It combines a traditional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source. Programmable switching frequency allows optimization of the external components for ef? The switching frequency of LT can be synchronized to an external clock signal. The LED current is externally programmable with a mV sense resistor. The CTRL pin provides further dimming ratio.
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Dt Sheet. LINER It combines a traditional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source. The switching frequency of the LT can be synchronized to an external clock signal. The LED current is externally programmable with a mV sense resistor. The CTRL pin provides further dimming ratio. The LT provides a complete solution for both constant-voltage and constant-current applications. Patent Pending. All other trademarks are the property of their respective owners.
Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Minimize trace at this pin to reduce EMI. Must be locally bypassed. Tie to 1. Do not leave this pin open. Tie an external clock signal here.
Tie the SYNC pin to ground if this feature is not used. SS: Soft-Start Pin. Place a soft-start capacitor here. Leave the pin open if not in use. Signal low turns off channel, disables the main switch and makes the TG pin high. There is an equivalent 50k resistor from PWM pin to ground internally.
Stabilize the loop with an RC network or compensating C. Works as overvoltage protection for LED drivers. If FB is higher than 1V, the main switch is turned off. There is an equivalent 40k resistor from TGEN pin to ground internally. Also serves as positive rail for TG pin driver. Leave TG unconnected if not used. Ground: Exposed Pad. Solder paddle directly to ground plane.
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, which turns on the Q1 power switch.
A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A4. When this voltage exceeds the level at the negative input of A4, the SR latch is reset, turning off the power switch.
A3 has two inputs, one from the voltage feedback loop and the other one from the current loop. Whichever feedback input is lower takes precedence, and forces the converter into either constant-current or constant-voltage mode.
The LT is designed to transition cleanly between these two modes of operation. Also, all internal loads on the VC pin are disabled so that the charge state of the VC pin will be saved on the external compensation capacitor. This feature reduces transient recovery time. When the PWM input again transitions high, the demand current for the switch returns to the value just before PWM last transitioned low.
To make this method of current control more accurate, the switch demand current is stored on the VC node during the quiescent phase. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch is used in the LED current path to prevent the output capacitor from discharging in the PWM signal low phase. Maximum PWM period is determined by the system and is unlikely to be longer than 12ms.
Table 1. A ceramic capacitor is usually the best choice. For the LED boost and buck-boost mode applications, a 2. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Table 2 shows some recommended capacitor vendors. Table 3. Schottky Diodes Table 2. The external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop stability. The high speed operation of the LT demands careful attention to board layout and component placement.
It is crucial to achieve a good electrical and thermal contact between the Exposed Pad and the ground plane of the board. Since there is a small DC input bias current to the ISN and ISP inputs, resistance in series with these inputs should be minimized and matched, otherwise there will be an offset.
Diode Selection Soft-Start The Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage. If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Table 3 has some recommended component vendors.
For many applications, it is necessary to minimize the inrush current at start-up. A typical value for the soft-start capacitor is 0. Both methods require a resistor connected at RT pin. Do not leave the RT pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. See Table 4 below or see the Oscillator Frequency vs RT graph in the Typical Performance Characteristics for resistor values and corresponding switching frequencies.
Table 4. Thermal Considerations The LT is rated to a maximum input voltage of 30V for continuous operation, and 40V for nonrepetitive one second transients. Careful attention must be paid to the internal power dissipation of the LT at higher input voltages to ensure that the maximum junction temperature is not exceeded. This junction limit is especially important when operating at high ambient temperatures.
The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should then be connected to an internal copper ground plane with thermal vias placed directly under the package to spread out the heat dissipated by the LT 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. Open as PDF.
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