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LM3402HV Datasheet(PDF) 11 Page - Texas Instruments |
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LM3402HV Datasheet(HTML) 11 Page - Texas Instruments |
11 / 36 page TSW 300 ns VO(MIN) = VIN x TSW TSW - 300 ns VO(MAX) = VIN x TSW = 1/fSW nMAX = VF(MAX) VO(max) - 200 mV DMAX = tON tON + tOFF-MIN VO(max) = DMAX x VIN IL-MIN = 0.2 RSNS L - VO x tSNS LM3402, LM3402HV www.ti.com SNVS450D – SEPTEMBER 2006 – REVISED FEBRUARY 2010 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 the following expression: (4) In this equation 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 Design Considerations section. 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 the following equations: (6) The maximum number of LEDs, nMAX, that can be placed in a single series string is governed by VO(MAX) and the maximum forward voltage of the LEDs used, VF(MAX), using the expression: (7) 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 voltage. The following equation relates switching frequency to maximum output voltage. (8) MINIMUM OUTPUT VOLTAGE The minimum recommended on-time for the LM3402/02HV is 300 ns. This lower limit for tON determines the minimum duty cycle and output voltage that can be regulated based on input voltage and switching frequency. The relationship is determined by the following equation: (9) HIGH VOLTAGE BIAS REGULATOR The LM3402/02HV contains an internal linear regulator with a 7V output, connected between the VIN and the VCC pins. The VCC pin should be bypassed to the GND pin with a 0.1 µF ceramic capacitor connected as close as possible to the pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical) and then regulates at 7V as VIN increases. Operation begins when VCC crosses 5.25V. INTERNAL MOSFET AND DRIVER The LM3402/02HV features an internal power MOSFET as well as a floating driver connected from the SW pin to the BOOT pin. Both rise time and fall time are 20 ns each (typical) and the approximate gate charge is 3 nC. The high-side rail for the driver circuitry uses a bootstrap circuit consisting of an internal high-voltage diode and an external 10 nF capacitor, CB. VCC charges CB through the internal diode while the power MOSFET is off. When the MOSFET turns on, the internal diode reverse biases. This creates a floating supply equal to the VCC voltage minus the diode drop to drive the MOSFET when its source voltage is equal to VIN. Copyright © 2006–2010, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3402 LM3402HV |
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