MAX4295EEE datasheet(11/15 Pages) MAXIM

11 / 15 page

Mono, 2W, Switch-Mode (Class D)

Audio Power Amplifier

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Output Filter

An output filter is required to attenuate the PWM switch-

ing frequency. Without the filter, the ripple in the load

can substantially degrade efficiency and may cause

interference problems with other electronic equipment.

A Butterworth lowpass filter is chosen for its flat

passband and nice phase response, though other filter

implementations may also be used. Three examples

are presented below. The filter parameters for bal-

anced 2-pole (Figure 4b) and 4-pole (Figure 4d)

Butterworth filters are taken from Electronic Filter

Design Handbook by Arthur B. Williams, McGraw Hill,

Inc. These filter designs assume that the load is purely

resistive and load impedance is constant over frequen-

cy. Calculation of filter component values should

include the DC resistance of the inductors and take into

account the worst-case load scenario:

• Single Ended 2-Pole Filter (Figure 4a)

C = 1 / (

√2 ✕ RL ✕ ωo), L = √2 ✕ RL / ωo

where

ωo = 2 ✕ π ✕ fo (fo = filter cutoff frequency);

choosing fo = 30kHz and RL = 4

Ω, C = 0.937µF, L =

30µH.

A single-ended 2-pole filter uses the minimum number

of external components, but the load (speaker) sees

the large common-mode switching voltage, which can

increase power dissipation and cause EMI problems.

• Balanced 2-Pole (Figure 4b):

A balanced 2-pole filter does not have the common-

mode swing problem of the single-ended filter.

C = 2 / (

√2 ✕ RL ✕ ωo), L = (√2 ✕ RL)/(2 ✕ ωo); choosing

fo = 30kHz and RL = 4

Ω, C1a = C1b = 2.0µF, L1a =

L1b = 15µH.

A single capacitor connected across RL, with a value of

CL = 1/(

√2 ✕ RL ✕ ωo), can be used in place of C1a and

C1b. However, the configuration as shown gives an

improved rejection to common-mode signal compo-

nents of OUT+_ and OUT-_. If the single capacitor

scheme is used, additional capacitors (Ca and Cb) can

be added from each side of RL, providing a high-fre-

quency short to ground (Figure 4c). These capacitors

should be approximately 0.2 ✕ CL.

• Balanced 4-Pole Filter (Figure 4d)

A balanced 4-pole filter is more effective in suppress-

ing the switching frequency and its harmonics.

For the 4-pole Butterworth filter, the normalized values

are: L1N = 1.5307, L2N = 1.0824, C1N = 1.5772, C2N =

0.3827.

The actual inductance and capacitance values for fO =

30kHz and a bridge-tied load of RL = 4

Ω are given by:

L1 = (L1N ✕ RL ) / (2 ✕

ωo) = 16.24µH, L2 = (L2N ✕ RL) /

(2 ✕

ωo) = 11.5µH, C1 = C1N / (RL ✕ ωo) = 2.1µF, C2a =

C2b = (2 ✕ C2N) / (RL ✕

ωo) = 1.0µF.

OUT+

OUT-

Figure 4a. Single-Ended 2-Pole Filter

OUT+

OUT-

C1a

C1b

Figure 4b. Balanced 2-Pole Filter

OUT+

OUT-

Figure 4c. Alternate Balanced 2-Pole Filter

OUT+

OUT-

L2a

C2a

L2b

C2b

L1b

L1a

Figure 4d. Balanced 4-Pole Filter

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