LM4920TLBD National Semiconductor, LM4920TLBD Datasheet - Page 14

• Manufacturer Part Number: LM4920TLBD
• Description: BOARD EVALUATION LM4920TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4920TLBD

• Amplifier Type: Class AB
• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 80mW x 2 @ 16 Ohm
• Voltage - Supply: 1.6 V ~ 4.2 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4920
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
SD_LC/SD_RC pins and V
the SD_LC/SD_RC pins and ground. Select normal amplifier
operation by opening the switch. Closing the switch con-
nects the SD_LC/SD_RC pins to ground, activating micro-
power shutdown. The switch and resistor guarantee that the
SD_LC/SD_RC pins will not float. This prevents unwanted
state changes. In a system with a microprocessor or micro-
controller, 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 LM4920’s performance requires properly se-
lecting external components. Though the LM4920 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
Charge Pump Capacitor Selection
Use low ESR (equivalent series resistance) (
ramic capacitors with an X7R dielectric for best perfor-
mance. 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 (C1). A larger
valued C1 (up to 3.3uF) improves load regulation and mini-
mizes charge pump output resistance. Beyond 3.3uF, the
switch-on resistance dominates the output impedance for
capacitor values above 2.2uF.
The output ripple is affected by the value and ESR of the
output capacitor (C2). Larger capacitors reduce output ripple
on the negative power supply. Lower ESR capacitors mini-
mize the output ripple and reduce the output impedance of
the charge pump.
DD
. Connect the switch between
(Continued)
<
100mΩ) ce-
14
The LM4920 charge pump design is optimized for 2.2uF, low
ESR, ceramic, flying, and output capacitors.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
pacitor can be expensive and may compromise space effi-
ciency in portable designs. In many cases, however, the
speakers used in portable systems, whether internal or ex-
ternal, have little ability to reproduce signals below 150Hz.
Applications using speakers with this limited frequency re-
sponse reap little improvement by using high value input and
output capacitors.
Besides affecting system cost and size, C
the LM4920’s click and pop performance. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Thus, pops can be minimized by selecting an input capacitor
value that is no higher than necessary to meet the desired
−3dB frequency.
As shown in Figure 1, the internal input resistor, R
input capacitor, C
frequency that is found using Equation (3). Conventional
headphone amplifiers require output capacitors; Equation (3)
can be used, along with the value of R
wards the value of output capacitor needed to produce a
–3dB high pass filter cutoff frequency.
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance. (See the section entitled Charge Pump
Capacitor Selection.)
i
, produce a -3dB high pass filter cutoff
f
i-3dB
= 1 / 2πR
i
in Figure 1). A high value ca-
i
C
i
L
, to determine to-
i
has an effect on
i
and the
(3)