MP7751 – 26V, 2X20W STEREO BTL FIX FREQUENCY CLASS D AUDIO AMPLIFIER

MP7751 Rev. 1.0

www.MonolithicPower.com

14

5/23/2014

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Figure 1b: enable on with audio signal present

Adjustable Power Limit

Power limit and anti-clipping feature control. The

voltage at pin 10 can used to limit the power to

levels below that which is possible based on the

supply rail.

Connect a resistor divider from AVCC to GND to

set power limit. Connect directly to AVCC for no

power limit.

An external reference may also be used if tighter

tolerance is required. Also add a 1μF capacitor

from pin 10 to ground.

Figure 2. PLIMT Circuit Operation)

The PLIMIT circuit sets a limit on the output

peak-to-peak voltage. The limiting is done by

limiting the duty cycle to fixed maximum value.

This limit can be thought of as a "virtual" voltage

rail which is lower than the supply connected to

PVCC. This "virtual" rail is 8 times the voltage at

the PLIMIT pin. This output voltage can be used

to calculate the maximum output power for a

given maximum input voltage and speaker

impedance.

2

L

P

LS

OUT

L

R

V

R2 R

P

2R

⎛⎞

⎛⎞

×

⎜⎟

⎜⎟

+×

⎝⎠

⎝⎠

=

×

for unclipped power

Where:

Rdson and the resistance of output filter.

And

OUT

OUT

P

@10%THD 1.25 unclipped P

≈

×

Table 1: Power Limit Typical Operation

Test

Condition

PLIMIT

Voltage

Output

Voltage

(Vp-p)

Output Power

(~10% THD)

VDD=12V,

Vin=1Vrms,

Load=8Ω,

Gain=20dB

0.80V

14.9

5.0W

VDD=18V,

Vin=1Vrms,

Load=8Ω,

Gain=20dB

0.80V

14.9

5.0W

VDD=18V,

Vin=1Vrms,

Load=8Ω,

Gain=20dB

1.44V

23.6

10.0W

VDD=24V,

Vin=1Vrms,

Load=8Ω,

Gain=20dB

1.44V

23.6

10.0W

Gain Setting

The gain of the MP7751 is set by the input

terminals, GAIN0 and GAIN1. The actual gain

settings are controlled by ratios of the internal

input and feedback resistors, details please see

the below table.