|Description||5-A Low Dropout Positive Regulators|
|LM1084ISX-3.3/NOPB Datasheet PDF : 29 Pages |
SNVS037G – SEPTEMBER 1999 – REVISED JANUARY 2015
Device Functional Modes (continued)
It is desirable to have large output capacitance for applications that entail large changes in load current
(microprocessors for example). The higher the capacitance, the larger the available charge per demand. It is also
desirable to provide low ESR to reduce the change in output voltage:
ΔV = ΔI x ESR
It is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the
output voltage by reducing the overall ESR.
Output capacitance can be increased indefinitely to improve transient response and stability.
7.4.3 Protection Diodes
Under normal operation, the LM1084 regulator does not need any protection diode. With the adjustable device,
the internal resistance between the adjustment and output terminals limits the current. No diode is needed to
divert the current around the regulator even with a capacitor on the adjustment pin. The adjust pin can take a
transient signal of ±25 V with respect to the output voltage without damaging the device.
When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge
into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage
of the regulator, and rate of decrease of VIN. In the LM1084 regulator, the internal diode between the output and
input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor (≥
1000 µf), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an
external diode is recommended between the output and input pins to protect the regulator, shown in Figure 15.
Figure 15. Regulator with Protection Diode
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