LP2985_15 Datasheet, PDF (13/29 Pages) Texas Instruments – 150-mA Low-noise Low-dropout Regulator With Shutdown

LP2985
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SLVS522O – JULY 2004 – REVISED JANUARY 2015
Application Information (continued)
8.1.3.2 Output Capacitor (C_OUT)
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 mΩ. 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.
• Minimum C_OUT: 2.2 μF (can be increased without limit to improve transient response stability margin)
• 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).
8.1.3.3 Noise Bypass Capacitor (C_BYPASS)
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
regulator – changes. Thus, for best output accuracy, dc leakage current through C_BYPASS should be minimized as
much as possible and never should exceed 100 nA.
A 10-nF capacitor is recommended for C_BYPASS. Ceramic and film capacitors are well suited for this purpose.
8.1.3.4 Reverse Input-Output Voltage
There is an inherent diode present across the PNP pass element of the LP2985.
V_IN
V_OUT
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.
Schottky
V_IN
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LP2985
V_OUT
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