V

+

V

-

VOCM

Input

60 k:

60 k:

Internal

Circuitry

LMP8350

www.ti.com

SNOSB80C – FEBRUARY 2011 – REVISED OCTOBER 2015

Values of RA and RB can be calculated to achieve the voltages in Table 2, however their sum should be below

50 k

Ω to keep the voltage at the enable pin stable. Recommended values for RA and RB are given in Table 3.

Table 3. Recommended RA and RB for Mode Selection

MODE

10 V

6.6 V

5 V

VEN

High Power

RA = 0

RA = 0

RA = 0

> 7/8 VS

RB = inf

RB = inf

RB = inf

Mid Power

RA = 18 K

RA = 18 K

RA = 18 K

5/8 VS

RB = 30 K

RB = 30 K

RB = 30 K

Low Power

RA = 33 K

RA = 33 K

RA = 33 K

3/8 VS

RB = 18 K

RB = 18 K

RB = 18 K

Shutdown

RA = Inf

RA = Inf

RA = Inf

< 1/8 VS

RB = 0

RB = 0

RB = 0

7.4.2 VOCM Pin and Output Common-Mode Setting

Output common-mode voltage is set by the VOCM pin. Both outputs will be offset in the same direction (phase) by

an amount equal to the applied VOCM voltage.

The VOCM pin, if left unconnected, will self-bias to mid-supply. Two internal 60-kΩ resistors set this midpoint.

These resistors are shown in Figure 28.

Figure 28. VOCM Internal Bias Circuit

The equivalent resistance looking into the VOCM pin will look like 30 kΩ to mid-supply, plus about ±700 nA for

internal base currents (which scales with power mode and supply current). If left floating, the VOCM input should

be bypassed to ground with a 0.1-µF ceramic capacitor.

If a different output common-mode voltage is desired, the VOCM pin should be driven by a clean, low impedance

source to override the internal divider resistors. The VOCM pin should be bypassed to ground with a 0.1-µF

ceramic capacitor. It should be noted that any signal or noise-coupling into the VOCM will be passed as common-

mode noise and may result in the loss of dynamic range, degraded CMRR, degraded balance and higher

distortion. The VOCM pin is primarily intended as a DC bias path and is not intended for use as a signal path.

For applications that can tolerate slight shifts in the VOCMvoltage over temperature, it is also possible to use a

single resistor to program the VOCM voltage by paralleling one of the internal resistors to change the ratio.

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