PARALLEL-BRIDGED CONFIGURATION USING LM3886
This Applicationlication note is from kits and is nothing more than attempt to share information resulting from experimentation and fooling around.I cannot and do not take any responsibility for anything anyone else does with this information - In other words you are on your own and take responsibility for your own actions in using any of this information. Nothing in this Application Note should be taken as an endorsement of or by National Semiconductor or anyone else...
INTRODUCTION
For audio (home or car) amplifiers up to about 50 watts per channel rms, a number of ICs are available that give excellent results. Advantages to the IC application over traditional discrete transistor designs with included "built-in" protection circuitry, ease of design, relative simplicity of support circuitry and components, quick assembly and high reliability. Over the 50 watt threshold, however, discrete transistor and now more popular switching (D and T class) designs still dominate.
This note shares findings at successfully using multiple LM3886 ICs to obtain power levels in excess of 200 watts into 8 ohms. While this application may not be the most cost-effective for high volume production, it preserves several advantages for the hobby or custom builder. Circuit board design is pretty simple and straightforward as relatively few support components are involved, typically it is no great inconvenience to design a single-sided board. Protection circuitry comes "built-in", eliminating issues of designing circuitry to cause shutdown in cases of excessive heat, shorts on the output, excessive output currents, turn-on delay to prevent thumping and so forth. Although a discrete transistor design might be "cheaper", in the long run ruining even one power transistor along the way could make the IC application more economical.
As much as possible this note is oriented toward the technician/builder perspective rather than engineering calculations. Some background is given addressing related issues like power supplies and heatsinking, for hard technical data the reader should consult the data sheet for LM3886, AN-898, (National's Applicationlication Note on their SPIKE protection circuitry) and BPA-200 (a note from National on using the LM3886 in parallel-bridged mode). In any event it is presumed the reader has these publications from National for reference, as well as my previous Application Note on bridging.
OUTPUT POWER AND LOADS
Let's first look at "rms" power into both 4 and 8 ohms. Keep in mind rms is .3535 the peak to peak voltage (since peak voltage is 1.414 times rms voltage and peak-to-peak is twice the peak), and that only one combination of rms voltage and current will produce specific power into a specific load. You cannot, for instance, use "more voltage and less current" to get 100 watts into 4 ohms. The load determines the current that will flow thru it at a particular voltage - good old Ohm's Law... P=I²×R, V=I×R (P is Power, I is current, V is voltage, R is Resistance)
| Output power |
4 ohm load |
8 ohm load |
| 50 watts rms |
3.5A, 14.14Vrms (40Vpp) |
2.5A , 20Vrms (56.58Vpp) |
| 100 watts rms |
5A, 20Vrms (56.58Vpp) |
3.5A , 28Vrms (79.21Vpp) |
| 200 watts rms |
7.07A, 28.28Vms (79.21Vpp) |
5A , 40Vms (113.1Vpp) |
| 400 watts rms |
10A, 40Vms (113Vpp) |
7.07A , 56.56Vms (160Vpp) |
| 800 watts rms |
14.14A, 56.57Vms (160Vpp) |
10A , 80Vrms (226.3Vpp) |
| 2000 watts rms |
22.36A,89.44Vrms (253 Vpp) |
15.81A , 126.49Vrms (357.82 Vpp) |
Formula for calculating RMS is : Wrms = Vp-p × 0.3535 / Rspeaker × I
I = Vp-p × 0.3535 / Rspeaker
Vrms = Vp-p × 0.3535
EXAMPLE : Wrms =60 Vp-p × 0.3535 × ( 60 Vpp × 0.3535 / 4 ohm )
= 21.21 Vrms × 5,3025 Amps = 112,466025 Wrms
These numbers may not be what one would expect, thinking that doubling the resistance cuts the current in half and therefore the power. Looking again, 5A@20V produces 100 watts into 4 ohms, the same 20V produces 2.5A into 8 ohms for 50 watts. Doubling the impedance cuts both current and power in half at the same voltage. At the same output power, increasing the impedance increases voltage but decreases current . For purposes of this ApplicationNote, "rms power" measurements are taken at the point where either clipping begins to occur or thermal protection begins to engage, whichever is smaller. Input signal used is a sine wave, constant amplitude driving the amplifier to the desired measurement point.
HEATSINKING
When it comes to heatsinking, bigger is better. Too small heat sink will result in premature thermal shutdowns, particularly when more than one IC is attached to the same heatsink. Production economics necessitate going with the minimum sink that will do the job, do-it-yourself builders should not use. Especially in multi-chip situations (parallel, bridged or parallel-bridged) heatsinking should be sufficient to ensure that thermal protection is engaged from the chip's limitations in transferring heat from the die to the heatsink rather than from the heatsink to the ambient air. The big advantage that comes from operating LM3886s in parallel is not so much having twice the output current available as it is having twice the heat dissipation capability (from the die to the heatsink). Given adequate heatsinking and proper voltage supply rails, 4 of LM3886 can deliver 230+ watts rms into 8 ohms without clipping or engaging the SPIKE protection. Arrange the board layout to spread the ICs as far apart as reasonable rather than mounting them immediately adjacent to one another. Orient the sink for maximum airflow across the finned area, and plan on a fan if the heatsink will be in a confined area.
POWER SUPPLY RAILS
Power supply voltage rails should be set at the minimum that will allow the voltage swing necessary for the desired output power. LM3886 will not swing "rail to rail", the clipping voltage can in range about 4V with+30V rails and a 4 ohm load down to around 3V with +35V rails and an 8 ohm load. The data sheet for the LM3886 uses these values/supply rails, which are pretty optimum for single-chip Application lications for experimenting have worked well in bridged applications. If the voltage supply rails are too low, the result will be excessive clipping and reduced output power. If the rails are too high, the result will be excessive heating with little or no increase in maximum power output. Thermal shutdown may engage partially or completely before clipping occurs, but the results will be unpredictable. Although LM3886 is rated to handle supply rails up to +42V, in experience realistic maximums are +28 to 30V for 4 ohm loads and +35V for 8 ohm loads, but only with oversized heatsink you can use +28 to 35V for 4 ohm loads and +42V for 8 ohm loads.
PARALLEL, BRIDGED (BTL), OR BOTH ?
Consider how much power you want to develop and the load impedance of the speakers you will be driving. The LM3886 does a fine job driving 4 ohm or 8 ohm loads but is not intended to drive 2 ohm loads (it will drive 2 ohm loads, but it takes very little output power to hit the maximum output current limit under those conditions). Remember that in a BTL (bridged) configuration each amp only sees half the load impedance, so using a BTL pair to drive a 4 ohm load should be avoided. The same pair will, however, do a fine job driving an 8 ohm load. Operating 2 or more LM3886s in parallel gives increased output current capability, but more importantly makes it easier to deal with thermal issues. In theory each chip in the parallel bank can still deliver its full rated power. Since the load is shared, each chip is only providing its share of the overall power, which means it is producing only its share of the overall heat. A pair of LM3886s pushing 50 watts into 4 ohms will have each chip generating half the power and half the heat, meaning each chip is running cooler and providing less current than it would acting alone. The parallel configuration is good for driving lower impedance loads (like 2 ohms) or for "beefing up" bridged applications (parallel-bridged).
For driving 8 ohm loads up to around 100 watts, a simple BTL configuration will do nicely. Power supply rails should not exceed + 35V. LM3886 can be driven from either the inverting or non-inverting input, National at one time published drawings showing the bridge being set up by driving the inverting input of one LM3886 and the non-inverting input on the other side of the bridge. We prefer driving the non-inverting input of each side of the bridge, which means we use a dual op amp (TLO72, NE5532 etc) to generate the "mirror image" input signals. This does require an additional parts, but it keeps the gain precisely the same on both halves of the bridge. Since the dual op amp is essentially a pre-amp, by using a pot for the feedback resistor in the first half of the network it is possible to control amplitude of the signal going to the main power section. This can be convenient for coordinating a subwoofer with the main audio channels, for instance. Refer to the drawing in ApplicationNote on bridging for the circuit and component values used.
Results are less satisfactory for bridging 4 ohm loads, even with reduced voltage supply rails. Two LM3886s each driving their own 4 ohm speaker will produce more audio power than they will driving a single 4 ohm load in a bridged configuration. For bridging 4 ohm loads, the parallel-bridged configuration should be used.
Parallel-bridging is nothing more than taking two paralleled banks and using the bridged configuration. The hard part is setting up the parallel banks, bridging is just like using a single pair of chips in the BTL configuration. When using the parallel-bridged combination to drive 4 ohm loads, attention must be paid to the voltage supply rails. Do not exceed +30V for best results, and in many applicationlications a slightly lower setting may go a long ways in preventing unexpected shutdown.
ENSURING COOPERATION AMONG THE CHIPS
The important areas are the two resistors that determine the gain (the feedback resistor and the resistor to ground on the inverting input) and the load-sharing resistor on the output. Matching the parallel circuits exactly is of utmost importance, much like using bi-polar power transistors in parallel. Two applications can be taken - using 0.1% resistors for setting the gain, or handpicking standard 5% resistors. We normally "handpick" since we have a large number of 5% resistors on hand anyway and since 0.1% tolerance units are relatively expensive, especially in small quantities. Setting the gain with such precision is not necessary, but it is vital that every amplifier circuit in the configuration is matched this closely with all other amp circuits in the configuration. Matching to this degree of precision is 1 ohm per 1000 for those who "handpick". I usually go with a gain of around 20, if needed more I obtain it "upstream" in either the bridging network or inverting buffer following the summing network (subwoofer amp).
Outputs are joined to the load through 1% 0.1 ohm precision resistors (3 watt minimum), similar to using low value emitter resistors when parallelling bi-polar power transistors. Precision matching is critical here also, to ensure each amp circuit drives the load rather than interacting with other amps sharing the load.
Since a parallel-bridged configuration counts as "extreme conditions", care must also be taken to ensure each amp in the configuration is completely stable and not subject to oscillation. Proper board layout is critical. On the inverting input, use a 100uF cap (to eliminate DC offset at the output) in series with a 1K resistor handpicked to match other amp circuits in the configuration. For matching purposes the cap can be ignored since it only has an effect at extremely low frequencies. We also use a 47pF-4700 ohm RC network in parallel with a 20K feedback resistor. Again, for matching purposes the RC leg can be ignored since it only comes into effect at frequencies well above the audio range.
An op amp should be used to drive the LM3886s to prevent overloading the signal source. In theory a bank could be constructed with 3, 4 or more ICs matched in parallel. Practical limits are determined by load impedance and voltage supply limitations presuming thermal isues have been adequately overcome. Developing a Vpp swing of 160V, for instance,application cannot handle even in a bridged configuration powered at +35V. An optimized design will hit the limits for current, voltage and thermal conditions at aproximately the same point. Power supply rails, load impedance and input signal levels should be coordinated such that a maximum input signal condition will drive the amplifier to just short of clipping, which should be reached before thermal or output current limitations are met.
EXPERIMENTAL AMPLIFER SETUP
Power supply is a switched mode unit regulated at +35V and capable of delivering in excess of 20A. knowing that it will produce up to 1400 watts, but can't keep enough power coming into it for a long period for more than about 800 watts. Since it starts at +12VDC, input currents are pretty large, easily exceeding 100A.
In this application there are running 8 of LM3886. Four of these are "full range" audio each driving their own 8 ohm, 3 way speaker box. The other 4 units are in a parallel-bridge configuration driving an 8 ohm subwoofer. The subwoofer circuitry and first pair of LM3886s are built on one of our LM3886 Experimental boards, additional pairs are built on boards identical to the power amp sections of the experimental board. The LM3886s are mounted on an 18" length of PEI682 extrusion, the switching power supply is mounted on an identical extrusion. Hearing protection is mandatory when driving the speakers near full power, which can only be done during off hours when surrounding businesses are closed.
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