TS4962M_07 Datasheet, PDF (29/41 Pages) STMicroelectronics – 3W filter-free class D audio power amplifier

TS4962M
Application information
5.3
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 V_icm limitation in the input stage (see Table 2: Operating conditions on
page 3), the common mode feedback loop can ensure 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 150kΩ resistor, it’s 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_ic
m
≤
-^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 V_icm calculation is not in the previous range, input coupling capacitors must
be used (with V_CC from 2.4V to 2.5V, input coupling capacitors are mandatory).
For example:
With V_CC = 3V, R_in = 150k and V_IC = 2.5V, we typically find V_icm = 2V and this is lower than
3V - 0.8V = 2.2V. With 136.5kΩ we find 1.97V, and with 163.5kΩ we have 2.02V. So, no input
coupling capacitors are required.
5.4
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 -3dB 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.
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