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LD2981ABXX Datasheet(PDF) 14 Page - STMicroelectronics

Part No. LD2981ABXX
Description  Ultra low drop voltage regulators with inhibit low ESR output capacitors compatible
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

LD2981ABXX Datasheet(HTML) 14 Page - STMicroelectronics

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Application notes
LD2981ABxx - LD2981Cxx
14/23
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 value 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 3 and Figure 4
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 1 and Figure 2 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 consumption 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.


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