поискавой системы для электроныых деталей |
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LM3405 датащи(PDF) 10 Page - Texas Instruments |
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LM3405 датащи(HTML) 10 Page - Texas Instruments |
10 / 29 page LM3405 SNVS429B – OCTOBER 2006 – REVISED MAY 2013 www.ti.com ENABLE MODE / SHUTDOWN MODE The LM3405 has both enable and shutdown modes that are controlled by the EN/DIM pin. Connecting a voltage source greater than 1.8V to the EN/DIM pin enables the operation of LM3405, while reducing this voltage below 0.4V places the part in a low quiescent current (0.3µA typical) shutdown mode. There is no internal pull-up on EN/DIM pin, therefore an external signal is required to initiate switching. Do not allow this pin to float or rise to 0.3V above VIN. It should be noted that when the EN/DIM pin voltage rises above 1.8V while the input voltage is greater than UVLO, there is a finite delay before switching starts. During this delay the LM3405 will go through a power on reset state after which the internal soft-start process commences. The soft-start process limits the inrush current and brings up the LED current (IF) in a smooth and controlled fashion. The total combined duration of the power on reset delay, soft-start delay and the delay to fully establish the LED current is in the order of 100µs (refer to Figure 23). The simplest way to enable the operation of LM3405 is to connect the EN/DIM pin to VIN which allows self start- up of LM3405 whenever the input voltage is applied. However, when an input voltage of slow rise time is used to power the application and if both the input voltage and the output voltage are not fully established before the soft- start time elapses, the control circuit will command maximum duty cycle operation of the internal power switch to bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.205V, the duty cycle will have to reduce from the maximum value accordingly, to maintain regulation. It takes a finite amount of time for this reduction of duty cycle and this will result in a spike in LED current for a short duration as shown in Figure 18. In applications where this LED current overshoot is undesirable, EN/DIM pin voltage can be delayed with respect to VIN such that VIN is fully established before the EN/DIM pin voltage reaches the enable threshold. This delay can be implemented by a simple Ra-Ca network as shown in Figure 19. The effect of adding this Ra-Ca network on the LED current is shown in Figure 20. For a fast rising input voltage (200µs for example), there is no need to delay the EN/DIM signal since soft-start can smoothly bring up the LED current as shown in Figure 21. Figure 18. Startup Response to VIN with 5ms rise time Figure 19. EN/DIM delayed with respect to VIN 10 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated Product Folder Links: LM3405 |
Аналогичный номер детали - LM3405 |
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Аналогичное описание - LM3405 |
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