Datasheet

LM48820
SNAS370B –MAY 2007–REVISED MAY 2013 www.ti.com
For this DSBGA package, θ
JA
= 86°C/W and T
JMAX
= 150°C. Depending on the ambient temperature, T
A
, of the
system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the
IC packaging. If the result of Equation 2 is greater than that of Equation 3, then either the supply voltage must be
decreased, the load impedance increased or T
A
reduced. For the typical application of a 3V power supply, with a
16Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is
approximately 127°C provided that device operation is around the maximum power dissipation point. Power
dissipation is a function of output power and thus, if typical operation is not around the maximum power
dissipation point, the ambient temperature may be increased accordingly.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. Applications that employ a 3V power supply typically use a 4.7µF capacitor in parallel with a 0.1µF
ceramic filter capacitor to stabilize the power supply output, reduce noise on the supply line, and improve the
supply's transient response. Keep the length of leads and traces that connect capacitors between the LM48820's
power supply pin and ground as short as possible.
MICRO POWER SHUTDOWN
The voltage applied to the SD_LC (shutdown left channel) pin and the SD_RC (shutdown right channel) pin
controls the LM48820’s shutdown function. When active, the LM48820’s micropower shutdown feature turns off
the amplifiers’ bias circuitry, reducing the supply current. The trigger point is 0.45V (max) for a logic-low level,
and 1.2V (min) for logic-high level. The low 0.05µA (typ) shutdown current is achieved by applying a voltage that
is as near as ground a possible to the SD_LC/SD_RC pins. A voltage that is higher than ground may increase
the shutdown current.
There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw
switch, a microprocessor, or a microcontroller. When using a switch, connect an external 100kΩ pull-up resistor
between the SD_LC/SD_RC pins and V
DD
. Connect the switch between the SD_LC/SD_RC pins and ground.
Select normal amplifier operation by opening the switch. Closing the switch connects the SD_LC/SD_RC pins to
ground, activating micro-power shutdown. The switch and resistor ensure that the SD_LC/SD_RC pins will not
float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digital
output to apply the control voltage to the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins with active
circuitry eliminates the pull-up resistor.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM48820's performance requires properly selecting external components. Though the LM48820
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
Charge Pump Capacitor Selection
Use low (<100mΩ) ESR (equivalent series resistance) ceramic capacitors with an X7R dielectric for best
performance. Low ESR capacitors keep the charge pump output impedance to a minimum, extending the
headroom on the negative supply. Higher ESR capacitors result in reduced output power from the audio
amplifiers.
Charge pump load regulation and output impedance are affected by the value of the flying capacitor (C
C
). A
larger valued C
C
(up to 3.3μF) improves load regulation and minimizes charge pump output resistance. The
switch-on resistance dominates the output impedance for capacitor values above 2.2μF.
The output ripple is affected by the value and ESR of the output capacitor (C
SS
). Larger capacitors reduce output
ripple on the negative power supply. Lower ESR capacitors minimize the output ripple and reduce the output
impedance of the charge pump.
The LM48820 charge pump design is optimized for 2.2μF, low ESR, ceramic, flying, and output capacitors.
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