Datasheet

LM4780, LM4780TABD
SNAS193B –JULY 2003–REVISED APRIL 2013 www.ti.com
APPLICATION INFORMATION
MUTE MODE
The muting function allows the user to mute the amplifier. This can be accomplished as shown in the Figure 1.
The resistor R
M
is chosen with reference to the negative supply voltage and is used in conjunction with a switch.
The switch, when opened or switched to GND, cuts off the current flow from the MUTE pins to −V
EE
, thus placing
the LM4780 into mute mode. Refer to the Mute Attenuation vs Mute Current curves in the Typical Performance
Characteristics section for values of attenuation per current out of each MUTE pin. The resistance R
M
is
calculated by the following equation:
R
M
≤ (|−V
EE
| − 2.6V) / I
MUTE
where
• I
MUTE
≥ 0.5mA for each MUTE pin. (1)
The MUTE pins can be tied together so that only one resistor is required for the mute function. The mute resistor
value must be chosen so that a minimum of 1mA is pulled through the resistor R
M
. This ensures that each
amplifier is fully operational. Taking into account supply line fluctuations, it is a good idea to pull out 1mA per
MUTE pin or 2mA total if both pins are tied together.
A turn-on MUTE or soft start circuit may also be used during power up. A simple circuit like the one shown below
may be used.
The RC combination of C
M
and R
M1
may cause the voltage at point A to change more slowly than the -V
EE
supply voltage. Until the voltage at point A is low enough to have 0.5mA of current per MUTE pin flow through
R
M2
, the IC will be in mute mode. The series combination of R
M1
and R
M2
needs to satisfy the mute equation
above for all operating voltages or mute mode may be activated during normal operation. For a longer turn-on
mute time, a larger time constant, τ = RC = R
M1
C
M
(sec), is needed. For the values show above and with the
MUTE pins tied together, the LM4780 will enter play mode when the voltage at point A is -17.6V. The voltage at
point A is found with Equation (1) below.
V
A
(t) = (V
f
- V
O
)e
-t/τ
(Volts)
where
• t = time (sec)
• τ = RC (sec)
• V
o
= Voltage on C at t = 0 (Volts)
• V
f
= Final voltage, -V
EE
in this circuit (Volts) (2)
UNDER-VOLTAGE PROTECTION
Upon system power-up, the under-voltage protection circuitry allows the power supplies and their corresponding
capacitors to come up close to their full values before turning on the LM4780. Since the supplies have essentially
settled to their final value, no DC output spikes occur. At power down, the outputs of the LM4780 are forced to
ground before the power supply voltages fully decay preventing transients on the output.
OVER-VOLTAGE PROTECTION
The LM4780 contains over-voltage protection circuitry that limits the output current while also providing voltage
clamping. The clamp does not, however, use internal clamping diodes. The clamping effect is quite the same
because the output transistors are designed to work alternately by sinking large current spikes.
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