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Because it directly regulates output current, the LT is ideal for driving light emitting diodes LEDs whose light intensity is proportional to the current passing through them, not the voltage across their terminals. With an input voltage range of 1V to 10V, the device works from a variety of input sources. When the LT is placed in shutdown, the LEDs are disconnected from the output, ensuring a quiescent current of under 1?
A for the entire circuit. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. A 34 26 This is the collector of the internal NPN power switch. Minimize the metal trace area connected to this pin to minimize EMI. Tie this pin directly to local ground plane.
Connect the cathode of the bottom LED to this pin. This pin is also used to provide LED dimming. Tie this pin higher than 0. Bypass this pin with a capacitor to ground as close to the device as possible. Operation can be best understood by referring to the block diagram in Figure 1. At the start of each oscillator cycle, the SR latch is set, turning on power switch Q1. The signal at the noninverting input of the PWM comparator A2 is proportional to the switch current, summed together with a portion of the oscillator ramp.
When this signal reaches the level set by the output of error amplifier A1, comparator A2 resets the latch and turns off the power switch. In this manner, A1 sets the correct peak current level to keep the LED current in regulation.
Many different sizes and shapes are available. Consult each manufacturer for more detailed information and for their entire selection of related parts. As core losses at 1.
Choose an inductor that can handle at least 0. H inductor will be a good choice for most LT designs. Table 1. Recommended Inductors L? There are 2mm tall inductors currently available that provide a low DCR and low core losses that help provide good overall efficiency.
Inductors with a height of 1mm and less are becoming more common, and a few companies have introduced chip inductors that are not only thin, but have a very small footprint as well.
While these smaller inductors will be a necessity in some designs, their smaller size gives higher DCR and core losses, resulting in lower efficiencies. Figure 2 shows efficiency for the Typical Application circuit on the front page of this data sheet, with several different inductors. Keep this in mind when choosing an inductor. The value of inductance also plays an important role in the overall system efficiency.
While a 1? H inductor will have a lower DCR and a higher current rating than the 6. H version of the same part, lower inductance will result in higher peak currents in the switch, inductor and diode. Efficiency will suffer if inductance is too small. Figure 3 shows the efficiency of the Typical Application on the front page of this data sheet, with several different values of the same type of inductor Panasonic ELJEA. H value. H Figure 3. Because they have an extremely low ESR and are available in very small packages, multilayer ceramic capacitors are an excellent choice.
X5R and X7R type capacitors are preferred because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. F output capacitor is sufficient for most applications. Always use a capacitor with a sufficient voltage rating. Ceramic capacitors do not need to be derated do not buy a capacitor with a rating twice what your application needs. A 16V ceramic capacitor is good to more than 16V, unlike a 16V tantalum, which may be good to only 8V when used in certain applications.
Low profile ceramic capacitors with a 1mm maximum thickness are available for designs having strict height requirements.
Ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the LT F input capacitor is sufficient for most applications. Table 2 shows a list of several ceramic capacitor manufacturers.
Consult the manufacturers for detailed information on their entire selection of ceramic parts. Table 2. Table 3 shows several different Schottky diodes that work well with the LT Make sure that the diode has a voltage rating greater than the output voltage. The diode conducts current only when the power switch is 6 U turned off typically less than one-third the time , so a 0. Table 3. Table 4. The LT will then switch at its maximum duty cycle, generating an output voltage 10 to 15 times greater than the input voltage.
Without the zener, the SW pin could see more than 36V and exceed its maximum rating. The zener voltage should be larger than the maximum forward voltage of the LED string. Using this method, the LEDs can be dimmed and turned off completely using the same control signal. MAX — 1? If the LED current is less than or more than this value, the emitted light becomes more blue.
For color LCDs, this often results in a noticeable and undesirable blue tint to the display. The current through them alternates between full current and zero current, so the average current changes with duty cycle. This ensures that when the LEDs are on, they can be driven at the appropriate current to give the purest white light. This operation may be undesirable for some systems, as it may reflect some noise to the input source at the PWM frequency.
The solution is to filter the control signal by adding a 10k resistor and a 0. The 10k resistor minimizes the capacitance seen by the RSET pin. Dimming Using a Logic Signal For applications that need to adjust the LED brightness in discrete steps, a logic signal can be used as shown in Figure 6.
VMAX — 0. Burst Mode is a trademark of Linear Technology Corporation. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent radiation and high frequency resonance problems, proper layout of the high frequency switching path is essential. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize interplane coupling.
The signal path including the switch, output diode D1 and output capacitor C2, contains nanosecond rise and fall times and should be kept as short as possible. In addition, the ground connection for the RSET resistor should be tied directly to the GND pin and not be shared with any other component, ensuring a clean, noise-free connection.
Recommended component placement is shown in Figure 7. F TA03a 5 7. H D1 VIN 1. H D1 85 80 75 70 65 60 55 0 TA10a C1 4. H D1 VIN 2.
F ? GND 2 ?
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