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A Look At The TDA 7377 Quad 12V Amplifier IC

The TDA7377 IC from ST Microelectronics is the main component of this month's 12V Mini Stereo Amplifier. It's not a new chip ? they've been making them since at least 1998 - but it is the first time we've used it so it deserves some elaboration. It comes in a 15-pin "Multiwatt" package similar to TO-218 and is available in both horizontal and vertical mounting packages.

By Nicholas Vinen

This IC is designed for use in car stereo systems and can provide four single-ended channels, two bridged channels or a combination of two single-ended and one bridged channel. Maximum power depends on speaker impedance, supply voltage and channel configuration but the most useful figures are 4 x 10W into 2Ω, 4 x 6W into 4Ω and 2 x 20W into 4Ω.

Noise performance and channel separation are also quite good. The S/N ratio is typically close to -100dB and channel separation is generally at least 60dB at 10kHz. This is surprisingly good when you consider that all four power amplifiers share the same package and power supply pins.

Click for larger image
The TDA7377 quad amplifier comes in a 15-pin Multiwatt package.

The best features of this IC are its low distortion (down to 0.02% or less) and high power. The most basic circuit for driving two speakers requires just the IC, five small capacitors, one large capacitor (for supply bypassing) and one resistor. It doesn’t get much easier than that!

Because there are no external gain-setting resistors, this means that the gain is internally fixed. This is both a blessing and a drawback – while it reduces the component count, we can’t adjust the gain to our liking. However, their choice of 20dB gain per circuit is reasonable.

This actually results in 26dB of gain in bridge mode. The reason is that in bridge mode, twice the voltage is placed across the speaker as in single-ended mode. This equates to +6dB of additional gain.

As is typical for integrated amplifiers, there is a standby pin which allows the amplifiers to be electronically shut down when not in use. In this condition, the quiescent current is around 1µA. The standby pin also prevents clicks and pops during turn-on and turn-off, because it either mutes or un-mutes the signal paths when it is switched.

Protection

The maximum supply voltage for the IC is 18V but it can withstand up to 28V when it is not operating and spikes of up to 50V for no longer than 50ms. Each channel can deliver up to 3.5A continuously (4.5A peak) and the maximum dissipation is 36W.

In fact, not only can the IC handle voltage spikes but it is virtually indestructible if kept within its limits. Output shorts, excessive current, overheating, inductive and capacitative loads, short-term open-circuit ground wiring, reversed battery – none of these will destroy it, thanks to internal protection circuitry.

The thermal limiting isn’t just a simple cut-out which disables the amplifier either. The current limiting gradually increases with die temperature, so that at first it creates only mild output distortion while reducing the dissipation in an attempt to prevent further temperature increases.

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