DocID6280 Rev 18 15/22
LD2980 Application notes
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7 Application notes
7.1 External capacitors
Like any low-dropout regulator, the LD2980 requires external capacitors for regulator
stability. This capacitor must be selected to meet the requirements of minimum capacitance
and equivalent series resistance (please refer to Figure 18 and Figure 19). 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 LD2980 (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 LD2980 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 and Figure 19
show the allowable ESR range as a function of the output capacitance. These curves
represent the stability region over the full temperature and I
O
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 and
Figure 19 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 V
I
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
