TS4855_04 Datasheet, PDF (20/27 Pages) STMicroelectronics – LOUDSPEAKER & HEADSET DRIVER WITH VOLUME CONTROL

TS4855
Application Information
6 APPLICATION INFORMATION
6.1 BTL Configuration Principle
The TS4855 integrates 3 monolithic power
amplifiers having BTL output. BTL (Bridge Tied
Load) means that each end of the load is
connected to two single-ended output amplifiers.
Thus, we have:
Single ended output 1 = Vout1 = Vout (V)
Single ended output 2 = Vout2 = -Vout (V)
and
Vout1 - Vout2 = 2Vout (V)
The output power is:
Pout = ^{(2 Vout RMS )2} (W)
R_L
For the same power supply voltage, the output
power in BTL configuration is 4 times higher than
the output power in single-ended configuration.
6.2 Power dissipation and efficiency
Hypotheses:
• Voltage and current in the load are sinusoidal
(Vout and Iout).
• Supply voltage is a pure DC source (Vcc).
Regarding the load we have:
VO UT = V_PEAK sin ω t (V)
and
IOUT = -V----^O----^U----^T--- (A)
RL
and
PO UT = ^V-----P---^E----^A---^K----²--- (W)
2RL
Therefore, the average current delivered by the
supply voltage is:
I CC _AVG =
2 ^V-----P----^E---^A---^K---- (A)
πRL
The power delivered by the supply voltage is:
Psupply = Vcc Icc_AVG (W)
Then, the power dissipated by each amplifier is
Pdiss = Psupply - Pout (W)
P_diss
=
2 2 V_CC
π R_L
P_OUT − P_OUT
(W)
and the maximum value is obtained when:
∂_{----P----d----i--s---s---- = 0}
∂POUT
and its value is:
Pdiss max
=
2 Vcc²
π²R_L
(W)
Note: This maximum value is only depending on
power supply voltage and load values.
The efficiency is the ratio between the output
power and the power supply
η = -----P-----^O----^U---^T------ = ---^π---V-----^P---^E----^A---^K--
Psupply 4VCC
The maximum theoretical value is reached when
Vpeak = Vcc, so
π_{---- = 78.5%}
4
The TS4855 has 3 independent power amplifiers
and each amplifier produces heat due to its power
dissipation. Therefore, the maximum die
temperature is the sum of the each amplifier’s
maximum power dissipation. It is calculated as
follows:
P_{diss speaker} = Power dissipation due to the
speaker power amplifier.
P_{diss head} = Power dissipation due to each
headphone’s power amplifier.
Total P_diss = P_{diss speaker} + P_{diss head1} + P_{diss head2} (W)
In most cases, P_{diss head1} = P_{diss head 2}, giving:
Total P_diss = P_{diss speaker} + 2P_{diss head} (W)
2
TotalP _diss =
2 V_CC


π _
P_{OUT SPEAKER}
R _{L SPEAKER}
+2
P_{OUT HEAD}


R_{L HEAD }
[ ] − P_{OUT SPEAKER} + 2 P_{OUT HEAD}
(W )
20/27