LP3872, LP3875

SNVS227H – FEBRUARY 2003 – REVISED JANUARY 2015

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Typical Application (continued)

8.2.1 Design Requirements

Table 1. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Input voltage range

2.5 V to 7 V

Output voltage

1.8 V

Output current

1.5 A

Output capacitor

10 µF

Input capacitor

10 µF

Output capacitor ESR range

100 m

Ω to 4 Ω

8.2.2 Detailed Design Procedure

8.2.2.1 Power Dissipation and Device Operation

The permissible power dissipation for any package is a measure of the capability of the device to pass heat from

the power source, the junctions of the IC, to the ultimate heat sink, the ambient environment. Thus, the power

dissipation depends on the ambient temperature and the thermal resistance across the various interfaces

between the die junction and ambient air.

The maximum allowable power dissipation for the device in a given package can be calculated using Equation 1:

(1)

The actual power being dissipated in the device can be represented by Equation 2:

(2)

Equation 1 and Equation 2 establish the relationship between the maximum power dissipation allowed due to

thermal consideration, the voltage drop across the device, and the continuous current capability of the device.

These two equations should be used to determine the optimum operating conditions for the device in the

application.

In applications where high power dissipation and/or poor package thermal resistance is present, the maximum

as given by Equation 3:

(3)

combination of the two.

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.

•

Input Capacitor: An input capacitor of at least 10 µF is required. Ceramic, tantalum, or Electrolytic capacitors

may be used, and capacitance may be increased without limit.

•

Output Capacitor: An output capacitor is required for loop stability. It must be located less than 1 cm from the

device and connected directly to the output and ground pins using traces which have no other currents

flowing through them (see Layout section).

The minimum value of output capacitance that can be used for stable full-load operation is 10 µF, but it may be

increased without limit. The output capacitor must have an equivalent series resistance (ESR) value as shown in

Figure 14. Tantalum capacitors are recommended for the output capacitor.

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