MP7722DF-LF-Z Datasheet MP7722DF-LF-Z | P7-P9

MP7722 – 2 x 20W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

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APPLICATION INFORMATION

COMPONENT SELECTION

The MP7722 uses a minimum number of

external components to complete a stereo

Class D audio amplifier. The circuit of Figure 1

is optimized for a 24V power supply and a 1.5V

RMS maximum input signal. This circuit should

be suitable for most applications. However, if

this circuit is not suitable, use the following

sections to determine how to customize the

amplifier for a particular application.

Setting the Voltage Gain

The maximum output voltage swing is limited by

the power supply. To achieve the maximum

power out of the MP7722 amplifier, the gain

resistors should be set such that the maximum

input signal results in an output voltage swing

that reaches the supply limit (clipping). The

maximum output voltage swing at clipping is

approximately ±V

/2. To achieve clipping for a

given input signal voltage, where V

IN(pk)

is the

peak input voltage, the voltage gain is:

)pk(V2

)MAX(A

The voltage gain setting results in the peak

output voltage approaching its maximum for the

maximum input signal. There are applications

where it is desirable to allow the amplifier to

overdrive slightly, allowing the THD to increase

at higher power levels (as the output signal

continues to go further into clipping), and so a

higher gain than A

V (max)

is required.

Setting the Switching Frequency

The idle switching frequency (the switching

frequency when no audio input is present) is a

function of several variables: the supply voltage

, the timing capacitor C

INT

and the feedback

resistor R

. For the MP7722, the idle switching

frequencies for CH1 and CH2 are independent

of each other and are a function of their own

associated components. The proper setting of

the “idle frequency” is important for obtaining

optimum performance. If the frequency is set

too high, the result will be more power loss and

high distortion, while setting the idle switching

frequency too low results in more inductor ripple,

causing more output voltage ripple with

increased the output noise.

The optimum quiescent switching frequency is

approximately 700KHz to 800KHz. Refer to

Table 1 for recommended values.

Table 1—Switching Frequency vs. V

, Timing

Capacitor and Feedback Resistor (see Figure 1)

Gain

(V/V)

Gain

(dB)

(kΩ)

INT

(V)

3.9 11.8 39 10 6.8nF 660KHz 12

8.2 18.3 82 10 3.3nF 660KHz 12

8.3 18.4 39 4.7 6.8nF 660KHz 12

17.4 24.8 82 4.7 3.3nF 660KHz 12

5.6 15.0 56 10 8.2nF 670KHz 24

8.2 18.3 82 10 5.6nF 720KHz 24

11.9 21.5 56 4.7 8.2nF 670KHz 24

17.4 24.8 82 4.7 5.6nF 720KHz 24

33.0 30.4 330 10 1.8nF 700KHz 24

Choosing the LC Filter

The Inductor-Capacitor (LC) filter converts the

pulse train at SW to the output voltage that

drives the speaker. Typical values for the LC

filter are a 10µH inductor and a 0.47µF

capacitor.

The characteristic frequency of the LC filter

needs to be high enough to allow high

frequency audio to the output, yet needs to be

low enough to filter out high frequency products

of the pulses from SW. The characteristic

frequency of the LC filter is:

LC2

The voltage ripple at the output is approximated

by the equation:

⎟

⎠

⎞

⎜

⎝

⎛

×≅

DDRIPPLE

MP7722 – 2 x 20W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

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The quality factor (Q) of the LC filter is important.

If this is too low output noise will increase, and if

this is too high then peaking may occur at high

signal frequencies reducing the passband

flatness. The circuit Q is set by the load

resistance (speaker resistance, typically 4 or

8) and is calculated as:

××π=×ω=

Where ω

is the characteristic frequency in

radians per second and f

is in Hz. Use a LC

filter with Q between 0.7 and 2.

The actual output ripple and noise is greatly

affected by the type of inductor and capacitor

used in the LC filter. Use a film capacitor and an

inductor with sufficient power handling capability

to supply the output current to the load. The

inductor should exhibit soft saturation

characteristics. If the inductor exhibits hard

saturation, it should operate well below the

saturation current. Gapped ferrite, MPP,

Powdered Iron or similar type toroidal cores are

recommended. If open or shielded bobbin ferrite

cores are used for multi-channel designs, make

sure that the start windings of each inductor line

up (all starts going toward SW pin or all starts

going toward the output) to prevent crosstalk or

other channel-to-channel interference.

Output Coupling Capacitors

The output AC coupling capacitors C

OUT1

and

OUT2

serve to block DC voltages and thus pass

only the amplified AC signal from the LC filter to

the load. The combination of the coupling

capacitor and the load resistance results in a

first-order high-pass filter. The values of C

OUT1

and C

OUT2

should be selected such that the

required minimum frequency is still allowed to

pass. The output corner frequency (-3dB point),

OUT

, can be calculated as:

OUTLOAD

OUT

CR2

××π×

Set the output corner frequency at or below the

minimum required frequency.

The output coupling capacitor carries the full

load current, so the capacitor should be chosen

such that its ripple current rating is greater than

the maximum load current. Low ESR aluminum

electrolytic capacitors are recommended.

Input Coupling Capacitors

The input coupling capacitors C

IN1

and C

IN2

are

used to pass only the AC signal at the input. In

a typical system application, the source input

signal is typically centered around the circuit

ground, while the MP7722 input is at half the

power supply voltage (V

/2). The input

coupling capacitors transmit the AC signal from

the source to the MP7722 while blocking the

DC voltage. Choose the input coupling

capacitors such that the corner frequency (f

) is

less than the passband frequency. The corner

frequency is calculated as:

ININ

CR2

××π×

Power Source

For maximum output power, the amplifier circuit

requires a regulated external power source to

supply the power to the amplifier. A higher

power supply voltage allows more power to be

delivered to a given load resistance. However if

the power source voltage exceeds the

maximum operating voltage of 24V, the

MP7722 may sustain permanent damage. It is

very important to bypass the power supply pins

with 1µF X7R ceramic capacitors.

Power Supply Pumping

It is also very important to bypass the power

supply with a large aluminum electrolytic

capacitor. It is recommended to use a value of

at least 2200µF. This is necessary to prevent

the supply voltage from getting pumped up to a

level that exceeds the absolute maximum rating.

Supply pumping occurs in single-ended Class D

amplifiers, and is caused by rapid switch

transitions where current is pumped back up

into the supply line. The large capacitor is

necessary to absorb this current and prevent

from rising too high.

MP7722 – 2 x 20W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

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PCB Layout

The circuit layout is critical for optimum

performance and low output distortion and

noise. Place the following components as close

to the MP7722 as possible:

Power Supply Bypass, C

BYP

BYP1

and C

BYP2

carry the transient current for

the switching power stage. To prevent

overstressing of the MP7722 and excessive

noise at the output, place C

BYP1

as close to pins

18 (VDD1) and 20 (PGND1) as possible and

also place C

BYP2

as close to pins 13 (VDD2) and

15 (PGND2) as possible.

Output Catch Diodes

SH1

, D

SL1

, D

SH2

and D

SL2

carry the current over

the dead-time while the MOSFET switches are

off. Place the diodes as close to the MP7722 as

possible.

Timing Capacitors

INT1

and C

INT2

are used to set the amplifier

switching frequencies and are typically on the

order of a few nF. Place C

INT1

as close to pins 2

and 3 as possible to reduce distortion and noise.

Likewise, place C

INT2

as close to pins 7 and 8

as possible.

Reference Bypass Capacitors

and C

filter the ½ V

reference voltages.

Place C

and C

as close to the IC as

possible to improve power supply rejection and

reduce distortion and noise at the output.

When laying out the PCB, use two separate

ground planes, analog ground (AGND) and

power ground (PGND), and connect the two

grounds together at a single point to prevent

noise injection into the amplifier input to reduce

distortion.

Make sure that any traces carrying the switch

node (SW) voltages are separated far from any

input signal traces. If it is required to run the

SW trace near the input, shield the input with a

ground plane between the traces. For multiple

channel applications, make sure that each

channel is physically separated to prevent

crosstalk. Make sure that all inductors used on

a single circuit board have the same orientation.

Also, make sure that the power supply is routed

from the source to each channel individually,

not serially. This prevents channel-to-channel

coupling through the power supply input.

High V

Operation

When operating at higher supply voltages,

special care must be taken to ensure that the

level does not exceed the absolute

maximum supply rating of the IC. Power supply

pumping is of significant concern when operating

near the maximum supply voltage. Supply

pumping is an effect where the V

voltage is

“pumped up” to a higher potential when charge

from the output DC blocking capacitor is

transferred to the power supply rail during switch

transitions. The simplest way to handle excess

pumping is to increase the size of the V

main

bulk capacitance such that the extra charge will

be absorbed by the increased capacitance, with

minimal supply increase. One way to eliminate

supply pumping altogether is to use a different

output configuration circuit. Figure 2 shows such

an alternate configuration for connecting the

speaker load. With this configuration, one side of

the speaker load is connected directly to the

output of the LC filter, while the other side is

connected to the mid-point of a series capacitor-

divider (C26, C28). Both the LC filter point and

the mid point of the capacitor divider will be at a

DC bias level of ½ V

, so the net DC across the

speaker is 0V

. With the speaker connected in

this fashion, there is no series capacitor to cause

supply pumping, and supply pumping is virtually

eliminated. If the output is connected in this way,

however, additional circuitry may be required to

protect the speaker from damage in the event of

a short circuit. Because both sides of the

speaker will be typically biased at ½ V

, a short-

circuit to GND on the negative side of the

speaker load will result in a large DC current

through the load. For example, if V

=24V and

=4Ω, there will be 12/4=3A of DC current

through the load. This current will be sustained

by the output FET stage of the IC as it will not

trigger the internal over-current protection sense

circuitry. A simple external sense circuit will be

required for those applications which may

experience an externally applied short circuit

under normal use. An example of such a circuit

is also shown in Figure 2.

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MP7722DF-LF-Z

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Description:

Audio Amplifiers 2x20W Class-D Single Ended Stereo Amp

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MP7722DF-LF-Z