LM48520, LM48520TLBD

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SNAS367C – FEBRUARY 2008 – REVISED APRIL 2013

DCR is the DC resistance of the inductor

Additionally, when the regulator is disabled, an external voltage between 2.4V and 5.5V can be applied directly to

PV1 and V1 to power the amplifier.

It is best to switch between ground and VDD for minimum current consumption while in shutdown. The LM48520

may be disabled with shutdown voltages in between GND and VDD, the idle current will be greater than the

typical 0.1µA value. Increased THD+N may also be observed when a voltage of less than VDD is applied to

AmpSD.

Proper Selection of External Components

Proper selection of external components in applications using integrated power amplifiers, and switching DC-DC

converters, is critical for optimizing device and system performance. Consideration to component values must be

used to maximize overall system quality.

The best capacitors for use with the switching converter portion of the LM48520 are multi-layer ceramic

capacitors. They have the lowest ESR (equivalent series resistance) and highest resonance frequency, which

makes them optimum for high frequency switching converters.

When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as

Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage,

they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor

manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from

Taiyo-Yuden, AVX, and Murata.

Power Supply Bypassing for Amplifier

As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply

rejection. The capacitor location on both PV1, V1 and VDD pins should be as close to the device as possible.

Selecting Input Capacitor for Audio Amplifier

Input capacitors, CIN, in conjunction with the input impedance of the LM48520 forms a high pass filter that

removes the DC bias from an incoming signal. The AC-coupling capacitor allows the amplifier to bias the signal

to an optimal DC level. Assuming zero source impedance, the -3dB point of the high pass filter is given by:

f(–3dB) = 1/2πRINCIN

(3)

Choose CIN such that f-3dB is well below that lowest frequency of interest. Setting f-3dB too high affects the low-

frequency responses of the amplifier. Use capacitors with low voltage coefficient dielectrics, such as tantalum or

aluminum electrolytic. Capacitors with high-voltage coefficients, such as ceramics, may result in increased

distortion at low frequencies. Other factors to consider when designing the input filter include the constraints of

the overall system. Although high fidelity audio requires a flat frequency response between 20Hz and 20kHz,

portable devices such as cell phones may only concentrate on the frequency range of the frequency range of the

spoken human voice (typically 300Hz to 4kHz). In addition, the physical size of the speakers used in such

portable devices limits the low frequency response; in this case, frequencies below 150Hz may be filtered out.

Selecting Output Capacitor (CO) for Boost Converter

A single 100µF low ESR tantalum capacitor provides sufficient output capacitance for most applications. Higher

capacitor values improve line regulation and transient response. Typical electrolytic capacitors are not suitable

for switching converters that operate above 500kHz because of significant ringing and temperature rise due to

self-heating from ripple current. An output capacitor with excessive ESR reduces phase margin and causes

instability.

Selecting Input Capacitor (Cs1) for Boost Converter

An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each

time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. We

recommend a nominal value of 2.2µF, but larger values can be used. Since this capacitor reduces the amount of

voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other

circuitry.

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