Application notes

LD2981

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DocID6279 Rev 17

7

Application notes

7.1

External capacitors

Like any low-dropout regulator, the LD2981 requires external capacitors for regulator

stability. This capacitor must be selected to meet the requirements of minimum capacitance

and equivalent series resistance. We suggest to solder input and output capacitors as

close as possible to the relative pins.

7.2

Input capacitor

An input capacitor whose valu

e is 1 μF is required with the LD2981 (amount of capacitance

can be increased without limit). This capacitor must be located a distance of not more than

0.5" from the input pin of the device and returned to a clean analog ground. Any good

quality ceramic, tantalum or film capacitors can be used for this capacitor.

7.3

Output capacitor

The LD2981 is designed specifically to work with ceramic output capacitors. It may also be

possible to use Tantalum capacitors, but these are not as attractive for reasons of size and

cost. By the way, the output capacitor must meet both the requirement for minimum

amount of capacitance and ESR (equivalent series resistance) value. The Figure 18: "Best

case: highest output version" and Figure 19: "Worst case: lowest output version" show the

allowable ESR range as a function of the output capacitance. These curves represent the

stability region over the full temperature and IO range. Due to the different loop gain, the

stability improves for higher output versions and so the suggested minimum output

capacitor value, if low ESR ceramic type is used, is 1 μF for output voltages equal or major

than 3.8 V, 2.2 μF for output voltages from 2.85 to 3.3 V, and 3.3 μF for the other versions.

However, if an output capacitor lower than the suggested one is used, it’s possible to make

stable the regulator adding a resistor in series to the capacitor (see Figure 18: "Best case:

highest output version" and Figure 19: "Worst case: lowest output version" to choose the

right value according to the used version and keeping in account that the ESR of ceramic

capacitors has been measured @ 100 kHz).

7.4

Important

The output capacitor must maintain its ESR in the stable region over the full operating

temperature to assure stability. Also, capacitor tolerance and variation with temperature

must be considered to assure the minimum amount of capacitance is provided at all times.

This capacitor should be located not more than 0.5" from the output pin of the device and

returned to a clean analog ground.

7.5

Inhibit input operation

The inhibit pin can be used to turn OFF the regulator when pulled low, so drastically

reducing the current consu

mption down to less than 1 μA. When the inhibit feature is not

used, this pin must be tied to VI to keep the regulator output ON at all times. To assure

proper operation, the signal source used to drive the inhibit pin must be able to swing

above and below the specified thresholds listed in the electrical characteristics section

under VIH VIL. Any slew rate can be used to drive the inhibit.