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LM3402HVMM Scheda tecnica(PDF) 10 Page - National Semiconductor (TI) |
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LM3402HVMM Scheda tecnica(HTML) 10 Page - National Semiconductor (TI) |
10 / 22 page Application Information (Continued) The LM3402/02HV regulators should be operated in continu- ous conduction mode (CCM), where inductor current stays positive throughout the switching cycle. During steady-state operationin the CCM, the converter maintains a constant switching frequency, which can be selected using the follow- ing equation: V F = forward voltage of each LED, n = number of LEDs in series AVERAGE LED CURRENT ACCURACY The COT architecture regulates the valley of ∆V SNS, the AC portion of V SNS. To determine the average LED current (which is also the average inductor current) the valley induc- tor current is calculated using the following expression: In this equation t SNS represents the propagation delay of the CS comparator, and is approximately 220 ns. The average inductor/LED current is equal to I L-MIN plus one-half of the inductor current ripple, ∆i L: I F =IL =IL-MIN + ∆i L /2 Detailed information for the calculation of ∆i L is given in the Design Considerations section. MAXIMUM OUTPUT VOLTAGE The 300 ns minimum off-time limits on the maximum duty cycle of the converter, D MAX, and in turn ,the maximum output voltage V O(MAX) is determined by the following equa- tions: The maximum number of LEDs, n MAX, that can be placed in a single series string is governed by V O(MAX) and the maxi- mum forward voltage of the LEDs used, V F(MAX), using the expression: At low switching frequency the maximum duty cycle and output voltage are higher, allowing the LM3402/02HV to regulate output voltages that are nearly equal to input volt- age. The following equation relates switching frequency to maximum output voltage. 20192105 FIGURE 1. Comparator and One-Shot www.national.com 10 |
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