Cirrus Logic CRD44800-ST-FB - Manual (Page 9)

CRD44800-ST-FB
9
System Issues
2.1 Power Supply Ratings
The required power supply current rating can be estimated as follows. 45 W is used as the
reference output power because this represents the typical full scale output with no clipping.
Assume the efficiency,
η , is approximately 85% (this accounts for power to supply control
electronics and overhead), then for 45 W / 8 channels:
Consequently the supply current is:
The factor of 2 in the denominator of the P
Supply
calculation arises from the fact that for typical
consumer applications in A/V or DVD receivers, the power supply should be capable of pro-
viding
½ the total requirement for all channels operating at full power. This design guide is
still quite conservative, and gives more that adequate headroom in real applications.
2.2 Power Supply Decoupling
Proper power supply decoupling is one key to maximizing the performance of a Class-D am-
plifier. Because the design uses an open loop output stage, noise on the power supply rail
will be coupled to the output. While the PSR functionality of the CS44800 helps reduce power
supply noise feedthrough to the output, careful decoupling of the power stage supply rails is
essential. Referring to Figure 11, the top side of the CRD44800-ST-FB PWM amplifier board,
good decoupling practice is shown. Notice that the 0.1
µ F ceramic capacitors are as close
as physically possible to the power pins of the STA500. The ground side of the capacitors is
connected directly to top side ground plane, which is also used by the power supply return
pins. This keeps the high frequency current loop small to minimize power supply variations
and EMI. 470
µ F electrolytic capacitors are also located in close proximity to the power sup-
ply pins to supply the current locally for each channel. These are not required to be expensive
low ESR capacitors. General purpose electrolytic capacitors that are specified to handle the
ripple current can be used. The real time PSR feedback of the CS44800/CS4461 can greatly
attenuate the induced voltages due to the power supply ripple current.
W
W P
P
Out
Total
424
85 . 0
360
= = =
η
W
W P
P
Total
Supply
212
2
424
2
= = =
A
V
W
V
P
I
Supply
Supply
Supply
1 . 7
30
212
= = =