TS4962_10 Datasheet, PDF (31/44 Pages) STMicroelectronics – 2.8 W filter-free mono class D audio power amplifier

TS4962
Application information
4.3
Common-mode feedback loop limitations
As explained previously, the common-mode feedback loop allows the output DC bias
voltage to be averaged at V_CC/2 for any DC common-mode bias input voltage.
However, due to a V_icm limitation in the input stage (see Table 3: Operating conditions on
page 4), the common-mode feedback loop can play its role only within a defined range. This
range depends upon the values of V_CC and R_in (A_Vdiff). To have a good estimation of the
V_icm value, we can apply this formula (no tolerance on R_in):
V_icm
=
V_{-----C---C-----}×_------R_{-----i-n----+-----}2_-----×_------V_----I--C----×_------1_----5_---0_----k_---Ω_--
2 × (R_in + 150kΩ)
(V)
with
V_IC
=
I_--n_----⁺_----+-----I_--n_----^-
2
(V)
And the result of the calculation must be in the range:
0.5V ≤ V_icm ≤ V_CC – 0.8V
Due to the +/-9% tolerance on the 150 kΩ resistor, it is also important to check V_icm in these
conditions.
-^V----^C---^C-----^×------^R-----ⁱ-ⁿ----+-----²-----^×------^V----^I--^C----^×------¹----³---⁶---^.--⁵----^k---^Ω--
2 × (R_in + 136.5kΩ)
≤
V_i
cm
≤
V_{-----C---C-----}×_------R_{-----i-n----+-----}2_-----×_------V_----I--C----×_------1_----6_---3_---._--5_----k_---Ω_--
2 × (R_in + 163.5kΩ)
If the result of the V_icm calculation is not in the previous range, input coupling capacitors
must be used. With V_CC between 2.4 and 2.5 V, input coupling capacitors are mandatory.
For example:
With V_CC = 3 V, R_in = 150 k and V_IC = 2.5 V, we typically find V_icm = 2 V, which is lower than
3 V-0.8 V = 2.2 V. With 136.5 kΩ we find 1.97 V and with 163.5 kΩ we have 2.02 V.
Therefore, no input coupling capacitors are required.
4.4
Low frequency response
If a low frequency bandwidth limitation is requested, it is possible to use input coupling
capacitors.
In the low frequency region, C_in (input coupling capacitor) starts to have an effect. C_in forms,
with R_in, a first order high-pass filter with a -3 dB cut-off frequency.
F_CL
=
------------------¹-------------------
2π × R_in × C_in
(Hz)
So, for a desired cut-off frequency we can calculate C_in,
C_in
=
-------------------¹--------------------
2π × R_in × F_CL
(F)
with R_in in Ω and F_CL in Hz.
Doc ID 10968 Rev 8
31/44