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.
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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. 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.
C to ? C LTI C LTH Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTE is guaranteed to meet performance speci? The LTH is guaranteed over the full —40? C operating junction temperature range. Operating lifetime is derated at junction temperatures greater than ?
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. Continuous operation above the speci? 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. Synchronization power switch turn-on occurs a? 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. VC: gm Error Ampli? 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. The inverting input to the current sense ampli? The noninverting input to the current sense ampli? 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. The level at the negative input of A4 is set by the error ampli? 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. The current sense ampli? The output of A1 is simply an ampli?
In this manner, the error ampli? If the error ampli? The FB voltage loop is implemented by the ampli? When the voltage loop dominates, the error ampli? When the PWM pin is low, switching is disabled and the error ampli? 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. In the buck mode, the inductor value can be estimated using the formula: L? In the boost con? Table 1. A ceramic capacitor is usually the best choice. In the buck mode con? D 8 To achieve the same LED ripple current, the required? For the LED boost and buck-boost mode applications, a 2. Use only ceramic capacitors with X7R, X5R or better dielectric as they are best for temperature and DC bias stability of the capacitor value.
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.
Always check that the voltage rating of the capacitor is suf? Table 2 shows some recommended capacitor vendors. Table 2. Output Capacitor Selection The selection of output capacitor depends on the load and converter con? Loop Compensation The LT uses an internal transconductance error ampli? The external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability. The compensation resistor and capacitor at VC are selected to optimize control loop stability.
Diode Selection 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. Therefore, choose the Schottky diode with suf? Table 3 has some recommended component vendors. Table 3. 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. Soft-Start For many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit signi?
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. Selection of a higher switching frequency will allow use of smaller value external components and yield a smaller solution size and pro? Thermal Considerations The LT is rated to a maximum input voltage of 30V for continuous operation, and 40V for nonrepetitive one second transients.
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