LP3921

SNVS580A – AUGUST 2008 – REVISED MAY 2013

www.ti.com

PDPMU is mainly the sum of power dissipated in the charger and LDO blocks as shown in Equation 5:

PDPMU = ICHG (VCHG_IN − VBATT) + (IOUT1 (VBATT − VOUT1) + (IOUT2 (VBATT − VOUT2) + (IOUT3 (VBATT − VOUT3) + ... (approx.) (5)

The LP3921's

θJA in an RTV0032A package is 30°C/W. Depending on the ambient temperature, TA, of the

system surroundings, 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 critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor location on both the bypass and power supply pins should be as close to

the device as possible. A larger half-supply bypass capacitor improves PSRR because it increases half-supply

stability. Typical applications 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 capacitor CB, although the PSRR may decrease. A 1 µF capacitor is

recommended for CB. This value maximizes PSRR performance. Lesser values may be used, but PSRR

decreases at frequencies below 1 kHz. The issue of CB selection is thus dependant upon desired PSRR and

click and pop performance as explained in PROPER SELECTION OF EXTERNAL 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 amplifier. (See Thermal Shutdown for recovery options.)

Independent 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 maximum 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. Input signals equal to or greater than 1

Vrms are available from sources such as audio codecs. Please refer to AUDIO POWER AMPLIFIER DESIGN for

a more complete explanation 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 VDD. Exact allowable input common-mode voltage levels are actually a function of VDD, Ri,

and Rf and may be determined by Equation 5:

VCMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri / 2Rf)

(6)

-RF / RI = AVD

(7)

Special care must be taken to match the values of the feedback resistors (RF1 and RF2) to each other as well as

matching the input resistors (Ri1 and Ri2) to each other (see Figure 10) 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 demonstrates 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 10, and assumes that VDD = 5V, RL = 8Ω, and the system has DC

coupled inputs tied to ground.

30

Submit Documentation Feedback

Copyright © 2008–2013, Texas Instruments Incorporated

Product Folder Links: LP3921