Cirrus Logic CS42L42 Manual page 37

4.4.2 Using External Output Switches
The CS42L42 can work with external switches for the headphone outputs along with mic inputs.
simplified, closed-loop example of supporting two separate headsets, including headphone and mic support. For
simplicity, tip sense and ring sense connectivity is not shown.
DACB−
DACB+
DACA−
DACA+
HS Bias
Generator
Fig. 4-10 shows HPSENSA and HPSENSB, pins not typically seen in the HP output. They allow the feedback point of the
HP output to include the switch impedance. This closed-loop method improves output performance, although the following
considerations must be adhered to when incorporating external switches:
• The combined switch ON-resistance (R
resistance in the signal path decreases output voltage swing, keeping the total resistance below 1  minimizes the
voltage loss along with reducing the effect on DC offsets. For example, for a 30- load, the full-scale output voltage
swing is reduced by the extent of the switches' ON-resistance.
• The switch ON-resistance flatness (R
of these switches. for SW6 and SW8 and less than 0.075  over the common-mode voltage swing of SW2 and
SW4. Failure to meet this requirements degrades THD performance.
Note that not just the value of the switches' R
performance. It is recommended that the shape be as linear as possible over the common-mode voltage swing
appearing at each switch. Shapes such as "W", "N", and "M" significantly affect THD, even if their R
meets the values defined here.
• The total capacitance placed on the HPOUTx pins is limited to 1 or 10 nF, depending on the
(see
p. 154). The combined switch capacitance (C
connector/cable/load capacitance must be within these limits, otherwise stability is reduced and THD is degraded.
Because the amplifier feedback path includes the switches, HP_PDN must be set if the switches are open.
DS1083F2
CS42L42
HPSENSB
–
HPOUTB
+
HPSENSA
–
HPOUTA
+
HS3_REF
HS4_REF
HSIN+
HSIN−
HS3
HS4
HS_CLAMP1
HS_CLAMP2
GNDHS
Figure 4-10. Closed-Loop External Output Switches
) and PCB trace resistance must be less than 1 . Although any added
ON
flatness) must be less than 0.02  over the common-mode voltage swing
ON
flatness, but also its shape has a considerable effect on THD
ON
ON
External Switch Bank 1
SW1
SW2
SW3
SW4
SW5
SW6
SW7
SW8
External Switch Bank 2
SW1
SW2
SW3
SW4
SW5
SW6
SW7
SW8
+ C ), PCB stray capacitance, and any headphone
OFF
CS42L42
4.4 Analog Output
Fig. 4-10 shows a
Headset
Connector 1
2 3
1 4
Headset
Connector 2
2 3
1 4
flatness
ON
setting
HPOUT_LOAD
37