LP4950C-5V, LP4951C

www.ti.com

SNVS208C – SEPTEMBER 2002 – REVISED APRIL 2013

TYPICAL PERFORMANCE CHARACTERISTICS (continued)

.

Thermal Response

Figure 35.

APPLICATION HINTS

EXTERNAL CAPACITORS

A 1.0

μF (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. 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 operation below

−25°C. The important parameters of the capacitor are an ESR of about 5 Ω 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 stability. The capacitor can be reduced

to 0.33

μF for currents below 10 mA or 0.1 μF for currents below 1 mA. Using the 8-pin version 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 LP4950C 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 LP4951C version with external resistors, a minimum load of 1

μA is recommended.

A 0.1

μF capacitor should be placed from the LP4950C/LP4951C 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 LP4951C 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

μF will fix this problem.

ERROR DETECTION COMPARATOR OUTPUT

The comparator produces a logic low output whenever the LP4951C output falls out of regulation by more than

approximately 5%. This figure is the comparator's built-in offset of about 60 mV divided by the 1.235 reference

voltage. (See to the block diagram in the front of the datasheet.) This trip level remains “5% below normal”

regardless of the programmed output voltage of the 4951C. 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 36 below gives a timing diagram depicting the ERROR signal and the regulated output voltage as the

LP4951C 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). Since the LP4951C's dropout voltage is 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 (approx. 4.75V) does not vary with load.

Copyright © 2002–2013, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LP4950C-5V LP4951C