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LD2980ABM33TR Datasheet(PDF) 11 Page - STMicroelectronics |
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LD2980ABM33TR Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 17 page LD2980 SERIES 11/17 APPLICATION NOTES 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. 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 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. 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 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 beusedto drive theinhibit. REVERSE CURRENT The power transistor used in the LD2980 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 K Ω. |
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