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LM4670SD bảng dữ liệu(PDF) 11 Page - Texas Instruments

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LM4670
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SNAS240C – DECEMBER 2004 – REVISED MAY 2013
The rising and falling edges are necessarily short in relation to the minimum pulse width (350ns), having
approximately 16ns 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 VDD 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.
As the distance from the LM4670 and the speaker increase, the amount of EMI radiation will increase since the
output wires or traces acting as antenna become more efficient with length. What is acceptable EMI is highly
application specific. Ferrite chip inductors placed close to the LM4670 may be needed to reduce EMI radiation.
The value of the ferrite chip is very application specific.
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 (CS) location should be as close as possible to the LM4670. 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 LM4670. A 1µF tantalum
capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4670 contains shutdown circuitry that reduces
current draw to less than 0.01µA. The trigger point for shutdown is shown as a typical value in Electrical
Characteristics and in the Shutdown Hysteresis Voltage graphs found in Typical Performance Characteristics. It
is best to switch between ground and supply for minimum current usage while in the shutdown state. While the
LM4670 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 less than VDD on the
Shutdown pin when in PLAY mode.
The LM4670 has an internal resistor connected between GND and Shutdown pins. The purpose of this resistor is
to eliminate any unwanted state changes when the Shutdown pin is floating. The LM4670 will enter the shutdown
state when the Shutdown pin is left floating or if not floating, when the shutdown voltage has crossed the
threshold. To minimize the supply current while in the shutdown state, the Shutdown pin should be driven to
GND or left floating. If the Shutdown pin is not driven to GND, the amount of additional resistor current due to the
internal shutdown resistor can be found by Equation 1.
(VSD - GND) / 300kΩ
(1)
With only a 0.5V difference, an additional 1.7µA of current will be drawn while in the shutdown state.
PROPER SELECTION OF EXTERNAL COMPONENTS
The gain of the LM4670 is set by the external resistors, Ri in Figure 1, The Gain is given by Equation 2. Best
THD+N performance is achieved with a gain of 2V/V (6dB).
AV = 2 * 150 kΩ / Ri (V/V)
(2)
It is recommended that resistors with 1% tolerance or better be used to set the gain of the LM4670. The Ri
resistors should be placed close to the input pins of the LM4670. Keeping the input traces close to each other
and of the same length in a high noise environment will aid in noise rejection due to the good CMRR of the
LM4670. Noise coupled onto input traces which are physically close to each other will be common mode and
easily rejected by the LM4670.
Input capacitors may be needed for some applications or when the source is single-ended (see Figure 27 and
Figure 29). Input capacitors are needed to block any DC voltage at the source so that the DC voltage seen
between the input terminals of the LM4670 is 0V. Input capacitors create a high-pass filter with the input
resistors, Ri. The –3dB point of the high-pass filter is found using Equation 3.
fC = 1 / (2πRi Ci ) (Hz)
(3)
Copyright © 2004–2013, Texas Instruments Incorporated
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