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LM3402 Datasheet(PDF) 11 Page - Texas Instruments |
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LM3402 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 39 page DMAX = tON tON + tOFF-MIN VO(max) = DMAX x VIN IL-MIN = 0.2 RSNS L - VO x tSNS fSW = VO 1.34 x 10 -10 x RON VO = n x VF + 200 mV LM3402/02HV CS VSNS IF RSNS IF + - VREF + - One-shot CS Comparator VO VF VF LED 1 LED n LM3402, LM3402HV www.ti.com SNVS450F – SEPTEMBER 2006 – REVISED OCTOBER 2015 Feature Description (continued) At the conclusion of tON the power MOSFET turns off for a minimum OFF-time, tOFF-MIN, of 300 ns. Once tOFF-MIN is complete the CS comparator compares VSNS and VREF again, waiting to begin the next cycle. Figure 17. Comparator and One-Shot The LM3402/HV regulators should be operated in continuous conduction mode (CCM), where inductor current stays positive throughout the switching cycle. During steady-state operation in the CCM, the converter maintains a constant switching frequency, which can be selected using Equation 2. (2) VF = forward voltage of each LED, n = number of LEDs in series (3) 7.3.2 Average LED Current Accuracy The COT architecture regulates the valley of ΔVSNS, the AC portion of VSNS. To determine the average LED current (which is also the average inductor current) the valley inductor current is calculated using Equation 4. (4) In Equation 4 tSNS represents the propagation delay of the CS comparator, and is approximately 220 ns. The average inductor/LED current is equal to IL-MIN plus one-half of the inductor current ripple, ΔiL: IF = IL = IL-MIN + ΔiL / 2 (5) Detailed information for the calculation of ΔiL is given in the Application and Implementation section. 7.3.3 Maximum Output Voltage The 300 ns minimum off-time limits on the maximum duty cycle of the converter, DMAX, and in turn ,the maximum output voltage VO(MAX) is determined by Equation 6: (6) Copyright © 2006–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3402 LM3402HV |
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