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Registered Member #3824
Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54
Hey, does anyone know how say, a cell phone, can keep working 100% correctly even when battery voltage is like 10% of nominal? More specifically, how do the individual transistors that make up the millions of logic gates in a computer chip stay in the proper mode of operation when the voltage to bias the pn junctions is insufficient?
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Easy, they use a certain voltage lockout to prevent overheating when the voltage gets too low. That's WHY they shut down after the battery gets so dry.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I have no idea what Inducktion is talking about, but it is not cellular phones or anything like it.
The chips have a limited voltage range where they work properly. Too low voltage and they get too slow, too high voltage and they overheat or are destroyed through other effects.
Voltage regulators are used to make sure the chips are safe and working. Both linear and switch mode regulators are used. A lithium battery can never go down to 10% of nominal voltage, it would destroy it. It is completely drained at 70% of nominal voltage. That is enough that a linear voltage regulator can still supply modern chips with their working voltage. In some cases a switch mode regulator is more efficient for example on CPUs that use 1 V, you have a 2.7 V drop that would be wasted as heat in a linear regulator. The backlight is often several LEDs in series so can use a 12 V boost regulator to make sure the backlight is constant brightness.
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Most modern power-sensitive equipment no longer uses linear regulators as they are very inefficient - SMPSs are used instead. Linear regulators can only reduce the input voltage and in doing so shed heat, e.g. to get 5V @ 1A from a 12V input, you dissipate 7V @ 1A in the regulator, i.e. you lose 7W in heat to get 5W out, an efficiency of about 42% (12W in, 5W out). A good SMPS will have an efficiency of about 85 to 90% (or more), so to get 5W out you'll only need about 5.6W in, i.e. you only dissipate about 0.6W as heat, so 12x more efficient on the heat front and under half the battery consumption.
There are two key modes for SMPS systems - "buck" & "boost" - "buck" configuration is where you drop down the voltage (as above), and "boost" is where the voltage is stepped up, e.g. a 5V battery can generate 10V out of the SMPS.
Some smart SMPS designs can operate in both boost & buck modes, so when a battery is fully charged, they operate in buck mode, but as the battery discharges below the output voltage, they switch to boost mode.
In extremis, designs such as the "joule thief" can suck the last remnants out of a battery and still light LEDs etc.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
A 3.3V chip running from a 3.6 V battery through a linear regulator is more efficient than many switch mode regulators. Some CPUs have built in linear regulators, that makes the circuit cheaper and more efficient. Modern CPUs use very little power a lot of the time and it is impossible to make a a SMPS that is quite efficient from 1 uA to 250 mA, starts up instantly and has zero cost like a built in linear regulator.
A common type of chip that you can find in something like a MP3 player or simple cellular phone is a 32 bit STM32
So there are a lot of linear regulators in use, you might not even see them because they are hidden away inside the chips, modern chips run different parts at different voltages too to save power. Often both types are used in the same design to get optimal performance at a given cost.
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transistor modes of opperation in computer chips
