LM4985 Stereo 135mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or
Capacitor-less (OCL) Output and Digitally Controlled (I
2
C) Volume Control
May 2006
LM4985
Stereo 135mW Low Noise Headphone Amplifier with
Selectable Capacitively Coupled or Output
Capacitor-less (OCL) Output and Digitally Controlled
(I
2
C) Volume Control
General Description
The LM4985 is a stereo audio power amplifier with internal
digitally controlled volume control. This amplifier is capable
of delivering 68mW
RMS
per channel into a 16鈩?load or
38mW
RMS
per channel into a 32鈩?load at 1% THD when
powered by a 3.6V power supply and operating in the OCL
mode.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. To that end, the LM4985 features two
functions that optimize system cost and minimize PCB area:
an integrated, digitally controlled (I
2
C bus) volume control
and an operational mode that eliminates output signal cou-
pling capacitors (OCL mode). Since the LM4985 does not
require bootstrap capacitors, snubber networks, or output
coupling capacitors, it is optimally suited for low-power, bat-
tery powered portable systems. For added design flexibility,
the LM4985 can also be configured for single-ended capaci-
tively coupled outputs.
The LM4985 features a current shutdown mode for mi-
cropower dissipation and thermal shutdown protection.
Key Specifications (V
DD
= 3.6V)
j
PSRR: 217Hz and 1kHz
Output Capacitor-less (OCL)
f
RIPPLE
= 217Hz
f
RIPPLE
= 1kHz
Capacitor Coupled (C-CUPL)
f
RIPPLE
= 217Hz
f
RIPPLE
= 1kHz
j
Output Power per channel
77dB (typ)
76dB (typ)
63dB (typ)
62dB (typ)
(f
IN
= 1kHz, THD+N = 1%),
R
L
= 16鈩?OCL
V
DD
= 2.5V
V
DD
= 3.6V
V
DD
= 5.0V
j
THD+N (f = 1kHz)
31mW (typ)
68mW (typ)
135mW (typ)
0.60
0.031
0.1碌A (typ)
R
LOAD
= 16鈩? OCL, P
OUT
= 60mW
R
LOAD
= 32鈩? OCL, P
OUT
= 33mW
j
Shutdown Current
Features
n
OCL or capacitively coupled outputs (patent pending)
n
I
2
C Digitally Controlled Volume Control
n
Available in space-saving 0.4mm lead-pitch micro SMD
package
n
Volume control range: 鈥?6dB to +18dB
n
Ultra low current shutdown mode
n
2.3V - 5.5V operation
n
Ultra low noise
Applications
n
n
n
n
Mobile Phones
PDAs
Portable electronics devices
MP3 Players
Boomer
庐
is a registered trademark of National Semiconductor Corporation.
漏 2006 National Semiconductor Corporation
DS201697
www.national.com