zxcd1000 Zetex Semiconductors plc., zxcd1000 Datasheet - Page 11

• Manufacturer Part Number: zxcd1000
• Description: High Fidelity Class D Audio Amplifier Solution
• Manufacturer: Zetex Semiconductors plc.

Class D 50W Mono Bridge Tied Load (BTL)
Solution with Feedback – Circuit Description
With the addition of feedback (hence closed loop
solution) it is possible to obtain even better THD
performance. A schematic diagram for this is shown in
Figure 9. Again proprietary circuit and special
magnetic design is necessary to yield the high THD
p e r f o r m a n c e a n d d e v i a t i o n f r o m t h i s c o u l d
significantly reduce performance.
Much of the circuitry is the same as described for the
open loop solution. The main differences being a
consequence of using the feedback circuitry. The audio
input is ac coupled and applied to an op-amp (1/2 of U3)
configured as a non–inverting amplifier with a gain of
approximately 4. Feedback is applied differentially
from the bridge outputs via the other half of U3
op-amp. A portion of the single ended output from this
op-amp is subtracted from the output of the
non-inverting op-amp output above. Overall negative
feedback is applied due to the polarity and connection
of the signals involved.
The audio signal from the above circuitry is applied to a
phase splitter as was done for the open loop solution.
This is built around the other 5532 dual op-amp (U2).
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 Audio A and Audio B inputs
respectively.
The output circuitry downstream of the ZXCD1000 is as
described for the open loop solution. In order to
support the 50W output power of this solution a 25V
rail is required for a 4
SOT223 packaged (ZXM64N035G and ZXM64P035).
Further information on this design is available through
Zetex applications.
ISSUE 2 - APRIL 2002
load. The MOSFETs used are
11
Higher Power Solutions
With some modifications the applications solutions
can be extended to give output power up to 100W. The
main differences being the supply voltage, the TO220
MOSFETs, and the output magnetics. The magnetics
for 100W are necessarily larger than required for
25/50W in order to handle the higher load currents. For
100W operation the supply voltage to the circuit is
nominally 35V with a 4 load. However the maximum
supply voltage to the ZXCD1000 class D controller IC is
18V, hence a voltage dropper is required. This could be
done, for example, as in the open loop solution
described previously. A 100W circuit is shown on
figure 10. This features a 35V bridge supply TO220
MOSFETs (ZXM64N035L3 and ZXM64P035L3) and
also proposed protection circuits for over current and
over temperature and an alternative anti pop circuit.
Further information on this 100W reference design can
be obtained through Zetex applications.
The ZXCD1000 class D controller IC is inherently
capable of driving even higher power solutions, with
the appropriate external circuitry. However as stated
above the maximum supply voltage to the ZXCD1000
class D controller IC is 18V and the higher supply
voltages must therefore be dropped. Also due
consideration must be given to the ZXCD1000 output
drive levels and the characteristics of the bridge
MOSFET’s. The latter must be sufficiently enhanced by
the OutA and OutB outputs to ensure the filter and load
network is driven properly. If the gate drive of the
ZXCD1000 is too low for the chosen MOSFET then the
OUTA and OUTB signal must be buffered using an
appropriate MOSFET driver circuit. Additionally,
suitable magnetics are essential to achieve good THD
performance.
Package details
The ZXCD1000 is available in a 16 pin exposed pad
QSOP package. The exposed pad on the underside of
the package should be soldered down to an area of
copper on the PCB, to function as a heatsink. The PCB
should have plated through vias to the underside of the
board, again connecting to an area of copper.
ZXCD1000