Schottky

LP2985

LP2985

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SLVS522O – JULY 2004 – REVISED JANUARY 2015

Application Information (continued)

As an advantage over other regulators, the LP2985 permits the use of low-ESR capacitors at the output,

including ceramic capacitors that can have an ESR as low as 5 m

Ω. Tantalum and film capacitors also can be

used if size and cost are not issues. The output capacitor also should be located within 1 cm of the output pin

and be returned to a clean analog ground.

As with other PNP LDOs, stability conditions require the output capacitor to have a minimum capacitance and an

ESR that falls within a certain range.

•

•

ESR range: see Figure 18 through Figure 20

It is critical that both the minimum capacitance and ESR requirement be met over the entire operating

temperature range. Depending on the type of capacitors used, both these parameters can vary significantly with

temperature (see capacitor characteristics).

The LP2985 allows for low-noise performance with the use of a bypass capacitor that is connected to the internal

bandgap reference via the BYPASS pin. This high-impedance bandgap circuitry is biased in the microampere

range and, thus, cannot be loaded significantly, otherwise, its output – and, correspondingly, the output of the

much as possible and never should exceed 100 nA.

8.1.3.4 Reverse Input-Output Voltage

There is an inherent diode present across the PNP pass element of the LP2985.

With the anode connected to the output, this diode is reverse biased during normal operation, since the input

voltage is higher than the output. However, if the output is pulled higher than the input for any reason, this diode

is forward biased and can cause a parasitic silicon-controlled rectifier (SCR) to latch, resulting in high current

flowing from the output to the input. Thus, to prevent possible damage to the regulator in any application where

the output may be pulled above the input, or the input may be shorted to ground, an external Schottky diode

should be connected between the output and input. With the anode on output, this Schottky limits the reverse

voltage across the output and input pins to

∼0.3 V, preventing the regulator’s internal diode from forward biasing.

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