LP3921 Datasheet, PDF (27/34 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

dissipated in the trace and not in the load as desired. This
problem of decreased load dissipation is exacerbated as load
impedance decreases. Therefore, to maintain the highest
load dissipation and widest output voltage swing, PCB traces
that connect the output pins to a load must be as wide as
possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply's output voltage de-
creases with increasing load current. Reduced supply voltage
causes decreased headroom, output signal clipping, and re-
duced output power. Even with tightly regulated supplies,
trace resistance creates the same effects as poor supply reg-
ulation. Therefore, making the power supply traces as wide
as possible helps maintain full output voltage swing.
POWER DISSIPATION
Power dissipation might be a major concern when designing
a successful amplifier, whether the amplifier is bridged or sin-
gle-ended. Equation 2 states the maximum power dissipation
point for a single-ended amplifier operating at a given supply
voltage and driving a specified output load.
P_DMAX = (V_DD)² / (2π²R_L) Single-Ended
(2)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation versus a single-ended amplifier op-
erating at the same conditions.
P_DMAX = 4 * (V_DD)² / (2π²R_L) Bridge Mode
(3)
Since the LP3921 has bridged outputs, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
Even with this substantial increase in power dissipation, the
LP3921 does not require additional heat sinking under most
operating conditions and output loading. From Equation 3,
assuming a 5V power supply and an 8Ω load, the maximum
power dissipation contribution from the audio amplifier is 625
mW. To this must be added the power dissipated from the
power management blocks. The maximum power dissipation
thus obtained (P_TOT) must not be greater than the power dis-
sipation results from Equation 4:
P_TOT = P_PDMU + P_DMAX = (T_JMAX - T_A) / θ_JA
(4)
possible. A larger half-supply bypass capacitor improves
PSRR because it increases half-supply stability. Typical ap-
plications employ a 5V regulator with 10 µF and 0.1 µF bypass
capacitors that increase supply stability. This, however, does
not eliminate the need for bypassing the supply nodes of the
LP3921. The LP3921 will operate without the bypass capac-
itor C_B, although the PSRR may decrease. A 1 µF capacitor
is recommended for C_B. This value maximizes PSRR perfor-
mance. Lesser values may be used, but PSRR decreases at
frequencies below 1 kHz. The issue of C_B selection is thus
dependant upon desired PSRR and click and pop perfor-
mance as explained in the section Proper Selection of Ex-
ternal Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
audio amplifier can be shut down by setting amp_en to 0 in
the Audio_Amp register. On power-up, the audio amplifier is
in shut down until enabled. (Contact NSC sales for a different
option.) (See Table 22.)
Thermal shutdown of the PMU will shut down the audio am-
plifier. (See Thermal Shutdown for recovery options.) Inde-
pendent temperature sensing within the audio amplifier may
also shut down the audio amplifier alone, without affecting
PMU control logic.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using
integrated power amplifiers is critical when optimizing device
and system performance. Although the LP3921 is tolerant to
a variety of external component combinations, consideration
of component values must be made when maximizing overall
system quality.
The LP3921 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LP3921 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. In-
put signals equal to or greater than 1 Vrms are available from
sources such as audio codecs. Please refer to the Audio
Power Amplifier Design section for a more complete expla-
nation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LP3921
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V_DD. Exact al-
lowable input common-mode voltage levels are actually a
function of V_DD, R_i, and R_f and may be determined by Equa-
tion 5:
P_DPMU is mainly the sum of power dissipated in the charger
and LDO blocks as shown in Equation 5:
P_DPMU = I_CHG (V_{CHG_IN} − V_BATT) + (I_OUT1 (V_BATT −
V_OUT1) + (I_OUT2 (V_BATT − V_OUT2) + (I_OUT3 (V_BATT −
V_OUT3) + ... (approx.)
(5)
The LP3921's θ_JA in an SQA32A package is 30°C/W. De-
pending on the ambient temperature, T_A, of the system sur-
roundings, Equation 4 can be used to find the maximum
internal power dissipation supported by the IC packaging.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion ratio (PSRR). The capacitor location on both the bypass
and power supply pins should be as close to the device as
V_CMi < (V_DD-1.2)*((R_f+(R_i)/(R_f)-V_DD*(R_i / 2R_f)
(6)
-R_F / R_I = A_VD
(7)
Special care must be taken to match the values of the feed-
back resistors (R_F1 and R_F2) to each other as well as matching
the input resistors (R_i1 and R_i2) to each other (see Figure 9)
more in front. Because of the balanced nature of differential
amplifiers, resistor matching differences can result in net DC
currents across the load. This DC current can increase power
consumption, internal IC power dissipation, reduce PSRR,
and possibly damaging the loudspeaker. Table 23 demon-
strates this problem by showing the effects of differing values
between the feedback resistors while assuming that the input
resistors are perfectly matched. The results below apply to
the application circuit shown in Figure 9, and assumes that
27
www.national.com