数字功放电感如何选择

Guide for choosing right inductor

for NTPxxxx

NeoFidelity, Inc.

Dec., 2014

Table of Contents

1General description

2Guide for selecting inductor

A.Inductor current

B.Example for selecting a right inductor

C.Calculation of L and C for a low pass filter

1. GENERAL DESCRIPTION

This document is made to help selecting a right inductor for a low pass filter when using NTPxxxx. NTPxxxx series are full digital amplifiers, which output PWM pulses. To reproduce analog audio signals, LC low pass filters are needed in general cases.

2. GUIDE FOR SELECTING INDUCTOR

A. Inductor current

Following Table 1 shows the currents on the inductor according to inductance, load resistance, and PVDD.

Table 1. Inductor current

In Table 1., inductor currents for 10 and 15uH are same as in the case for 8.2uH/15V/10W.

B. Example for selecting a right inductor

Here, the example illustrates how to select a proper inductor using the inductor specification, AE032X100048. Once the inductance and current for an inductor are set,

other specifications like dc inductance tolerance, saturation current, etc. should be considered. Let’s say we are looking for an inductor and the requirements are 10uH, 20V, 15W, and 6Ω. According to Table 1, the current on the inductor is 1.59Arms. With these conditions in mind, let’s look at the electrical characteristics in Table 2, which is from AE032X10048 documented by TAIYO YUDEN. The current on the inductor will be 1.59Arms, so the saturation current should be larger than 1.59Arms. The second factor for consideration is the temperature rise current (Idc2), for the worse condition. For NT6045T100M of 10uH, Idc2 is 2500mA, which is 1.77Arms. This value meets the above requirement, so NR6045T100M is a good candidate for the LC filter. In addition, self-resonant frequency, 12MHz, is high enough compared to the PWM carrier frequency of 384kHz. Also, temperature on the inductor is considered, which is dependent on not only DC resistance but PCB patterns.

Table 2. Electrical characteristics of inductors

C. Calculation of L and C for a low pass filter

The characteristic frequency of the LC filter needs to be high enough to allow high frequency audio to the output, yet needs to be low enough to filter out high frequency

products of the pulses from the amplifier.

The characteristic frequency of the LC filter in Figure 1 can be calculated like the following:

Figure 1. LC low pass filter

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