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LD2981

ULTRA LOW DROP VOLTAGE REGULATORS WITH INHIBIT LOW ESR OUTPUT CAPACITORS COMPATIBLE

STMicroelectronics 

LD2981 SERIES

APPLICATION NOTES

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.

INPUT CAPACITOR

An input capacitor whose value is 1µF 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.

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 figures 1 and 2 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 1µF for output voltages equal or major than

3.8V, 2.2µF for output voltages from 2.85 to 3.3V, and 3.3µF 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 Figure1 & Figure2 to choose the right value according to the used version

and keeping in account that the ESR of ceramic capacitors has been measured @ 100KHz).

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.

INHIBIT INPUT OPERATION

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

consumption down to less than 1µA. 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.

REVERSE CURRENT

The power transistor used in the LD2981 has not an inherent diode connected between the regulator input

and output. If the output is forced above the input, no current will flow from the output to the input across

the series pass transistor. When a VREV voltage is applied on the output, the reverse current measured,

according to the test circuit in figure 19, flows to the GND across the two feedback resistors. This current

typical value is 160µA. R1 and R2 resistors are implanted type; typical values are, respectively, 42.6 KΩ

and 51.150 KΩ.

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