LM48823TL datasheet(13/20 Pages) NSC

13 / 20 page

SHUTDOWN FUNCTION

The LM48823 features a low-power shutdown mode that dis-

ables the device, lowering the quiescent current to 0.01µA.

Set bits B1 (ENABLE_A) and B2 (ENABLE_B) to 0 to disable

the amplifiers and charge pump. Set both ENABLE_A and

ENABLE_B to 1 for normal operation. Shutdown mode does

not clear the I2C register. When re-enabled, the device returns

to its previous volume setting. To clear the I2C register, either

remove power from the device, or toggle RESET (see RE-

SET section).

RESET

The LM48823 features an active low reset input. Driving RE-

SET low clears the I2C register. Volume control is set to 00000

(-70dB) and both ENABLE_A and ENABLE_B are set to 0,

disabling the device. While RESET is low, the LM48823 ig-

nores any I2C data. After the device is reset, and RESET is

driven high, the LM48823 remains in shutdown mode with the

volume set to -70dB. Re-enable the device by writing to the

I2C register.

PROPER SELECTION OF EXTERNAL COMPONENTS

Power Supply Bypassing/Filtering

Proper power supply bypassing is critical for low noise per-

formance and high PSRR. Place the supply bypass capaci-

tors as close to the device as possible. Place a 1µF ceramic

capacitor from V

DD to GND. Additional bulk capacitance may

be added as required.

Bypass Capacitor Selection

The BYPASS capacitor, C

BYPASS, improves PSRR, noise re-

jection and output offset. For best results, use a capacitor of

identical value to the input coupling capacitors

Charge Pump Capacitor Selection

Use low ESR ceramic capacitors (less than 100m

Ω) for opti-

mum performance.

Charge Pump Flying Capacitor (C1)

The flying capacitor (C1) affects the load regulation and out-

put impedance of the charge pump. A C1 value that is too low

results in a loss of current drive, leading to a loss of amplifier

headroom. A higher valued C1 improves load regulation and

lowers charge pump output impedance to an extent. Above

2.2µF, the R

DS(ON) of the charge pump switches and the ESR

of C1 and C2 dominate the output impedance. A lower value

capacitor can be used in systems with low maximum output

power requirements.

Charge Pump Hold Capacitor (C2)

The value and ESR of the hold capacitor (C2) directly affects

the ripple on CPV

SS. Increasing the value of C2 reduces out-

put ripple. Decreasing the ESR of C2 reduces both output

ripple and charge pump output impedance. A lower value ca-

pacitor can be used in systems with low maximum output

power requirements.

Input Capacitor Selection

Input capacitors block the DC component of the audio signal,

eliminating any conflict between the DC component of the

audio source and the bias voltage of the LM48823. The input

capacitors create a high-pass filter with the input resistors

IN. The -3dB point of the high pass filter is found using Equa-

tion (1) below.

f = 1 / 2

πR

INCIN

(Hz)

(1)

Where the value of R

IN is given in the Electrical Characteris-

tics Table.

High pass filtering the audio signal helps protect the speakers.

When the LM48823 is using a single-ended source, power

supply noise on the ground is seen as an input signal. Setting

the high-pass filter point above the power supply noise fre-

quencies, 217Hz in a GSM phone, for example, filters out the

noise such that it is not amplified and heard on the output.

Capacitors with a tolerance of 10% or better are recommend-

ed for impedance matching and improved CMRR and PSRR.

LM48823TL Datasheet(PDF) 13 Page - National Semiconductor (TI)

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