tda2500 Tripath Technology Inc., tda2500 Datasheet - Page 23

• Manufacturer Part Number: tda2500
• Description: Stereo Class-t Digital Audio Amplifier Driver Using Digital Power Processing Dpp
• Manufacturer: Tripath Technology Inc.

The tradeoff involved in making this setting is that as the delay is reduced, distortion levels improve but
shoot-through and power dissipation increase. The actual amount of BBM required is dependent upon
components such as MOSFET type and gate resistor value as well as circuit board layout. The BBM
value selected should be verified in the actual application circuit board. It should also be verified under
maximum temperature and power conditions since shoot-through in the output MOSFETs can increase
under these conditions, possibly requiring a higher BBM setting than at room temperature.
OU T PU T FIL T ER D ESIGN
One advantage of Tripath amplifiers over PWM solutions is the ability to use higher-cutoff-frequency
filters. This means load-dependent peaking/droop in the 20kHz audio band potentially caused by the filter
can be made negligible. Furthermore, speakers are not purely resistive loads and the impedance they
present changes over frequency and from speaker model to speaker model.
Tripath recommends designing the filter as a 2nd order, LC filter. Tripath has obtained good results with
L
the ripple current caused by the high frequency switching. Thus, a high quality film capacitor is strongly
recommended.
The typical application of the TDA2500 is driving “high impedance” loads from 12.5 ohms and above.
This dictates the use of a larger value output inductor, L
minimize in band output filter peaking and match better to the intended load impedance.
There is a compromise between inductor value and amplifier efficiency. Tripath amplifiers count on the
inductor current making “free” transitions. Take the case where the inductor current is flowing out towards
the load. This is the case where there is a positive going output waveform. When the top side device
turns off, the output voltage will “flip” to keep the inductor current in the same direction. If the entire
transition of the output voltage (from VPP to VNN) occurs before the bottom side device is enhanced,
then the transition is free. This has a positive effect on amplifier efficiency. If the bottom side device
turns on before the transition is completed then power is wasted and the amplifier efficiency suffers. The
output transition time is directly proportional to the inductor value and the supply voltage. Thus, larger
values of inductance (for a given fet output capacitance) will result in longer transition times and
decreased efficiency for a fixed supply rail. The value of L
a reasonable compromise between efficiency and load “damping.” An upper bound on L
sacrificing efficiency, is 47uH for typical TDA2500 supply voltages and the STW20NM50FD fets. Above
this value, the designer should fully characterize the amplifier efficiency before settling on the inductor
value. The peaking exhibited by a lightly loaded LC filter can be equalized out (to some degree) by an
input RC filter located before the input coupling capacitor, C
response over a wider range of output loads. In addition, it will provide additional protection (beyond that
provided by the zobel network) against high frequency signals that can cause the output filter to resonate.
The core material of the output filter inductor has an effect on the distortion levels produced by a
TDA2500 amplifier. Tripath recommends low-mu type-2 iron powder cores because of their low loss and
high linearity (available from Micrometals, www.micrometals.com). The specific core used on the RB-
TDA2500 was a T106-2 wound with 29 turns of 16AWG wire.
Tripath also recommends that an RC damper be used after the LC low-pass filter. No-load operation of a
TDA2500 amplifier can create significant peaking in the LC filter, which produces strong resonant
23
O
= 11uH and C
O
= 0.22uF (resonant frequency of 59kHz). The filter capacitor must be able of sustain
BBM1
Table 2: BBM Delay
0
0
1
1
BBM0
0
1
0
1
145 ns
105 ns
Delay
65 ns
25 ns
O
, as compared to other Tripath amplifiers to
T r i p a t h T e c h n o l o g y , I n c . - T e c h n i c a l I n f o r m a t i o n
O
, 33uH, recommended above was chosen as
I
. This will result in a flatter magnitude
TDA2500 – KL/ Rev. 0.9/05.05
O
without totally