8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

8.1.1 Recommended Capacitor Types

The device is designed to be stable using low equivalent series resistance (ESR) ceramic capacitors at the input

and output. Multilayer ceramic capacitors have become the industry standard for these types of applications and

are recommended, but must be used with good judgment. Ceramic capacitors that employ X7R-, X5R-, and

C0G-rated dielectric materials provide relatively good capacitive stability across temperature, whereas the use of

Y5V-rated capacitors is discouraged because of large variations in capacitance.

Regardless of the ceramic capacitor type selected, the effective capacitance varies with operating voltage

and temperature. Generally, expect the effective capacitance to decrease by as much as 50%. The input and

output capacitors listed in the Recommended Operating Conditions table account for an effective capacitance of

approximately 50% of the nominal value.

8.1.2 Input and Output Capacitor Requirements

Although an input capacitor is not required for stability, good analog design practice is to connect a capacitor

from IN to GND. This capacitor counteracts reactive input sources and improves transient response, input ripple,

and PSRR. Use an input capacitor if the source impedance is more than 0.5 Ω. A higher value capacitor can be

necessary if large, fast rise-time load or line transients are anticipated or if the device is located several inches

from the input power source.

Dynamic performance of the device is improved with the use of an output capacitor. Use an output capacitor

within the range specified in the Recommended Operating Conditions table for stability.

8.1.3 Noise Bypass Capacitor (CBYPASS)

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

band-gap reference with the BYPASS pin. This high-impedance band-gap circuitry is biased in the microampere

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

regulator) changes. Thus, for best output accuracy, dc leakage current through CBYPASS must be minimized as

much as possible and must never exceed 100 nA. The CBYPASS capacitor also impacts the start-up behavior of

the regulator. Inrush current and start-up time increase with larger bypass capacitor values.

Use a 10-nF capacitor for CBYPASS. Ceramic and film capacitors are good choices for this purpose.

8.1.4 Reverse Current

Excessive reverse current can damage this device. Reverse current flows through the intrinsic body diode of the

pass transistor instead of the normal conducting channel. At high magnitudes, this current flow degrades the

long-term reliability of the device.

Conditions where reverse current can occur are outlined in this section, all of which can exceed the absolute

maximum rating of VOUT ≤ VIN + 0.3 V.

• If the device has a large COUT and the input supply collapses with little or no load current

• The output is biased when the input supply is not established

• The output is biased above the input supply

If reverse current flow is expected in the application, use external protection to protect the device. Reverse

current is not limited in the device, so external limiting is required if extended reverse voltage operation is

anticipated.

LP2985

SLVS522Q – JULY 2004 – REVISED DECEMBER 2022

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