Help us learn something new. I definitely appreciate all you do around here, I guess I'm just used to my real life where people specifically do NOT want to know the boring nerdy details of how I fix stuff. Anyways here we go, full version. First of all the BD is going to have issues running at ma.
While a good device for the original circuit, Its gain drops drastically after ma and could be as low as at ma down from 80 or so originally. We need a higher gain transistor or a smaller resistor or both. The second problem is what actually happens when the overcurrent protection kicks in.
When the voltage across the 1. This limits the current through the leds by increasing the voltage drop across the left BD The Meanwell driver will try to compensate to make the circuit "take" the full ma by pegging its voltage output to the maximum it can. When you scale it up by a factor of 3 like we are planning to there is a potential for a whole lot of heat when the protection kicks in.
The two transistors should be screwed down right next to each other on a large heatsink such as the one your LEDs are on with an isolating thermal pad. A quick mouser search found me the KSD, which looks like it should work much better at this current. Also it is in an insulated package so it can be mounted straight to the heatsink without an isolating pad, just some good thermal grease.
It should have a gain of 65 or so, which means for ma we want around 17ma of base current. The other resistors need to drop just under. Thanks for explaining I understand the purpose of using a current mirror in parallel strings of LEDs, and the general purpose of each component.
I lack the "tools" to determine the accurate components. Does this change things? Thanks for your explantion. I'm also abit lost as I'm new in this industry, however, I would like to try out! Thanks again! Thank you for the explanaition.
This makes a lot more sense now that it is explained out completely, and I see the flaws with it. It's still useable, but definitely not as simple as the author of the article made it out to be. It can still be implemented effectively, but adding it to the heatsink adds a little more headache to the matter. Npain, if you are going to run that driver, use the values of components that cptbjorn listed at the end of his post.
This will set your max current to a little over 1A. Make sure the resistors are sized correctly. The 0. It will work best with 4 or more parallel strings though as a single failed LEDs current is spread over the remaining 3 good ones.
Wouldn't work with just 2 strings and 3 is cutting it close especially for the royal blue XR-Es as I have yet to find any data on how they do at ma. The version with the series resistors doesn't make much sense as only the first parallel set of LEDs will "see" the resistors. In order to balance current mismatches using resistors you would need a smaller series resistor on each individual LED.
This shouldn't be necessary though, as I assume the Vf's of a group of LEDs from the same run will be close enough to not need it. And if they DO have large Vf variances it would be much more effective to match the LEDs by their Vf values into groups of 4 or however many parallel strings you are doing.
All it would take is a LM on a small heatsink, a 1. The schematic is in the LM datasheet, basically all you do is run each individual LED at ma-1A and measure the voltage across it, then either make piles or stick post-it notes. I had a pm asking how to adapt this current mirror to run 2 LED strings in parallel from one of the 1.
This circuit is named for its inventor, Robert Widlar, and was patented in The Widlar circuit may be used with bipolar transistors or MOS transistors. An example application is in the now famous uA operational amplifier, and Widlar used the circuit in many of his designs. The key to this circuit is that the voltage drop across the resistor R 2 subtracts from the base-emitter voltage of transistor Q 2 , thereby reducing the collector current compared to transistor Q 1.
A simulation plot showing this reduction in I C2 is presented in figure This observation is expressed by using KVL around the base emitter loop of the circuit in Figure Suppose we want to create a uA output current from a uA input current as in the simulation plot of figure V T is 26mV times ln 3 is This equation makes the approximation that the currents are both much larger than the saturation currents I S1 , I S2 , an approximation valid except for very low current levels.
In the following the distinction between the two scale currents is dropped, although the difference can be important, for example, if the two transistors are designed with different emitter areas. An important property of a current source is its small signal incremental output impedance, which should ideally be infinite. The emitter degeneration resistance introduces local current feedback for transistor Q 2. Any increase in the current in Q 2 increases the voltage drop across R 2 , reducing the V BE for Q 2 , thereby countering the increase in current.
This feedback means the output impedance of the circuit is increased, because the feedback involving R 2 forces use of a larger voltage to drive a given current. Output resistance is found using a small-signal model for the circuit, shown in Figure The transistor Q 1 is replaced by its small-signal emitter resistance r E because it is diode connected.
In a diode-connected transistor the collector is short-circuited to the base, so the transistor collector-base junction has no time-varying voltage across it. The transistor Q 2 is replaced with its hybrid-pi model. A test current I x is attached at the output. Using the figure, the output resistance is determined using Kirchhoff's laws.
Using Kirchhoff's voltage law from the ground on the left to the ground connection of R 2 :. Using Kirchhoff's voltage law from the ground connection of R 2 to the ground of the test current:. According to Eq. The output transistor carries a low current, making r p large, and increase in R 2 tends to reduce this current further, causing a correlated increase in r p. Therefore, a goal of R 2 » r p can be unrealistic, and further discussion is provided below.
When designing a circuit it is important to take into account the wide variation in certain device values from one to another. A central objective of the designer is to desensitize the circuit to these variations to produce a circuit which meets the specifications across all possible conditions.
One aspect of design which is common to nearly all circuits is the establishment of stable bias or operating point levels. This seemingly minor portion of a design can provide the most challenging and interesting circuit problems.
Many bias generators are centered around the generation of currents to operate the core of the circuit. Current generated from simple resistors and diodes or diode connected transistors connected across the power supply will vary approximately proportional to the variation of the supply voltage.
This variation in the resulting bias current is frequently undesirable. This is to introduce another kind of current mirror, actually a stabilized current source, which has an output which had been desensitized to variation in input current.
To understand this configuration, it is helpful to examine the behavior of a zero gain amplifier. A NMOS version is shown in figure For a given drain current, if the drain resistor R L is set equal to r s then the gain A will be minus 1.
If we now connect the gate to the top of resistor R L as in figure A zero gain amplifier made using an enhancement mode NMOS 2N transistor was simulated where the small signal AC gain and phase was calculated as the drain current was swept.
As can be seen in figure This also occurs at the point where the phase makes a sharp transition from 0 degrees to degrees. Now that we understand the concept of the zero gain amplifier, the objective is to investigate its use to produce an output current which is stabilized less sensitive to variations of the input current level. This current source configuration, figure Because the collector voltage V C of transistor Q 1 is now more constant with changes in the input supply voltage as represented by V IN , it can be used as the base voltage of Q 2 to produce a much more constant collector current in transistor Q 2.
More background on current mirrors can be found in this Wikipedia page. Return to Previous Chapter. Go to Next Chapter. Return to Table of Contents. Analog Devices Wiki. Analog Devices Wiki Resources and Tools. Quick Start Guides. Linux Software Drivers.
Microcontroller Software Drivers. ACE Software. Technical Guides. Education Content. Consequently, the current mirror can generate relatively accurate current output. Each of the LED diode strings - 1 ,.
The LED driver circuits, - 1 ,. In some embodiments, the LED system includes an optional feedback voltage generator that is configured to generate a feedback voltage for the boost regulator based on the output currents, iLED 1 , iLED 2 ,. Although not shown in FIG. The frequency chopping unit is configured to in perform frequency chopping to reduce or even remove the input offset of the error amplifier.
The chopping frequency of the frequency chopping unit can be the same as the switching frequency of the boost regulator such that the noise generated by the frequency chopping unit is synchronized with the switching noise of the boost regulator, which can be filtered out together if necessary. The constant-current LED driver IC device may include more or less circuit components to realize more or less functionalities.
Although specific embodiments of the invention that have been described or depicted include several components described or depicted herein, other embodiments of the invention may include fewer or more components to implement less or more features.
In addition, although specific embodiments of the invention have been described and depicted, the invention is not to be limited to the specific forms or arrangements of parts so described and depicted. The scope of the invention is to be defined by the claims appended hereto and their equivalents.
All rights reserved. Login Sign up. Search Expert Search Quick Search. United States Patent Embodiments of a current mirror for a constant-current light-emitting diode LED driver system and a constant-current LED driver integrated circuit IC device having the current mirror are described. Each of the at least one current mirror cell includes semiconductor circuits configured to generate an output current based on a reference current and a control module configured to alternately and continuously charge the semiconductor circuits in response to non-overlapping clock signals.
Click for automatic bibliography generation. NXP B. Eindhoven, NL. Download PDF CNU Accurate dimming circuit having low quiescent current EP Low noise apparatus for receiving an input current and producing an output current which mirrors the input current.
Kaur, Parneet et al. What is claimed is: 1. The current mirror of claim 1, wherein when one of the first and second semiconductor circuits is charged by the reference current, the other one of the first and second semiconductor circuits generates the output current.
The current mirror of claim 2, wherein the output current is equal to the reference current over process, voltage, and temperature variations.
The current mirror of claim 1, wherein each of the first and second sets of switches are configured to be controlled by a first clock signal and a second clock signal, and wherein the first clock signal does not overlap with the second clock signal. The current mirror of claim 4, wherein the first semiconductor circuit comprises a first PMOS device, and wherein the second semiconductor circuit comprises a second PMOS device.
The current mirror of claim 5, wherein the first control circuit comprises: a first switch connected between a gate terminal of the first PMOS device and a drain terminal of the first PMOS device; a second switch connected between the drain terminal of the first PMOS device and the reference current signal path; and a third switch connected between the drain terminal of the first PMOS device and the current output signal path.
The current mirror of claim 6, wherein the second control circuit comprises: a fourth switch connected between a gate terminal of the second PMOS device and a drain terminal of the second PMOS device; a fifth switch connected between the drain terminal of the second PMOS device and the reference current signal path; and a sixth switch connected between the drain terminal of the second PMOS device and the current output signal path. The current mirror of claim 6, wherein the first, second, and sixth switches are configured to be controlled by the first clock signal, and wherein the third, fourth, and fifth switches are configured to be controlled by the second clock signal.
A constant-current LED driver integrated circuit IC device comprising the current mirror of claim 1, a reference current generator, and a plurality of LED driver circuits. The constant-current LED driver IC device of claim 9, wherein each of the LED driver circuits comprises a plurality of resistors, an error amplifier, and a plurality of switches.
The constant-current LED driver IC device of claim 10, wherein the error amplifier comprises a frequency chopping unit configured to perform frequency chopping to reduce an input offset of the error amplifier. A constant-current light-emitting diode LED driver integrated circuit IC device comprising: a current mirror comprising a plurality of current mirror cells, wherein each of the current mirror cells comprises: a first PMOS device and a second PMOS device configured to generate an output current based on a reference current; and a control module configured to alternately and continuously charge the first and second PMOS devices in response to a plurality of non-overlapping clock signals; a reference current generator configured to generate the reference current; and a plurality of LED driver circuits configured to generate LED driving currents based on output currents generated by the current mirror.
The constant-current LED driver IC device of claim 13, wherein each of the LED driver circuits comprises a plurality of resistors, an error amplifier, and a plurality of switches.
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