24 September 2004: LM4780 is a dual LM3886
Refer to the LM4780 / 3886 / 3875 / 1875 Comparison page which contains
electrical characteristic tables, equivalent schematics and distortion graphs for these four chips.
Based purely on the electrical characteristics, the LM4780 appears to be dual version of the LM3886. The
following parameters have exactly the same values:
- Power Supply Voltage
- Slew Rate
- Input Offset Voltage
- Input Bias Current
- Output Current Limit
- Output Dropout Voltage, both Io= +100mA and -100mA
- Power Supply Rejection Ratio on V+ Rail and V- Rail
- Common Mode Rejection Ratio
- Open Loop Voltage Gain
- Gain Bandwidth Product
- Input Noise
- Intermodulation Distortion at F=60Hz,7KHz, 4:1 (SMPTE)
The LM4780 has a Signal-to-Noise ratio (Po=1 W, A-Weighted, Measured at 1 KHz, Rs=25 Ohm) of 97 dB, which is
slightly better than the LM3886 Signal-to-Noise ratio of 92.5 dB.
With +/- 28 Volt supply rails the LM3886 is specified to deliver
68 Watts into 4 Ohms, at 0.1% THD+N and 20 Hz - 20 KHz bandwidth.
The LM4780 is only specified at +/-25 Volts rails into 4 Ohms,
and delivers 55 Watts, at 1% THD. However, according to
the graph at middle left on page 10 of the PDF spec sheet,
the power supply rails can be lifted to +/-28 Volts, where upon
it will deliver approximately 66 Watts.
I don't think LM4780 is any less capable than the LM3886
when driving 4 Ohm loads (eg. they have the same peak
current output capability), its just that the sample performance
specifications are expressed differently.
The LM3886 is specified at +/-28 V (for 4 Ohm and 8 Ohm loads)
and +/-35 V (for 8 Ohm), whereas the LM4780 is specified at
+/-25 V (4 Ohm), +/-30V (6 Ohm) and +/-35V (8 Ohm).
I had more email correspondence with the Design Support Group on 24 August 2004 where I wrote
'The LM3886 has almost identical characteristics to half of an LM4780, so I assume
their design is very similar'.
The response on 26 August 2004 was 'The LM4780 is two LM3886 die in a single package. So the equivalent
schematic from the LM3886 DS can be used as an equivalent schematic for each channel (amplifier) in the LM4780. With the
LM4780 being two die the GND pins are NOT
internally connected'. A further response on 27 August 2004 was 'There is
no difference in the two die used in the LM4780 and the single die used in the LM3886.
Using two die is more efficient thermally as the distance between the die helps improve heat flow
out of the package to the heat sink. Cost is also reduced as smaller die have higher yields
than larger die due to defect density so a single, large dual die design would not have any
benefits over two separate die'.
I wrote again on 30 August 2004 'From the connection diagram on page 2 of the LM4780 PDF data sheet
it is fairly clear how the lead-outs from the two LM3886 dice are allocated to pins 7 & 12-16 for die "A"
and pins 19-25 for die "B", but I am confused about the five pairs of V+ and V- pins.
Are the five V+ pins connected together internally, and the five V- pins connected together internally, or are they
individually distributed between the two dice?'.
The Design Support Group responsed on 14 September 2004 'Here is the connection of each die:
Pin Connection
1 N/C
2 V- (Vee) Down bond to the DAP of the package
3 N/C
4 V- (Vee) to die A 1st V- pad
5 V+ (Vcc) to die A 1st V+pad
6 N/C
7 Output from die A pad
8 V+ (Vcc) to die A 2nd V+ pad
9 V+ (Vcc) to die A 2nd V+ pad
10 V- (Vee) to die A 2nd V- pad
11 V- (Vee) to die A 2nd V- pad
12 GND to die A GND pad
13 N/C
14 Mute to die A Mute pad
15 INA- to die A INA- pad
16 INA+ to die A INA+ pad
17 V+ (Vcc) to die B 2nd V+ pad
18 V- (Vee) to die B 2nd V- pad
19 GND to die B GND pad
20 Mute to die B Mute pad
21 INB- to die B INB- pad
22 INB+ to die B INB+ pad
23 N/C
24 N/C
25 Output from die B pad
26 N/C
27 V+ (Vcc) to die B 1st V+pad
Internal Downbond to DAP from die B 1st V- pad
The reason sometimes there are two V- or V+ pins to a pad is that there is
one bond wire from each pin. This makes for better connection to the PCB.
When there is only one pin then there are two bond wires from the pin to the
pad. Downbonds are all double wires as well. Inputs, Mute and GND are all single
bond wires since they are low current'
.
After I asked for some clarification on a couple of the connections, there
was a further response on 15 September 2004 'DAP means Die Attach Pad, it is the flat area where the die is
attached to the package. .... There is only one wire from pin 4 to the V-
pad because of spacing it was not possible to do two wires. Also, the output pins have two wires
to them but each die actually has two output pads. One wire from each pad goes to pin 7 or pin 25
respectively. So two wires to each output pin'.
I have summarised this information concerning the LM4780 internal connection scheme to the two LM3886 die
in two annotated pin connection diagrams on the LM4780 Dual 60W Audio Power Amplifier page.
I have also added my version of an annotated equivalent schematic for the LM4780 on the
LM4780 / 3886 / 3875 / 1875 Comparison page.
The slightly different Distortion versus Power Output characteristics
between the LM4780 and LM3886 would be mostly due to the different packaging and layout of the Reference PCB.
The characteristic curves are measured and plotted by National Semiconductor with the devices
installed on their Reference PCBs.
17 January 2005: Prototype demonstrated to Melbourne Audio Club
I demonstrated my prototype LM4780 stereo power amplifier on the
15th December 2004 at the General Meeting of
the Melbourne Audio Club, which is the annual "DIY night".
A report on the meeting was published in "Melbourne Audio News", Issue 353, January 2005. The MAC Program Co-ordinator, Bob Toll, wrote:
Next Glenn Baddeley presented a complete system with a TEAC CD
player modified with a two valve output section, a chip amp he constructed
using the LM4780, and two way floor standing speakers using Focal and Dynaudio drivers.
The cabinets were completely inert being constructed using five different layers including
lead, Masonite and MDF; and the drivers were time-aligned. Great audio.
Another LM4780 amp was demonstrated at the same meeting with a custom PCB design based on the National Semiconductor evaluation design. Bob wrote:
Chris Mogford presented a similar amplifier with a computer style Creative CD player (which
refused to select tracks so the TEAC from the previous system was commandeered) with a pair of largish
book-shelf speakers built around German Visaton and Jaycar drivers by Bob Toll, with crossovers
by Ron Newbound. Ron also supplied a small powere subwoofer to augment the bass. Very detailed.
