regulated output voltage. For

example, the error flag trip level is 4.75 V for a normal 5.0 V

regulated output, or 9.50 V for a 10 V output voltage.

Figure 2 is a timing diagram which shows the ERROR

signal and the regulated output voltage as the input voltage

to the LP2951 is ramped up and down. The ERROR signal

becomes valid (low) at about 1.3 V input. It goes high when

Since the LP2951’s dropout voltage is dependent upon the

load current (refer to the curve in the Typical Performance

Characteristics), the input voltage trip point will vary with

load current. The output voltage trip point does not vary

with load.

The error comparator output is an open collector which

requires an external pullup resistor. This resistor may be

returned to the output or some other voltage within the

system. The resistance value should be chosen to be

consistent with the 400

mA sink capability of the error

comparator. A value between 100 k and 1.0 M

W is

suggested. No pullup resistance is required if this output is

unused.

When operated in the shutdown mode, the error

comparator output will go high if it has been pulled up to an

external supply. To avoid this invalid response, the error

Figure 18).

Figure 18. ERROR Output Timing

5.0 V

4.75 V

4.70 V

1.3 V

1.3 V

Not

Valid

Pullup

Pullup

to Ext

Output

Voltage

ERROR

Input

Voltage

Not

Valid

Programming the Output Voltage (LP2951)

The LP2951CX may be pin−strapped for the nominal

fixed output voltage using its internal voltage divider by

tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback) to

Pin 6 (5.0 V tap). Alternatively, it may be programmed for

any output voltage between its 1.235 reference voltage and

its 30 V maximum rating. An external pair of resistors is

required, as shown in Figure 19.