NCP2821

http://onsemi.com

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DESCRIPTION INFORMATION

Detailed Description

The basic structure of the NCP2821 is composed of one

analog pre−amplifier, a pulse width modulator and an

H−bridge CMOS power stage. The first stage is externally

the ratios of these resistors) and the other stage is fixed. The

load is driven differentially through two output stages.

The differential PWM output signal is a digital image of

the analog audio input signal. The human ear is a band pass

filter regarding acoustic waveforms, the typical values of

which are 20 Hz and 20 kHz. Thus, the user will hear only

the amplified audio input signal within the frequency range.

The switching frequency and its harmonics are fully filtered.

The inductive parasitic element of the loudspeaker helps to

guarantee a superior distortion value.

Power Amplifier

The output PMOS and NMOS transistors of the amplifier

have been designed to deliver the output power of the

specifications without clipping. The channel resistance

Turn On and Turn Off Transitions

In order to eliminate “pop and click” noises during

transition, the output power in the load must not be

established or cutoff suddenly. When a logic high is applied

to the shutdown pin, the internal biasing voltage rises

quickly and, 4 ms later, once the output DC level is around

the common mode voltage, the gain is established slowly

(5.0 ms). This method to turn on the device is optimized in

terms of rejection of “pop and click” noises. Thus, the total

turn on time to get full power to the load is 9 ms (typical)

(see Figure 35).

The device has the same behavior when it is turned−off by

a logic low on the shutdown pin. No power is delivered to the

load 5 ms after a falling edge on the shutdown pin

(see Figure 36). Due to the fast turn on and off times, the

shutdown signal can be used as a mute signal as well.

Shutdown Function

The device enters shutdown mode when the shutdown

signal is low. During the shutdown mode, the DC quiescent

current of the circuit does not exceed 1.5

mA.

Current Breaker Circuit

The maximum output power of the circuit corresponds to

an average current in the load of 820 mA.

In order to limit the excessive power dissipation in the

load if a short−circuit occurs, a current breaker cell shuts

down the output stage. The current in the four output MOS

transistors are real−time controlled, and if one current

exceeds the threshold set to 1.5 A, the MOS transistor is

opened and the current is reduced to zero. As soon as the

short−circuit is removed, the circuit is able to deliver the

expected output power.

This patented structure protects the NCP2821. Since it

completely turns off the load, it minimizes the risk of the

chip overheating which could occur if a soft current limiting

circuit was used.