The popular SSM2164 quad VCA
(datasheet, PDF, 330k)
Analog Devices, and its clone the V2164 from
CoolAudio (we'll stick to the original name).
Power Rail Failure
Unfortunately the SSM2164 exhibits a rather bad failure mode:
- The SSM2164 exhibits a catastrophic failure mode when the V+
pin is powered and the V- pin is disconnected. The symptom is massive current
flow into the V+ pin resulting in internal heating which, if not limited, results
in destruction of the die.
- In a modular synthesizer system there is the possibility of accidentally
disconnecting the negative rail. For example, the power supply could trip its
negative output and disconnect it, or a power connector could become partially
disconnected while moving the cases. If this should ever occur then
all the SSM2164 devices that
are in the modular synth will be destroyed.
It is therefore especially important to protect the SSM2164 in these types of
open systems (i.e., where users can add and remove modules themselves); in closed
systems, where a supply connection cannot be so easily tampered with, this
situation is very much less likely, but we feel that the information presented on this
page may still be relevant.
Note: this page is the joint work of myself and
By far the best solution we have devised consists of adding a single
diode with its anode connected to the V- pin (pin 9) and its cathode to Ground
It appears that if the V- rail is disconnected the diode
provides a safe path for current to safely flow out of pin 9 (V-) to
ground without causing any damage to the SSM2164. The suspicion is that there is some
parasitic crow-bar that is triggered when the V- pin rises one diode drop (700mV or thereabouts) above GND. The
Schottky stops this happening by hard-limiting the V- pin to about half that (300mV or so).
This is the solution we recommend for all SSM2164 applications.
To prove this solution the following experiment was conducted:
- A 100Ω resistor was placed in series in the positive supply to pin 16
to limit the fault current to about 94mA
- Current flow into pin 16 under test conditions (worst-case signal and control voltages) was
measured at 10mA (typical is about 5mA)
- When the negative supply was disconnected, the pin 16 current immediately
rose to 94mA indicating FAULT condition
- Adding a BAT85 Schottky diode connected with its anode to pin 9 and
cathode to pin 8 the current into pin 16 dropped to 0mA.
Note: if the V- rail drops to 0V as opposed to disconnected, so that current can still flow into
the V- rail, the SSM2164 behaves the same safe way. It is only in the situation where the V- rail is
completely disconnected that this fault occurs.
Reverse Voltage Protection
The SSM2164 is sensitive to reverse voltages. Experiments have shown that certain
fault power conditions will
immediately destroy an SSM2164 (JPG, 124k ... may be upsetting if you are
of a sensitive chip-loving nature).
The BAT85, as described above, could be combined with a standard reverse polarity protection
(polyfuses in series for each rail and 1N4002 from each rail to ground).
If polyfuses are not present the Schottky should be sized accordingly; for example a 1A Schottky like the 1N5817.
A better approach is to use two 1N5817 Schottky diodes in place of 1N4002 diodes, providing both
reverse polarity protection and SSM2164 negative supply fault protection. Please note that at the time of writing
the 1N5817 solution has not yet been tested.
The following solutions were also evaluated in this study. They all provide
some level of protection, but none are as satisfactory as the preferred solution
1. Simple series resistor
- Lowest component count: 1xR
- Very easy to implement on existing modules
|Considerable voltage drop under normal operating conditions
could adversely affect performance
||Useful for monitoring fault current during experimenting, but
probably not useful in practical applications.
2. PNP current source
||Quite easy to implement on existing modules.
- High component count: 1xQ, 2xR, 2xD
- In fault mode the transistor will get warm.
There is also the small issue that there is about 1V drop
in the V+ supply rail to the SSM2164 which may be important in
2b. LM334 current source
||Low component count: 1xR, 1xQ
||Positive supply rail reduced by about 500-600mV.
||Has been tested, 10mA that the LM334 provides
will cover normal operation, but it might be better to consider a source
that provides something like 20mA.
This solution was suggested by Steve Lenham on the synth-diy list.
3. Transistor switch
||Very low supply voltage drop (typ. 300mV)
||High component count: 2xR, 1xQ, 1xD
||Probably the second-best solution if not for the component count.
Could use a low-RDS(on) MOSFET for even lower voltage drop.
This information is provided "as-is". If you find an error please
let me know.