Datasheet

TS419-TS421
28/32
The efficiency is the ratio between the output
power and the power supply
The maximum theoretical value is reached when
Vpeak = Vcc, so
■ Decoupling of the circuit
Two capacitors are needed to bypass properly the
TS419/TS421. A power supply bypass capacitor
C
S
and a bias voltage bypass capacitor C
B
.
C
S
has particular influence on the THD+N in the
high frequency region (above 7kHz) and an
indirect influence on power supply disturbances.
With 1µF, you can expect similar THD+N
performances to those shown in the datasheet.
In the high frequency region, if C
S
is lower than
1µF, it increases THD+N and disturbances on the
power supply rail are less filtered.
On the other hand, if C
S
is higher than 1µF, those
disturbances on the power supply rail are more
filtered.
C
B
has an influence on THD+N at lower
frequencies, but its function is critical to the final
result of PSRR (with input grounded and in the
lower frequency region).
If C
B
is lower than 1µF, THD+N increases at lower
frequencies and PSRR worsens.
If C
B
is higher than 1µF, the benefit on THD+N at
lower frequencies is small, but the benefit to PSRR
is substantial.
Note that C
IN
has a non-negligible effect on PSRR
at lower frequencies. The lower the value of C
IN
,
the higher the PSRR.
■ Wake-up Time: T
WU
When standby is released to put the device ON,
the bypass capacitor C
B
will not be charged
immediatly. As C
B
is directly linked to the bias of
the amplifier, the bias will not work properly until
the C
B
voltage is correct. The time to reach this
voltage is called wake-up time or T
WU
and typically
equal to:
T
WU
=0.15xC
B
(s) with C
B
in µF.
Due to process tolerances, the range of the
wake-up time is :
0.12xCb < T
WU
< 0.18xC
B
(s) with C
B
in µF
Note : When the standby command is set, the time
to put the device in shutdown mode is a few
microseconds.
■ Pop performance
Pop performance is intimately linked with the size
of the input capacitor Cin and the bias voltage
bypass capacitor C
B
.
The size of C
IN
is dependent on the lower cut-off
frequency and PSRR values requested. The size
of C
B
is dependent on THD+N and PSRR values
requested at lower frequencies.
Moreover, C
B
determines the speed with which the
amplifier turns ON. The slower the speed is, the
softer the turn ON noise is.
The charge time of C
B
is directly proportional to
the internal generator resistance 150kΩ..
Then, the charge time constant for C
B
is
τ
B
= 150kΩxC
B
(s)
As C
B
is directly connected to the non-inverting
input (pin 2 & 3) and if we want to minimize, in
amplitude and duration, the output spike on Vout1
(pin 5), C
IN
must be charged faster than C
B
. The
equivalent charge time constant of C
IN
is:
τ
IN
= (Rin+Rfeed)xC
IN
(s)
Thus we have the relation:
τ
IN
< τ
B
(s)
Proper respect of this relation allows to minimize
the pop noise.
Remark
: Minimizing C
IN
and C
B
benefits both the
pop phenomena, and the cost and size of the
application.
■ Application : Differential inputs BTL power
amplifier.
The schematic on figure 98, shows how to design
the TS419/21 to work in a differential input mode.
The gain of the amplifier is:
In order to reach optimal
performances of the differential function, R
1
and
R
2
should be matched at 1% max.
Vcc4
V
plysupP
P
PEAKOUT
π
==η
%5.78
4
=
π
1
2
VDIFF
R
R
2G
=