EVAL-AD1990EB【PDF 13/16 页】Audio Switching Amplifier

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AD1990

POWER STAGE

The H-Bridge

The output stage of the AD1990 includes four integrated

MOSFET devices arranged in a full H-bridge, as shown in

Figure 23. The P-Type, high-side transistor of one leg and the

N-Type, low-side transistor of the opposite leg switch on and off

as a pair producing a total voltage swing across the load of

to +PV

. The drive is floating and differential, and it is

important that neither output terminal be shorted to ground.

Rev. 0 | Page 13 of 16

The power supply for the output stage of the AD1990, PV

should be in the 8 V to 20 V range and should be capable of

supplying enough current to drive the load. Connect the power

supply across the PVDD and PGND pins. The feedback pins,

NFR+, NFR, NFL+, and NFL, supply negative feedback to the

modulator as described in the Setting the Modulator Gain section.

For reactive loads, the impedance can only be below the

recommended threshold over a small portion of the amplifier’s

bandwidth. In these cases, the amplifier can enter overcurrent

shutdown in response to even small input signals in those

frequency bands. When designing a system, use the minimum

load impedance over the entire range of amplified frequencies

when calculating current output rather than the average or

nominal load impedance ratings often cited by loudspeaker

driver manufacturers.

Output Transistor Nonoverlap Time

The AD1990 allows the user to select from one of eight different

nonoverlap times, as shown in Figure 24. Nonoverlap time

prevents or minimizes the period during which both the high-

side and low-side devices are on simultaneously due to propagation

delays and nonzero rise and fall times. If both the upper and

lower portions of a half-bridge conduct simultaneously, there is a

path directly from the power supply to ground and an induced

current flow known as shoot-through. However, introducing

this delay increases distortion by pushing the switching pattern

further from an ideal two-state waveform. Selecting the

nonoverlap delay requires a compromise between distortion

and efficiency. The logic levels on the three delay control pins,

DCTRL2, DCTRL1, and DCTRL0, set the nonoverlap time

according to Table 12. The state of DCTRL[2:0] is read on the

rising edge of RESET and should not be changed while RESET

is logic high.

Table 12. Nonoverlap Time Settings

DCTRL2

DCTRL1

DCTRL0

Nonoverlap Time (ns)

13.5

Values are typical and are not production tested.

HIGH-SIDE

GATE DRIVE

LOW-SIDE

GATE DRIVE

NOL

Figure 24. Half-Bridge Nonoverlap Delay Timing

The shortest setting (DCTRL[2:0] = 111) or the second shortest

setting (DCTRL[2:0] = 111) is recommended for most applications.

These two settings allow a small trade-off between efficiency

and distortion. Longer nonoverlap times generally increase

distortion while providing little or no decrease in shoot-

through current.

CLOCKING

The AD1990 Σ-Δ modulator requires an external clock source

with a nominal frequency of 12.288 MHz. This clock can come

from a crystal or from an existing clock signal in the application

circuit. The discrete time portions of the modulator run internally

at 6.144 MHz, corresponding to 128 × f

, where f

= 48 kHz.

As mentioned in the Σ-Δ Modulator section, the modulator has

a noise-shaping effect such that SNR is increased within the

audio band by shifting modulator quantization noise upward in

frequency. For an external clock frequency of 12.288 MHz, the

modulator’s noise-shaping works in a manner that results in a

flat noise floor at the amplifier output for frequencies 20 kHz

and below. Above 20 kHz, the amplifier noise rises due to the

spectral shaping of the modulator quantization noise. At very

high frequencies, the noise floor levels off and decreases due to

poles in the modulator noise-transfer function and in the

external LC filter.

The clock frequency does not have to be exactly equal to

12.288 kHz and can vary by up to ±10%. For other rates, the

noise corner scales linearly with frequency. When the modulator

runs at a rate lower than nominal, the average power stage

switching frequency decreases, the efficiency increases slightly,

and the noise floor begins to rise at a slightly lower frequency.

Likewise, a faster clock gives slightly increased bandwidth and

slightly lower efficiency.

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