Thermal Response

20052858

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. 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

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 volt-

age of the LP4951C version with external resistors, a mini-

mum load of 1µA is recommended.

A 0.1µF 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 3.3µF 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 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 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 limit-

ing.

Figure 1 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 V

OUT

= 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.

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.

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