International Rectifier IRAUDAMP4, Руководство пользователя

Страница: 10 / 44

www.irf.com 9
RD-0617
0
100
200
300
400
500
600
10% 20% 30% 40% 50% 60% 70% 80% 90%
Duty Cycle
O
p
er
at
in
g Fr
eq
u
e
n
cy (
k
H
z
)
Figure 5. Typical Lock Frequency Range vs. PWM Duty Ratio
(Self-oscillating frequency set to 400 kHz with no input)
As illustrated by the THD+N Ratio vs. Output Power results (Figure 6) , the noise levels
increase slightly when all channels are driven (ACD) with the self oscillator, especially
below the 5 W range. Residual noise typically increases by a third or more (see
“Specifications – Audio Performance”) compared to a single-channel driven (SCD)
configuration. Locking the oscillator frequency results in lowering the residual noise to
that of a single-channel-driven system. The output power range, for which the frequency-
locking is successful, depends on what the locking frequency is with respect to the self-
oscillating frequency. As illustrated in Figure 6, the locking frequency is lowered (from
450 kHz to 400 kHz to 350 kHz and then 300 kHz) as the output power range (where
locking is achieved) is extended. Once locking is lost, however, the audio performance
degrades, but the increase in THD seems independent from the clock frequency.
Therefore, a 300 kHz clock frequency is recommended.
It is possible to improve the THD performance by increasing the corner frequency of the
high pass filter (HPF) (R17 and C15 for Ch1) that is used to inject the clock signal. This
drop in THD, however, comes at the cost of reducing the locking range. Resistor values
of up to 100 k
Ω
and capacitor values down to 10 pF can be used.
In the IRAUDAMP4, this switching frequency lock/synchronization feature is achieved
with either an internal or external clock input (selectable through S3). If an internal (INT)
clock is selected, an internally-generated clock signal will be used, adjusted by setting
potentiometer R113 “INT FREQ.” If external (EXT) clock signal is selected, a 0 V to 5 V
square-wave (~50% duty ratio) logic signal must be applied to BNC connector J17.
Locking range
Self-oscillating frequency
Suggested clock frequency
for maximum locking range