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LM3017 Datenblatt(PDF) 11 Page - Texas Instruments |
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LM3017 Datenblatt(HTML) 11 Page - Texas Instruments |
11 / 34 page LM3017 www.ti.com SNOSC66C – MARCH 2012 – REVISED MARCH 2013 CURRENT LIMIT AND SHORT CIRCUIT PROTECTION Boost Mode The LM3017 implements current limit protection by controlling the pass FET, Q2. In boost mode the LM3017 features both cycle-by-cycle current limit and short circuit protection. Unlike most boost regulators, the LM3017 can protect itself from short circuits on the output by shutting off the pass FET. The boost current limit, defined by VCL=170mV in the electrical characteristics table, turns off the boost FET for normal overloads on a cycle-by-cycle basis. The current is limited to VCL/RSEN until the overload is removed. Should the output be shorted, or otherwise pulled below VIN, the inductor current will have a tendency to “run- away”. This is prevented by the short circuit protection feature, defined as VSC = 200mV in the electrical characteristics table. When this current limit is tripped, the current is limited to VSC/RSEN by controlling the pass FET. If the short persists for TSC > 450µs the pass FET will be latched off. In this way, the current is limited to VSC/Rsen until the short is removed or the time of TSC = 450µs is completed. Pulling the EN/MODE pin low (<0.4V, typ) is required to reset this short circuit latch-off mode. The delay of TSC = 450µs helps to prevent nuisance latch-off during a momentary short on the output. Start-up Boost Mode During start-up in boost mode, peak inductor current may be higher compared to normal operation. To allow for this, current limit levels and timing are different during startup. The current limit is defined by VLIM2 = 100mV (typ.) in the electrical characteristics table, for the first TLIM2 = 3.6ms (typ.). The current is limited to VLIM2/RSEN, for this period . Once the TLIM2 = 3.6ms (typ.) timer has finished, the current limit is increased to VSC = 200mV (typ.). For the first TLIM2 = 3.6ms (typ.) of the start-up, the latch-off feature is not enabled, however the current will always be limited to VLIM2/RSEN. This allows the part to start-up normally. If the current limit is still tripped at the end of TLIM2 = 3.6ms (typ.) , the TSC = 900µs (typ.) timer is started. Once the TSC = 900µs (typ.) time has expired, the pass FET (Q2) is latched off. This gives a total current-limited time of TSC + TLIM2 = 4.05ms (typ.), in cases where the LM3017 is started into a short circuit at the output. Pass-Through Mode In pass-through mode the power path is protected from shorts and overloads by the current limit defined as VLIM1 = 85mV (typ.) in the electrical characteristics table. When this current limit is tripped, the current is limited to VLIM1/RSEN by controlling the pass FET. If the short persists for TLIM1 > 900µs (typ.) the pass FET (Q2) will be latched off. In this way, the current is limited to VLIM1/RSEN until the short is removed or the time of TLIM1 = 900µs (typ.) is completed. Pulling the EN/MODE pin low (0.4V, typ.) is required to reset this latch-off mode. Start-up Pass-Through Mode During start-up in pass mode, the current limit is defined by VLIM2 = 100mV (typ.) in the electrical characteristics table, for the first TLIM2 = 3.6ms (typ.). The current is limited to VLIM2/RSEN, for this period . Once the TLIM2 = 3.6ms (typ.) timer has finished, the current limit is reduced to VLIM1 = 85mV (typ.). For the first TLIM2 = 3.6ms (typ.) of the start-up, the latch-off feature is not enabled, however the current will always be limited to VLIM2/RSEN. This higher limit allows the part to start-up normally. If the current limit is still tripped at the end of TLIM2 = 3.6ms (typ.), the TLIM1 = 900µs (typ.) timer is started. Once the TLIM1= 900µs time has expired, the pass FET(Q2) is latched off. This gives a total current-limited time of TLIM1+ TLIM2 = 4.5ms (typ.), in cases where the LM3017 is started into a short circuit at the output. Copyright © 2012–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links :LM3017 |
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