LP2986

SNVS137I – MARCH 1999 – REVISED SEPTEMBER 2015

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Typical Applications (continued)

8.2.1 Design Requirements

For typical ultra-low-dropout linear regulator applications, use the parameters listed in Table 1.

Table 1. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Input voltage

4.3 V

Output voltage

3.3 V

Output current

200 mA (maximum)

RMS noise, 300 Hz to 50 kHz

PSRR at 1 kHz

65 dB typical

8.2.2 Detailed Design Procedure

8.2.2.1 Using an External Resistive Divider

The LP2986 output voltage can be programmed using an external resistive divider. Figure 30 shows a typical

circuit application using external resistive divider.

The resistor connected between the FEEDBACK pin and ground should be 51.1 k

Ω. The value for the other

resistor (R1) connected between the FEEDBACK pin and the regulated output is found using the formula:

(1)

It should be noted that the 25 µA of current flowing through the external divider is approximately equal to the

current saved by not connecting the internal divider, which means the quiescent current is not increased by using

external resistors.

(2)

significantly over temperature. Mica or ceramic capacitors can be used, assuming a tolerance of ±20% or better

is selected.

If a ceramic is used, select one with a temperature coefficient of NPO, COG, Y5P, or X7R. Capacitor types Z5U,

Y5V, and Z4V can not be used because their value varies more that 50% over the

−25°C to +85°C temperature

range.

8.2.2.2 External Capacitors

Like any low-dropout regulator, external capacitors are required to assure stability. These capacitors must be

correctly selected for proper performance.

8.2.2.2.1

Input Capacitor

An input capacitor (

≥ 2.2 µF) is required between the LP2986 input and ground (amount of capacitance may be

increased without limit).

This capacitor must be located a distance of not more than 0.5 inches from the input pin and returned to a clean

analog ground. Any good quality ceramic or tantalum may be used for this capacitor.

8.2.2.2.2

Output Capacitor

The output capacitor must meet the requirement for minimum amount of capacitance and also have an

appropriate equivalent series resistance (ESR) value.

Curves are provided which show the allowable ESR range as a function of load current for various output

voltages and capacitor values (see Figure 31 and Figure 32).

16

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