APA2057A
ApplicationInformation
Amplifier Mode Operation
Headphone Mode Operation
The APA2057A has two pairs of operational amplifiers
internally, which allows different amplifier configurations.
HVDD
VOUT
HVDD/2
GND
-
Pre-amplifier
OUT+
+
Output signal
OP1
Conventional Headphone amplifier
Vbias
HVDD
GND
-
DIFF_AM P_CONFIG
VOUT
+
OUT-
OP2
Figure 1: APA2057A internal configuration
(each channel)
The OP1 and OP2 are all differential drive configurations.
The differential drive configurations doubling the voltage
swing on the load. Compare with the single-ending
configuration, the differential gain for each channel is 2X
(Gain of SE mode).
VSS
Cap-free Headphone amplifier
Figure 2: Cap-free Operation
The APA2057A’s headphone amplifiers uses a charge
pump to invert the positive power supply (CVDD) to negative
power supply (CVSS), see Figure2. The headphone am-
plifiers operate at this bipolar power supply (HVDD & VSS),
and the outputs reference refers to the ground. This fea-
ture eliminates the output capacitor that is using in con-
ventional single-ended headphone amplifier. The head-
phone amplifier internal supply voltage comes from HVDD
and VSS. For good AC performance, the HVDD connected to
3.3V is recommended. It can avoid the output over volt-
age for line out application.
By driving the load differentially through outputs OUT+
and OUT-, an amplifier configuration commonly referred
to all differential mode is established. All differential mode
operation is different from the classical single-ended SE
amplifier configuration where one side of its load is con-
nected to ground.
A differential amplifier design has a few distinct advan-
tages over the SE configuration, as it provides differential
drive to the load, thus it is doubling the output swing
for a specified supply voltage. The output power can be
4 times greater than the SE amplifier working under the
same condition. A differential configuration, similar as the
one used in APA2057A, also creates a second advantage
over SE amplifiers. Since the differential outputs, ROUT+,
ROUT-, LOUT+, and LOUT-, are biased at half-supply,
there is no need for DC voltage across the load. This
eliminates the need for an output coupling capacitor which
is required in a single supply, SE configuration.
Charge Pump Flying Capacitor
The flying capacitor (CCPF) affects the load transient of the
charge pump. If the capacitor’s value is too small, then
that will degrade the charge pump’s current driver capa-
bility and the performance of headphone amplifier.
Increasing the flying capacitor’s value will improve the
load transient of charge pump. It is recommend to use
the low ESR ceramic capacitors (X7R type is recommended)
above 1mf.
Copyright ã ANPEC Electronics Corp.
21
www.anpec.com.tw
Rev. A.1 - Aug., 2007
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
