LM4651 datasheet(11/20 Pages) NSC

11 / 20 page

Application Information (Continued)

reduced as the desired bandwidth of operation increases.

The dead time can be adjusted with the R

DLY resistor by

Equation (4):

DLY = 1.7x10

−12 (500 + R

DLY)

(Seconds)

(4)

Currently, the recommended value is 5k

Ω.

Oscillator Control: The modulation frequency is set by an

external resistor, R

OSC,

connected between pin 16 and

GND. The modulation frequency can be set within the range

of 50kHz to 225kHz according to the design requirements.

The values of R

OSC and fOSC can be found by Equation (5)

and (6):

OSC = 1x10

9/ (4000 + R

OSC)

(Hz)

(5)

OSC = (1x10

9/f

OSC) − 4000

(

Ω)

(6)

Equations (5) and (6) are for R

DLY = 0. Using a value of RDLY

greater than zero will increase the value needed for R

OSC.

For R

DLY =5kΩ,ROSC will need to be increased by about

Ω. As the graphs show, increasing the switching frequency

will reduce the THD but also decreases the efficiency and

maximum output power level before clipping. Increasing the

switching frequency increases the amount of loss because

switching currents lower the efficiency across the output

power range. A higher switching frequency also lowers the

maximum output power before clipping or the 1% THD point

occur.

Over-Modulation Protection: The over-modulation protec-

tion is an internally generated fixed pulse width signal that

prevents any side of the H-bridge power MOSFETs from

remaining active for an extended period of time. This condi-

tion can result when the input signal amplitude is higher than

the internal triangle waveform. Lack of an over modulation

signal can increase distortion when the amplifier’s output is

clipping. Figure 4 shows how the over modulation protection

works.

The over modulation protection also provides a ’soft clip’

type response on the top of a sine wave. This minimum

pulse time is internally set and cannot be adjusted. As the

switching frequency increases this minimum time becomes a

higher percentage of the period (T

PERIOD = 1/fSW). Because

the over modulation protection time is a higher percentage of

the period, the peak output voltage is lower and, therefore,

the output power at clipping is lower for the same given

supply rails and load.

Feedback Amplifier and Filter: The purpose of the feed-

back amplifier is to differentially sample the output and pro-

vide a single-ended feedback signal to the error amplifier to

close the feedback loop. The feedback is taken directly from

the switching output before the demodulating LC filter to

avoid the phase shift caused by the output filter. The signal

fed back is first low pass filtered with a single pole or dual

pole RC filter to remove the switching frequency and its

harmonics. The differential signal, derived from the bridge

output, goes into the high input impedance instrumentation

amplifier that is used as the feedback amplifier. The instru-

mentation amplifier has an internally fixed gain of 1. The use

of an instrumentation amplifier serves two purposes. First,

it’s input are high impedance so it doesn’t load down the

output stage. Secondly, an IA has excellent common-mode

rejection when its gain setting resistors are properly

matched. This feature allows the IA to derive the true feed-

back signal from the differential output, which aids in improv-

ing the system performance.

Error Amplifier: The purpose of the error amplifier is to sum

the input audio signal with the feedback signal derived from

the output. This inverting amplifier’s gain is externally con-

figurable by resistors Rf and R1. The parallel feedback ca-

pacitor and resistor form a low pass filter that limits the

frequency content of the input audio signal and the feedback

signal. The pole of the filter is set by Equation (7).

IP = 1/(2πRfCf)

(Hz)

(7)

On-Board Regulators: The LM4651 has its own internal

supply regulators for both analog and digital circuits. Sepa-

rate ±6V regulators exist solely for the analog amplifiers,

oscillator and PWM comparators. A separate voltage regu-

lator powers the digital logic that controls the protection,

level shifting, and high−/low−side driver circuits. System per-

formance is enhanced by bypassing each regulator’s output.

The ±6V regulator outputs, labeled +6V

BYP (pin 6) and

−6V

BYP (pin 8) should be bypassed to ground. The digital

regulator output, −V

DDBYP (pins 20 & 21) should be by-

passed to −V

EE (pins 22 & 23). The voltage level of −VDDBYP

should be always be 6V closer to ground than the negative

rail, −V

EE. As an example, if −VEE = −20V, then −VDDBYP

should equal −14V. Recommended capacitor values and

type can be found in Figure 1, Typical audio Application

Circuit.

APPLICATIONS HINTS

DS101277-2

FIGURE 4. Over Modulation Protection

DS101277-3

FIGURE 5. Feedback instrumentation Amplifier

Schematic

LM4651 Datasheet(PDF) 11 Page - National Semiconductor (TI)

LM4651 Datasheet(HTML) 11 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL