LM4665 datasheet(10/18 Pages) NSC

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Application Information (Continued)

the output pins to the load and the supply pins to the power

supply should be as wide as possible to minimize trace

resistance.

The rising and falling edges are necessarily short in relation

to the minimum pulse width (160ns), having approximately

2ns rise and fall times, typical, depending on parasitic output

capacitance. The inductive nature of the transducer load can

also result in overshoot on one or both edges, clamped by

the parasitic diodes to GND and V

DD in each case. From an

EMI standpoint, this is an aggressive waveform that can

radiate or conduct to other components in the system and

cause interference. It is essential to keep the power and

output traces short and well shielded if possible. Use of

ground planes, beads, and micro-strip layout techniques are

all useful in preventing unwanted interference.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor (C

S) location should be

as close as possible to the LM4665. Typical applications

employ a voltage regulator with a 10µF and a 0.1µF bypass

capacitors that increase supply stability. These capacitors do

not eliminate the need for bypassing on the supply pin of the

LM4665. A 1µF tantalum capacitor is recommended.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4665 contains shutdown circuitry that reduces current

draw to less than 0.01µA. In addition, the LM4665 contains a

Shutdown Mode pin allowing the designer to designate

whether the shutdown circuitry is activated by either a High

level logic signal or a Low level logic signal. The Shutdown

Mode pin should be permanently connected to either GND

(Low) or V

DD (High). The LM4665 may then be placed into

shutdown by toggling the Shutdown pin to the same state as

the Shutdown Mode pin. For simplicity’s sake, this is called

"Shutdown same", as the LM4665 enters into a shutdown

state whenever the two pins are in the same logic state. The

trigger point for either shutdown high or shutdown low is

shown as a typical value in the Electrical Characteristics

Tables and in the Shutdown Hysteresis Voltage graphs

found in the Typical Performance Characteristics section.

It is best to switch between ground and supply for minimum

current usage while in the shutdown state. While the

LM4665 may be disabled with shutdown voltages in between

ground and supply, the idle current will be greater than the

typical 0.01µA value. Increased THD may also be observed

with voltages greater than GND and less than V

DD on the

Shutdown pin when in PLAY mode.

The LM4665 has an internal resistor connected between the

Shutdown Mode and Shutdown pins. The purpose of this

resistor is to eliminate any unwanted state changes when

the Shutdown pin is floating, as long as the Shutdown Mode

pin is connected to GND or V

DD. When the Shutdown Mode

pin is properly connected, the LM4665 will enter the shut-

down state when the Shutdown pin is left floating or if not

floating, when the shutdown voltage has crossed the corre-

sponding threshold for the logic level assigned by the Shut-

down Mode pin voltage. To minimize the supply current while

in the shutdown state, the Shutdown pin should be driven to

the same potential as the Shutdown Mode pin or left floating.

The amount of additional current due to the internal shut-

down resistor can be found by Equation (1) below.

SD MODE -VSD) / 60k

Ω

(1)

With only a 0.5V difference between the Shutdown Mode

voltage and the Shutdown voltage an additional 8.3µA of

current will be drawn while in the shutdown state.

GAIN SELECTION FUNCTION

The LM4665 has fixed selectable gain to minimize external

components, increase flexibility and simplify design. For a

differential gain of 6dB (2V/V), the Gain Select pin should be

permanently connected to V

DD or driven to a logic high level.

For a differential gain of 12dB (4V/V), the Gain Select pin

should be permanently connected to GND or driven to a

logic low level. The gain of the LM4665 can be switched

while the amplifier is in PLAY mode driving a load with a

signal without damage to the IC. The voltage on the Gain

Select pin should be switched quickly between GND (logic

low) and V

DD (logic high) to eliminate any possible audible

artifacts from appearing at the output. For typical threshold

voltages for the Gain Select function, refer to the Gain

Threshold Voltages graph in the Typical Performance

Characteristics section.

INCREASING SUPPLY VOLTAGE RANGE

When using the micro SMD package (ITL), the operating

supply voltage range is 2.7V - 3.8V with an 8

Ω speaker load.

To increase the operating supply voltage range, four Schot-

tky diodes (D

1 -D4) can be used to control the over and

undershoot of the output pulse waveform (See Figure 2

below). To reduce THD+N, small value capacitors in the

range of 10pF - 33pF (C

N1 &CN2) can also be added as

needed. The diodes should be placed as close to the micro

SMD package as possible.

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