1.5-W Loudspeaker Amplifier Delivers Sound Performance
by Troy Murphy
The SSM2211 speaker amplifier, from the Analog Devices audio amplifier group, is an operational power amplifier designed to deliver up to 1.5 W of power into a 4-
speaker when powered by a +5-V single supply. Its current drive, sound quality, and heat dissipation are substantially improved over earlier integrated speaker amplifiers. Its SO-8 package uses a patented Thermal Coastline technique for significantly improved heat dissipation in a small space. This allows the device to deliver power at elevated ambient temperatures.
| The pushpull-output SSM2211 consists of an input amplifier (Figure 1), that can be configured for gain like a standard op amp, and a unity-gain inverting amplifier--with appropriate biasing--producing a differential output voltage across a floating "bridge-tied" load (BTL) with maximum swing approaching twice the supply voltage (hence four times the single-ended power output into a resistive load). Both amplifiers have high-current output stages (to within 400 mV of the rails at full power). A reference voltage is available to bias the two amplifiers for single-supply use, and the device can be put into a low-current shutdown mode, drawing typically less than 10 nA; this makes it very suitable for battery-powered applications, such as portable PC audio and mobile radios. |  |
| Figure 1. SSM2211 Simplified Schematic. |
At maximum output power, the total harmonic distortion (THD) is only 0.1%, a significant improvement over IC speaker amplifiers currently on the market.
Design objectives There are two major challenges in designing a power amplifier to be housed in a small-outline (SOIC) package. One is to deliver the maximum power efficiently from a single supply voltage. The other is to dissipate the heat the device generates at high output power levels without excessive temperature rise.
To drive a load connected from the single-ended output of an amplifier to ground, the maximum sine-wave power available is simply VP2/(2R), where VP is the peak voltage. In the ideal case (rail-to-rail), VP would be half the supply voltage, and max output would be Vs2/(8R). With the amplifier biased halfway in a single-supply application, a capacitor must be used to couple a speaker to a single-ended output to block direct current from the speaker. Because the typical resistance of a speaker can be 8 or less, the capacitance must be at least several hundred microfarads to minimize attenuation at low frequencies. The capacitor adds cost to the system design and takes up precious board space. The efficiency of this arrangement is low.
By connecting a speaker across both outputs in a pushpull, or bridge-tied load (BTL) configuration, the need for a coupling capacitor is eliminated, because both output terminals are biased to the same dc voltage. The BTL configuration also doubles the voltage swing across the output. Because the output power is proportional to the square of the voltage, this allows four times as much power to be delivered to the speaker, a loudness increase of 12 dB. In addition, efficiency can be greater.
The maximum power dissipation of the SSM2211 is a function of the supply voltage and the resistance of the speaker it is driving. It can be found by the formula:
where VDD is the supply voltage and RL is the speaker resistance. With a +5-V supply and an 8-ohm speaker, the maximum power dissipation of the device is 633 mW. This can result in a significant heat increase in a standard SO-8 package. To improve the heat dissipation from the package, the SSM2211 uses a modified package for lowered thermal resistance. This proprietary package, developed by Analog Devices, uses an internal modification called a Thermal Coastline® to improve the thermal resistance in a SOIC package by more than 30%.
| The modification, done inside the package, is invisible to the user. In a standard package, the die sits on a rectangular paddle with the bonding pads coming out to the die. In a package with a Thermal Coastline, the area of the paddle is increased; the bonding pads are extended and curve around the paddle, shown in Figure 2. This provides a path with increased thermal conductivity for heat to flow from the die into the package case, thereby lowering the thermal resistance from the die to the ambient surroundings. |  |
| Figure 2. Thermal Coastline. |
For standard SOIC package, typical junction-to-ambient-termperature thermal resistance (thetaJA) is 158 °C/W. In a Thermal Coastline SOIC package, thetaJA is 98°C/W. Thus, a die in a Thermal Coastline package will not get as hot as a die in a standard package with the same power dissipation.
As a result of this packaging, SSM2211 can deliver 1 W into an 8-
load at temperatures up to +85°C. This is a significant improvement over IC power amplifiers in conventional small-outline packages, which can only deliver this magnitude of output power at temperatures less than +44°C.
Analog Devices Termal Coastline technology is not limited to small outline packages; it can be applied to practically any package type. Besides high-power audio, these new thermally efficent packages have useful applications in power management and temperature sensing devices. You can expect to see more small packages of this sort with greater power output on increasing numbers of new products from Analog Devices.