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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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SP7601_EB Arkusz danych(PDF) 6 Page - Exar Corporation |
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SP7601_EB Arkusz danych(HTML) 6 Page - Exar Corporation |
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6 / 9 page  Sept25-07 Rev D SP7601 29V High Voltage LED Driver © 2007 Exar Corporation Page 6 GENERAL OVERVIEW Overcurrent Programming Resistor Rs can be used to program Overcurrent Protection (OCP). Use equation 2 for calculating the Rs value: () uA on Rds Iocp V Rs 33 ) ( 05 . 1 5 . 1 35 . 0 × × × − = .… (2) Where Iocp is the programmed overcurrent and is generally set 50% above nominal output current. The maximum value of Rs that can be used for programming OCP is 3kΩ. Inductor Selection Select the inductor L1 for inductance, IRMS and ISAT. Calculate inductance from () L I f Vin Vo Vin Vo L Δ × × − × = ………………………. (3) Where: VIN is converter input voltage Vo is converter output voltage. Since voltage across RSET is small, Vo approximately equals VF (for a string of series connected LEDs Vo equals total VF) L I Δ is inductor current ripple (nominally set to 10% of Io) Inductor ISAT and IRMS must allow sufficient safeguard over output current Io. As a rule of thumb these parameters should be 50% higher than Io. Where high efficiency is required, a low DCR inductor should be used. Input capacitor selection Select the input capacitor for capacitance and ripple current rating. Use the capacitances listed in table 1 as a starting point and, if needed, increase CIN. Table1- CIN selection Calculate the ripple current requirement from: ( ) D D Io Irip − × = 1 ………..………. (4) Where D is converter duty cycle: Vin Vo D = Ceramic capacitors are recommended for input filtering due to their low Equivalent Series Resistance (ESR), Equivalent Series Inductance (ESL) and small form factor. MOSFET selection Select the MOSFET M1 for ON resistance RDS(ON), voltage rating (BVDSS) and gate-to- drain charge (QGD). The recommended MOSFET voltage rating for 12V and 24V applications is 30V and 40V respectively. As a rule of thumb QGD should be less than 10nC. RDS(ON) must be selected such that, when operating at peak current and junction temperature, the Overcurrent threshold of the SP7601 is not exceeded. Allowing 50% margin for temperature coefficient of RDS(ON), 5% margin for inductor current ripple and 50% margin for OCP, the following expression can be used: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × × × ≤ Io mV ON Rds 05 . 1 5 . 1 5 . 1 350 ) ( ……….. (5) Within this constraint, selecting MOSFETs with lower RDS(ON) will reduce conduction losses at the expense of increased switching losses. If a choice of several MOSFETs is available, select the highest RDS(ON) MOSFET that meets the above criteria. Schottky Rectifier selection Select the Schottky D1 for Voltage rating VR and current rating If. Voltage rating should be selected using the same guidelines outlined for MOSFET voltage selection. Current rating can be calculated from: Io Vin Vo If × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ≥ 1 ……………………… (6) Io (A) Cin (uF) < 0.7 2.2 0.71 to 1.2 4.7 > 1.2 2 x 4.7 |
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