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LP2985LV-N

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SNOS510Q – NOVEMBER 1999 – REVISED OCTOBER 2016

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Copyright © 1999–2016, Texas Instruments Incorporated

The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic

capacitors with either NPO or COG dielectric typically have very low leakage. 10-nF polypropolene and

polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low

leakage current.

8.2.2.2 Capacitor Characteristics

The LP2985LV-N is designed to work with ceramic capacitors on the output to take advantage of the benefits

they offer: for capacitance values in the 2.2-µF to 4.7-µF range, ceramics are the least expensive and also have

the lowest ESR values (making them best for eliminating high-frequency noise). The ESR of a typical 2.2-µF

ceramic capacitor is in the range of 10 mΩ to 20 mΩ, which easily meets the ESR limits required for stability by

the device.

One disadvantage of ceramic capacitors is that their capacitance can vary with temperature. Most large value

ceramic capacitors (

≥ 2.2 µF) are manufactured with the Z5U or Y5V temperature characteristic, which results in

the capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.

Problems may ensue if a 2.2-µF capacitor is used on the output because it drops down to approximately 1 µF at

high ambient temperatures (which could cause the LM2985 to oscillate). If Z5U or Y5V capacitors are used on

the output, a minimum capacitance value of 4.7 µF must be observed.

A better choice for temperature coefficient in ceramic capacitors is X7R, which holds the capacitance within

±15%. Unfortunately, the larger values of capacitance are not offered by all manufacturers in the X7R dielectric.

8.2.2.2.1

Tantalum

Tantalum capacitors are less desirable than ceramics for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 1 µF to 4.7 µF range.

An additional important consideration is that tantalum capacitors have higher ESR values than equivalent size

ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the

stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic

capacitor with the same ESR value.

Note that the ESR of a typical tantalum increases about 2:1 as the temperature goes from 25°C down to

−40°C,

so some guard band must be allowed.

8.2.2.3 On/OFF Input Operation

The LP2985LV-N is shut off by driving the ON/OFF input low, and turned on by pulling it high. If this feature is

not to be used, the ON/OFF input must be tied to VIN to keep the regulator output on at all times.

To assure proper operation, the signal source used to drive the ON/OFF input must be able to swing above and

below the specified turnon/turnoff voltage thresholds listed inElectrical Characteristics under VON/OFF. To prevent

mis-operation, the turnon (and turnoff) voltage signals applied to the ON/OFF input must have a slew rate which

is

≥ 40 mV/µs.

CAUTION

The regulator output voltage cannot be ensured if a slow-moving AC (or DC) signal is

applied that is in the range between the specified turnon and turnoff voltages listed

under the electrical specification VON/OFF (see Electrical Characteristics).

8.2.2.4 Reverse Input-Output Voltage

The PNP power transistor used as the pass element in the LP2985LV-N has an inherent diode connected

between the regulator output and input. During normal operation (where the input voltage is higher than the

output) this diode is reverse-biased.