Doug S
Flashlight Enthusiast
I am normally a big advocate of paying attention to the "Absolute maximum limits" on datasheets but there are times when I am tempted to consciously exceed them where the decision is at least semi-informed and it is not an application where I would required to defend my decision before a Design Review Board or a court of law.
It seems that with the trend towards lower voltage electronics there are lots of switching voltage regulator ICs being released with Vin and Vout maximum voltage ratings of 6 volts. Some of these do not have truly comparable equivalents with higher maximum ratings. When designing switcher circuits for 1 and 5W luxeons there are battery choices that tempt one to fudge the voltage limits of these 6V ICs a bit. In the case of stepup switchers powering 5W luxeons, output voltages on the order of 7V [depending on current and bin] are required. For stepdown applications a battery of 2 CR123 cells would yield a Vin of 6.5V when fresh. For the really daring, 2 Li-ion cells gives 8.4V when fully charged, and for the downright crazed, 6 alkalines would be 9V+ when fresh.
Here is my personal list of 6V rated ICs that I have evaluated as attractive for luxeon power supply applications.
STEPDOWN: LTC1701, LTC3406, LTC3412, LTC3411, MAX1920, MAX1927, MAX1951, MAX1973, MAX1820, MAX1821, TPS62000
STEPUP: LTC3400, LTC3401, MAX1674, MAX1675, MAX1832, MAX1834
All of the above devices are CMOS devices. I can think of three mechanisms whereby higher voltages could adversely affect the ICs. These are:
1) Zener diodes incorporated in the IC for ESD protection where the avalanche voltages are slightly above the max voltage ratings
2) Exceeding the gate to source voltage capabilities of the mosfets of the IC
3) Exceeding the drain to source breakdown voltage of the mosfets in the IC.
I have *absolutely no experience or data* concerning exceeding the max voltage ratings of the above listed ICs. I do have some experience with other Mosfet devices that suggests that the limits can likely be exceeded to some degree without degrading reliability excessively for our purposes. In the case of gate to source breakdown, assuming that the voltage for instantaneous failure is not exceeded, per some IRF applications notes that I have, a 50% increase in Vgs gives a factor of 10 increase in Vgs related failures. For devices that have MTBFs in the 10s of millions of hours, a factor of 10 increase seems acceptable. For IRF GEN V mosfets, per a telephone conversion I had in 1995 with one of their technical people, the voltage for instanaeous Vgs failure is about 2X the max rated.
In the case of Vds breakdown, I once tested a small sample of 20V rated IRF mosfets for this parameter and found they all ran at least 40% over rated.
Question for members on the forum: is anyone running any of the above 6V rated parts over the max limits and what are the particulars of your experience with them? I would suspect that there must be some folks running 5W luxeons off of Waynes Madmax modules which use the Max1674 IC. If anyone with a madmax module wants to do an experiment for the benefit of the forum, here is a way to test the Vds breakdown of the switching transistor that *has a chance* of not distroying your module: Power the module through a resistance of 100-200 ohms at less than its 6V maximum, load the output with a 1K ohm resistor only. While monitoring the output voltage slowly adjust the pot on the module until the voltage stops increasing. The maximum voltage attained is the Vds breakdown voltage of the switching transistor. The resistor on the input prevents the IC from drawing excess power at Vds breakdown. Perhaps Wayne has already done the equivalent testing and can share his results. Understandably, Wayne does not endorse the use of the Madmax to power 5W luxeons but there is no reason why the more gutsy folks here might not want to give it a try. This is certainly *much tamer* than some of the stuff people here admit to doing!
It seems that with the trend towards lower voltage electronics there are lots of switching voltage regulator ICs being released with Vin and Vout maximum voltage ratings of 6 volts. Some of these do not have truly comparable equivalents with higher maximum ratings. When designing switcher circuits for 1 and 5W luxeons there are battery choices that tempt one to fudge the voltage limits of these 6V ICs a bit. In the case of stepup switchers powering 5W luxeons, output voltages on the order of 7V [depending on current and bin] are required. For stepdown applications a battery of 2 CR123 cells would yield a Vin of 6.5V when fresh. For the really daring, 2 Li-ion cells gives 8.4V when fully charged, and for the downright crazed, 6 alkalines would be 9V+ when fresh.
Here is my personal list of 6V rated ICs that I have evaluated as attractive for luxeon power supply applications.
STEPDOWN: LTC1701, LTC3406, LTC3412, LTC3411, MAX1920, MAX1927, MAX1951, MAX1973, MAX1820, MAX1821, TPS62000
STEPUP: LTC3400, LTC3401, MAX1674, MAX1675, MAX1832, MAX1834
All of the above devices are CMOS devices. I can think of three mechanisms whereby higher voltages could adversely affect the ICs. These are:
1) Zener diodes incorporated in the IC for ESD protection where the avalanche voltages are slightly above the max voltage ratings
2) Exceeding the gate to source voltage capabilities of the mosfets of the IC
3) Exceeding the drain to source breakdown voltage of the mosfets in the IC.
I have *absolutely no experience or data* concerning exceeding the max voltage ratings of the above listed ICs. I do have some experience with other Mosfet devices that suggests that the limits can likely be exceeded to some degree without degrading reliability excessively for our purposes. In the case of gate to source breakdown, assuming that the voltage for instantaneous failure is not exceeded, per some IRF applications notes that I have, a 50% increase in Vgs gives a factor of 10 increase in Vgs related failures. For devices that have MTBFs in the 10s of millions of hours, a factor of 10 increase seems acceptable. For IRF GEN V mosfets, per a telephone conversion I had in 1995 with one of their technical people, the voltage for instanaeous Vgs failure is about 2X the max rated.
In the case of Vds breakdown, I once tested a small sample of 20V rated IRF mosfets for this parameter and found they all ran at least 40% over rated.
Question for members on the forum: is anyone running any of the above 6V rated parts over the max limits and what are the particulars of your experience with them? I would suspect that there must be some folks running 5W luxeons off of Waynes Madmax modules which use the Max1674 IC. If anyone with a madmax module wants to do an experiment for the benefit of the forum, here is a way to test the Vds breakdown of the switching transistor that *has a chance* of not distroying your module: Power the module through a resistance of 100-200 ohms at less than its 6V maximum, load the output with a 1K ohm resistor only. While monitoring the output voltage slowly adjust the pot on the module until the voltage stops increasing. The maximum voltage attained is the Vds breakdown voltage of the switching transistor. The resistor on the input prevents the IC from drawing excess power at Vds breakdown. Perhaps Wayne has already done the equivalent testing and can share his results. Understandably, Wayne does not endorse the use of the Madmax to power 5W luxeons but there is no reason why the more gutsy folks here might not want to give it a try. This is certainly *much tamer* than some of the stuff people here admit to doing!
