LM4781TA/NOPB National Semiconductor, LM4781TA/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM4781TA/NOPB
• Description: IC AMP AUDIO PWR 35W AB TO220-27
• Manufacturer: National Semiconductor
• Series: Overture™r
• Type: Class ABr

Specifications of LM4781TA/NOPB

• Output Type: 3-Channel
• Max Output Power X Channels @ Load: 35W x 3 @ 8 Ohm
• Voltage - Supply: 20 V ~ 70 V, ±10 V ~ 35 V
• Features: Depop, Mute, Short-Circuit and Thermal Protection
• Mounting Type: Through Hole
• Package / Case: TO-220-27 (Bent and Staggered Leads)
• For Use With: LM4781TABD - BOARD EVALUATION LM4781TA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM4781TA; *LM4781TA/NOPB; LM4781TA

Typical Performance Characteristics
Note 17: Bridge mode graphs were taken using the demo board and inverting the signal to the channel B input.
Note 18: Parallel mode graphs were taken using the demo board connecting each output through a 0.1Ω/3W resistor to the load.
Application Information
MUTE MODE
The muting function allows the user to mute the amplifier.
This can be accomplished as shown in the Typical Applica-
tion Circuit. The resistor R
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
the LM4781 into mute mode. Refer to the Mute Attenuation
vs Mute Current curves in the Typical Performance Char-
acteristics section for values of attenuation per current out
of each MUTE pin. The resistance R
following equation:
Where I
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 1.5mA is pulled
through the resistor R
fully operational. Taking into account supply line fluctuations,
it is a good idea to pull out 1mA per MUTE pin or 3mA total
if all 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
at point A to change more slowly than the -V
voltage. Until the voltage at point A is low enough to have
0.5mA of current per MUTE pin flow through R
MUTE
≥ 0.5mA for each MUTE pin.
R
M
≤ (|−V
M
. This ensures that each amplifier is
EE
M
M
| − 2.6V) / I
and R
is chosen with reference to the
M1
may cause the voltage
M
MUTE
is calculated by the
P
Frequency Response of Demo Board
OUT
EE
200673B7
M2
, thus placing
= 10W/Channel = 0dB, R
, the IC will
EE
supply
R
L
= 8Ω, No Filters
15
(Continued)
be in mute mode. The series combination of R
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
show above and with the MUTE pins tied together, the
LM4781 will enter play mode when the voltage at point A is
-25.1V. The voltage at point A is found with Equation (1)
below.
where:
UNDER-VOLTAGE PROTECTION
Upon system power-up, the under-voltage protection cir-
cuitry allows the power supplies and their corresponding
capacitors to come up close to their full values before turning
on the LM4781. Since the supplies have essentially settled
to their final value, no DC output spikes occur. At power
down, the outputs of the LM4781 are forced to ground before
the power supply voltages fully decay preventing transients
on the output.
OVER-VOLTAGE PROTECTION
The LM4781 contains over-voltage protection circuitry that
limits the output current while also providing voltage clamp-
ing. 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.
SPiKe PROTECTION
The LM4781 is protected from instantaneous peak-
temperature stressing of the power transistor array. The Safe
Operating graph in the Typical Performance Characteris-
t = time (sec)
τ = RC (sec)
V
V
o
f
= Final voltage, -V
= Voltage on C at t = 0 (Volts)
IN
= 47kΩ
200673C9
V
A
(t) = (V
M1
EE
C
M
f
in this circuit (Volts)
- V
(sec), is needed. For the values
O
)e
-t/τ
(Volts)
M1
www.national.com
and R
(1)
M2