Class D 25W Mono Open Loop (Bridge Tied Load - BTL) Solution – Circuit Description

Proprietary circuit design and high quality magnetics

are necessary to yield the high THD performance

specified. Deviation from the Zetex recommended

solution could significantly degrade performance.

The speaker is connected as a Bridge Tied Load (BTL).

This means that both sides of the speaker are driven

from the output bridge and therefore neither side of the

speaker connects to ground. This allows maximum

power to be delivered to the load, from a given supply

voltage.

The supply voltage for this solution is

nominally 16V for 25W into a 4

load.

A schematic diagram for the solution is shown in

Figure 4. The audio input is AC coupled and applied to a

low pass filter and a phase splitter built around the

NE5532 dual op-amp. One of these op-amps is

configured as a voltage follower and the other as a X1

inverting amplifier. This produces in phase and

inverted signals for application to the ZXCD1000. The

op-amp outputs are AC coupled into the ZXCD1000

Audio A and Audio B inputs via simple R-C low pass

filters (R16/C3 and R15/C7). The op-amps are biased to

a DC level of approximately 6V by R11 and R12.

The Pulse Width Modulated (PWM) outputs, OutA and

OutB, which contain the audio information, are AC

coupled and DC restored before driving the Zetex

ZXM64P03X and ZXM64N03X PMOS and NMOS

output bridge FET’s. AC coupling is via C17, C18, C19

and C20. DC restoration is provided by the D2(A1a)/R4,

D1(A4a)/R2 and D3(A1a)/R6, D4(A4a)/R9 components.

This technique allows the output stage supply voltage

to be higher than the high level of the OutA and OutB

outputs (approximately 8.5V),

whilst still supplying

almost the full output voltage swing to the gates of the

bridge FET’s (thereby ensuring good turn on). This can

be exploited to yield higher power solutions with

higher supply voltages – this is discussed later.

The resistor/diode combinations (R5/D2(A16b),

R3/D1(A4b), R7/D3(A1b) and R8/D4(A4b)) in series with

the bridge FET gates, assist in controlling the switching

of the bridge FET’s. This design minimises shoot

through currents whilst still achieving the low

distortion characteristics of the system.

The purpose of the special inductors in conjunction

with the output capacitors C23, C24, C25 and C26 is to

low pass filter the high frequency switching PWM

signal that comes from the bridge. Thus the lower

frequency audio signal is recovered and is available at

the speakerA and speakerB outputs across which the

speaker should be connected. Zetex can offer advice on

suitable source for the specialist magnetics.

The optional components R17 and C3 form a Zobel

network. The applicability of these depends upon the

application and speaker characteristics. Suggested

values are 47nF and 10 ohms

Efficiency

The following plots show the measured efficiency of

the Zetex solution at various power levels into both 4

Ω

and 8

Ω loads. As a comparison, typical efficiency is

plotted for a class A-B amplifier. They clearly

demonstrate the major efficiency benefits available

from the Zetex class D solution.

ZXCD1000

ISSUE 2 - APRIL 2002

7

Figure 5