8.2 Typical Application

V

IN

GND

ON/OFF

1

2

3

5

4

BYPASS

V

OUT

1 μF

GND

2.2 μF

GND

LP2985-N

**

*

**

***

  *The ON/OFF input must be actively terminated. Tie to VIN if this function is not used. **Minimum capacitance is shown to ensure

stability (can be increased without limit). Ceramic capacitor required for output (see the Input and Output Capacitor Requirements

section). ***Reduces output noise (can be omitted if application is not noise critical). Use ceramic or film type with very low leakage

current (see the Noise Bypass Capacitor (CBYPASS) section).

Figure 8-2. Typical Application Schematic

8.2.1 Design Requirements

Table 8-1 lists the typical design parameters.

Table 8-1. Design Parameters for the New Chip

DESIGN PARAMETERS

VALUE

Input voltage

4.3 V, ±10% provided by the DC/DC converter switching at 1 MHz

Output voltage

3.3 V, ±1%

Output current

0 mA–150 mA (maximum)

RMS noise, 300 Hz to 50 kHz

< 30 µVRMS

PSRR at 100 kHz

> 40 dB

8.2.2 Detailed Design Procedure

8.2.2.1 Capacitor Characteristics

The LP2985-N was designed to work with ceramic capacitors on the output to take advantage of the benefits

these capacitors 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 (which makes these components the most efficient at 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 LP2985-N.

One disadvantage of ceramic capacitors is that the 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.

This capacitance drop can cause problems if a 2.2-µF capacitor is used on the output because that capacitor

can down to approximately 1 µF at high ambient temperatures (which can cause the LM2985 to oscillate). If Z5U

or Y5V capacitors are used on the output, a minimum capacitance value of 2.2 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.

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.

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

ceramics. Which means that although a tantalum capacitor can possibly have an ESR value within the stable

range, the capacitor must be larger in capacitance (which means bigger and more costly) than a ceramic

capacitor with the same ESR value.

LP2985-N

SNVS018Z – MARCH 2000 – REVISED MAY 2023

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