LM is an adjustable voltage regulator IC used for shunt regulator, current regulator, and temperature controller. The LM LMC is a voltage regulator designed primar- ily for series regulator applications By itself it will supply output currents up to mA but external. Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. 2All trademarks are the property of their respective.
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The LM is a classic, now available for 50 continuous years and still going strong. Naturally, it is frowned upon by a lot of designers that are just half as old.
They see this chip as an old-age part that has no merits today. If we pragmatically summarize the pros and cons of this methusalem, we end up with something like this:.
So, in case you have decided for an LM to make a good power supply, there are some rules of thumb that will make your life easier:. Click here for a larger resolution picture ….
A lot of basic application circuits can be found in the LM datasheef. The net and some manufacturers application notes are also a good source of information.
A Collection of Proper Design Practices using the LM723 IC Regulator
Its a self-heated reference. When you ask for the same basic parameters 40V, mA, … there is simply no better IC today. There are only better ones with low voltage 10V and lessand they cost a lot more than a LM Its comparing apples and oranges. The LM is not discontinued at all, sold by TI in the millions, available from all electronic distributors and dirt cheap.
The performance of an LM is not bad at all, especially for noise and long term stability. Which comparable ICs are so much better now? First, ST went out of the LM business. They dont make this part anymore. As I found out and others did as well their consumer parts where a bit drifty within spec limitsprobably by a conversion to a new manufacturing process with not just improvements as a consequence. For your bread-and-butter PSU, they are still fine and noise is quite low.
Both have been making s for decades, and after the sale TI took over the National Semiconductor designation of LM and used their own of uA From a power supply manufacturer I read some email correspondence with TI tech support where he also claimed more drift in recent parts. After some ping-pong TI recommended the MIL-spec part and guaranteed that this would be free from drift. You are commenting using your WordPress. You are commenting using your Twitter account.
You are commenting using your Facebook account. Notify me of new comments via email. A schematic with all suggestions implemented can be seen at the bottom of this page. I added a reference study that compares voltage regulator noise. The interior design of an LM looks like this: On a transistor level, you can see some design tricks by the legendary Bob Widlar: If we pragmatically summarize the pros and cons of this methusalem, we end up with something like this: No wonder after 50 years, it is known what you may expect and what not.
Relying on a buried Zener low noise reference, it can compete with a lot of newer chips when it comes to stability it does have a long term stability rating in the datasheet, few new chips have that. Tempco is also very good, also when compared to newer chips.
One of the best low noise regulators even today when hooked up properly. Reason one is the low noise buried Zener reference, reason two is the fact that the reference voltage can be noise filtered before going into the error amp. It can be found here. Input voltage goes up tp 40V constant, with 50V pulses allowed. There is even an obsolete 80V version, the LCB. Support for external pass transistorskeeping on-chip dissipation and therefore drift low.
Compensation by the user. You can tailor the frequency response to your application but you also have to do that because there is no internal compensation. Can serve as positive, negative, shunt, pass, floating or switching regulator or as a temperature controller.
Extended Temperature Lm72 available. People used to 3-terminal regulators are in for a culture shock caused by the amount of design aspects to consider.
Not a very high PSRR. The current limiting feature is implemented as a transistor that is turned on by the voltage drop on a current sensing resistor. Current limiting sensitive to overload. If you have a heavily overcompensated i.
Minimum dropout voltage is ca. This is due to the 7. If you need to regulate lower voltages, the LM chip itself needs a separate power input at 10V minimum. Minimum regulated output voltage is ca.
With less accurracy, you may go below that, but not by much. Moderate error amplifer gain. We have only a factor of ca. The LM values translate to about 0. Error amp bias current. The superbeta bipolars in the error amp have a low, but nonzero bias current that can cause voltage shifts of a few millivolts across voltage divider or RC filter resistors.
LM Datasheet(PDF) – National Semiconductor (TI)
Intolerant to missing or misdesigned compensation. Yes, it will oscillate if you forget or undercompensate. Some extra bleeders are needed to make falling edge output regulations fast enough. Gate parasitic suppression resistors of a few Ohms are also a good idea. No microscopic SMD cases available. This also acts as a soft start feature. Take all networks into account, and make sure that the reference divider and the output sampling network present the same impedance to the error amplifier inputs.
This cancels the error amps base bias currents. The collector voltage Vc driving the output does not need to be stabilized. If you use current limiting, protect the base of the limiting transistor by a 1K, e. Although the LM can dissipate a few mW and deliver up to mA, better use an external pass transistor to do the hard work and keep the regulator chip cool. A PNP pass transistor can further improve noise performance.
Provide enough voltage headroom. This wastes some power, but speed, noise and other performance criteria are a lot better with a few volts across the pass transistor. Before you specify a compensation cap, run a few tests using SPICE to see if gain and phase margins are in order.
National Semiconductor – datasheet pdf
Try with different output voltages, load currents and input voltages. Especially when driving MOSFETs, you might need a bleeder resistor to make the falling edges fast enough for your needs. Minimum Output for Floating Regulators.
The datasheet is not very instructive on this, but the floating regulator variants will fail to regulate when the load resistance becomes to high. The load must be able to drain at least the supply current of the circuit without exhibiting a voltage higher than the desired output voltage. If you want precision current control, dont try to do this with a pot sampling a fraction of dataeheet across a shunt measurement resistor. The curves of the limiting transistors Vbe voltages over temperature in the LM datasheet should datwsheet it absolutely clear that no precision can be expected here.
The way to go is either high side sensing with an op amp that can do that, or a floating design like the famous Elektor one from 1 x LM, 2 x LM A schematic where all this has been implemented can be see below: Click here for a larger resolution picture … A lot of basic application circuits can be found in the LM datasheet.
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