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LP3997

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SNVS272B – MAY 2004 – REVISED MAY 2013

Applications Information

External Capacitors

In common with most regulators, the LP3997 requires the inclusion of external capacitors.

An input capacitor is required for stability. It is recommended that a minimum of 1.0µF capacitor is connected

between the LP3997 input pin and ground (this capacitance value may be increased without limit).

This capacitor must be located a distance of not more than 1cm from the input pin and returned to a clean

analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.

Important: To ensure stable operation it is essential that good PCB design practices are employed to minimize

ground impedance and keep input inductance low. If these conditions cannot be met, or if long wire leads are

used to connect the battery or other power source to the LP3997, then it is recommended to increase the input

capacitor to at least 2.2µF. Also, tantalum capacitors can suffer catastrophic failures due to surge current when

connected to a low-impedance source of power (like a battery or a very large capacitor). If a tantalum capacitor is

used at the input, it must be ensured by the manufacturer to have a surge current rating sufficient for the

application.

There are no requirements for the ESR (Equivalent Series Resistance) on the input capacitor, but tolerance and

temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain

≊

1.0µF over the entire operating temperature range.

ensure stability the capacitor must meet the minimum value for capacitance and have an ESR in the range 5m

Ω

to 500m

Ω. Ceramic X7R types are recommended. If an output capacitor larger than 4.7µF is fitted then checks

on in-rush current, transient performance and stability, should be made.

SENSE

SHUTDOWN

ERROR

external pull up resistor is required on this pin. When a capacitor is connected from DELAY to GROUND, the

error signal is delayed (see DELAY section). This delayed error signal can be used as the power-on reset signal

for the application system. The ERROR pin is disconnected when not used.

DELAY

A capacitor from DELAY to GROUND sets the time delay for ERROR changing from low to high state. The delay

time is set by the following formula.

The DELAY pin should be open circuit if not used.

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