LM48520, LM48520TLBD

SNAS367C – FEBRUARY 2008 – REVISED APRIL 2013

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APPLICATION INFORMATION

General Amplifier Function

The LM48520 features a Class D audio power amplifier that utilizes a filterless modulation scheme, reducing

external component count, conserving board space and reducing system cost. The outputs of the device

transition from PV1 to GND with a 300kHz switching frequency. With no signal applied, the outputs (VLS+ and

VLS-) switch with a 50% duty cycle, in phase, causing the two outputs to cancel. This cancellation results in no

net voltage across the speaker, thus there is no current to the load in the idle state.

With the input signal applied, the duty cycle (pulse width) of the LM48520 outputs changes. For increasing output

voltage, the duty cycle of VLS+ increases, while the duty cycle of VLS-decreases. For decreasing output voltages,

the converse occurs. The difference between the two pulse widths yields the differential output voltage.

Differential Amplifier Explanation

The amplifier portion of the LM48520 is a fully differential amplifier that features differential input and output

stages. A differential amplifier amplifies the difference between the two input signals. Traditional audio power

amplifiers have typically offered only single-ended inputs resulting in a 6dB reduction in signal to noise ratio

relative to differential inputs. The amplifier also offers the possibility of DC input coupling which eliminates the

two external AC coupling, DC blocking capacitors. The amplifier can be used, however, as a single ended input

amplifier while still retaining it's fully differential benefits. In fact, completely unrelated signals may be placed on

the input pins. The amplifier portion of the LM48520 simply amplifies the difference between the signals. A major

benefit of a differential amplifier is the improved common mode rejection ratio (CMRR) over single input

amplifiers. The common-mode rejection characteristic of the differential amplifier reduces sensitivity to ground

offset related noise injection, especially important in high noise applications.

Amplifier Dissipation and Efficiency

The major benefit of a Class D amplifier is increased efficiency versus a Class AB. The efficiency of the

LM48520 is attributed to the region of operation of the transistors in the output stage. The Class D output stage

acts as current steering switches, consuming negligible amounts of power compared to their Class AB

counterparts. Most of the power loss associated with the output stage is due to the IR loss of the MOSFET on-

resistance, along with switching losses due to gate charge.

Regulator Power Dissipation

At higher duty cycles, the increased ON-time of the switch FET means the maximum output current will be

determined by power dissipation within the LM48520 FET switch. The switch power dissipation from ON-time

conduction is calculated by:

PD(SWITCH) = DC x (IINDUCTOR(AVE))

2 x R

DS(ON)

(W)

(1)

where:

Where DC is the duty cycle

Shutdown Function

The LM48520 features independent amplifier and regulator shutdown controls, allowing each portion of the

device to be disabled or enabled independently. AmpSD controls the Class D amplifiers, while BstSD controls

the regulator. Driving either inputs low disables the corresponding portion of the device, and reducing supply

current.

When the regulator is disabled, both FB_GND switches open, further reducing shutdown current by eliminating

the current path to GND through the regulator feedback network. With the regulator disabled, there is still a

current path from VDD, through the inductor and diode, to the amplifier power supply. This allows the amplifier to

operate even when the regulator is disabled. The voltage at PV1 and V1 will be:

VDD — [VD + (IL x DCR)]

(2)

where:

VD is the forward voltage of the Schottky diode

IL is the current through the inductor

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