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LM1771 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LM1771 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 31 page LM1771 www.ti.com SNVS446D – JUNE 2006 – REVISED JANUARY 2016 Table 2. Frequency vs VOUT for Variants TIMING OPTIONS (1) VOUT 500 ns 1000 ns 2000 ns 0.8 485 242 121 1 606 303 152 1.2 727 364 182 1.5 909 455 227 1.8 1091 545 273 2.5 1515 758 379 3.3 2000 1000 500 (1) Switching Frequency (kHz) of LM1771 based on output voltage and timing option. 7.3.2 Short-Circuit Protection The LM1771 has an internal short-circuit comparator that constantly monitors the feedback node (except during soft-start). If the feedback voltage drops below 0.55 V (equivalent to the output voltage dropping below 68% of nominal), the comparator trips causing the part to latch off. The LM1771 does not resume switching until the input voltage is taken below the UVLO threshold and then brought back into its normal operating range, or the part is disabled then re-enabled through the enable pin. The purpose of this function is to prevent a severe short circuit from causing damage to the application. Due to the fast transient response of the LM1771 a severe short on the output causing the feedback to drop would only occur if the load applied had an effective resistance that approaches the PMOS RDS(ON). 7.3.3 Precision Enable The LM1771 features a precision enable circuit. If the voltage on the EN pin is 1.2 V or greater, the part is enabled and switching occurs. If the enable voltage falls below 1.2 V, the part is placed into a shutdown state and the drivers is tri-stated. This allows the LM1771 to be easily sequenced using a resistive divider from the output of another regulator, or the working input voltage range of the LM1771 to be set using a resistive divider on VIN. There is no internal pullup connected to the EN pin, so an external signal is required to initiate switching. NOTE When power is first applied to the LM1771, there is a slight delay before the enable comparator is functional. During this delay, typically on the order of 400 µs, the part is disabled regardless of the voltage on the EN pin. The falling enable threshold features 50 mV of hysteresis 7.3.4 Soft-Start To limit in-rush current and allow for a controlled start-up the LM1771 incorporates an internal soft-start scheme. Every time the enable voltage rises above 1.2 V while VIN is greater than the UVLO threshold, the LM1771 goes through an adaptive soft-start that limits the ON-time and expands the minimum OFF-time. In addition the part only activates the PMOS allowing a discontinuous mode of operation enabling a prebiased start-up. The time spent in soft-start depends on the load applied to the output, but is usually close to a set time that is dependent on the timing option. The approximate soft-start time can be seen in Table 3 for each timing option. Table 3. Soft-Start Timing for All Variants PRODUCT ID TIMING TSS LM1771S 0.5 µs 1 ms LM1771T 1.0 µs 1.2 ms LM1771U 2.0 µs 1.8 ms Copyright © 2006–2016, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM1771 |
Số phần tương tự - LM1771_16 |
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Mô tả tương tự - LM1771_16 |
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