ZXCD1000
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.
A schematic diagram for the solution is shown in
Figure 4. The audio input is AC coupled and applied to a
simple R-C (R1 and C1) 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/C8 and R15/C9). 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(A16),
R3/D1(A46), R7/D3(A16) and R8/D4(A46)) 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 (ZXFN1000) inductors in
conjunction with the output capacitors C23, C24, C25
and C26 is to low pass filter the high frequency
ISSUE 1 -MARCH 2001
7
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. The
ZXFN1000 magnetics form an integral part of, and are
specially designed for, the Zetex solution.
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 4Ω
and 8Ω 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.
ETC [ ETC ]
