LM4702 Audio Power Amplifier Driver - Glenn's Design Log

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6 Dec 2005	In the beginning
22 March 2006	Sample chips
4 June 2006	Web page set up
20 June 2006	Heatsinking tests
13 July 2006	GND pin tests
31 May 2007	Added information on AN-1645 and LME chips

6 December 2005: In the beginning

I first became aware of the LM4702 chip on the Chip Amps forum of DiyAudio.com. The two threads, "Is this any good?" and "Anyone investigate this part yet?", had been going since October 2005 with a moderate amount of discussion. I downloaded, printed and studied the PDF Data Sheet.

22 March 2006: Sample chips

Photograph Copyright Glenn Baddeley 2006

4 June 2006: Web page set up

The main page was intitally set up with an overview, pin connections and links. A reference to the page first surfaced in the "Anyone investigate this part yet?" thread on 11 June 2006 and it was ranked 3rd on a Google search for LM4702 the same day. Also on that day, the desiderata section was added to the main page, containing the first tip about selecting a preferred value for the mute resistor.

20 June 2006: Heatsinking tests

Information on heatsinking requirements was added to the desiderata section of the main page. Here are some photos of the test rig:

     
Photographs Copyright Glenn Baddeley 2006

A K-type thermocouple probe was inserted in the hole in the tab of the LM4702C with some silicone thermal grease applied. The temperature in degrees Celsius was measured on a Jaytech QM-1530 digital multimeter to an accuracy of about +/- 1 °C. All the 1% metal-film resistors are soldered point-to-point beneath the prototyping board so that probes can be easily attached at various points. The 100 VA transformer and power supply can be seen in the background. The transformer is connected to a variac so that the power supply voltage can be set.

The test circuit is a simple non-inverting amplifier with a gain of 23 and the 'source' and 'sink' outputs tied together with no load. The input is shunted to ground via a 100 Ohm resistor to prevent spurious noise on the output.

13 July 2006: GND pin tests

After a series of tests, information on the role of the GND pin and recommendations for its connection to the "star earth" were added to the desiderata section of the main page.

31 May 2007: Added information on AN-1645 and LME chips

A new application note was published this month by National Semiconductor, AN-1645 LM4702 Driving a MOSFET Output Stage. I have also made mention on the main page of the recent additions to the high performance audio family, the LM4562 (not really so new), LME49710, LME49720 and LME49740 op amps, and also the LME49810 driver which will probably replace the LM4702. I have samples of the LM4562, LME49710 and LME49720 op amps and have been using them in the audio output stage of a SACD player for more than 6 months. LME49810 samples were ordered today.