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3517HFE Datasheet(PDF) 9 Page - Linear Technology |
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3517HFE Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LT3517 9 3517fa APPLICATIONS INFORMATION than 1V, the LED current is: I V R LED CTRL SENSE = 10 • (2) When VCTRL is higher than 1V, the LED current is clamped to be: I mV R LED SENSE = 100 (3) The LED current programming feature possibly increases total dimming range by a factor of ten. The CTRL pin should not be left open (tie to VREF if not used). The CTRL pin can also be used in conjunction with a PTC thermistor to provide overtemperature protection for the LED load. For a buck or a buck-boost configuration, the output voltage is typically level-shifted to a signal with respect to GND as illustrated in the Figure 4. The output can be expressed as: V R R VV OUT BE Q =+ 1 2 101 2 •. () (5) 49.9k 3517 F02 45.3k 2V VREF 5k PTC CTRL Figure 2 Setting Output Voltage For a boost application, the output voltage can be set by selecting the values of R1 and R2 (see Figure 3) according to the following equation: V R R V OUT =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 1 2 11 01 •. (4) LT3517 FB R1 VOUT R2 3517 F03 Figure 3 LT3517 FB R1 Q2 + – LED ARRAY VOUT R2 3517 F04 RSENSE Figure 4 Inductor Selection The inductor used with the LT3517 should have a satura- tion current rating of 2A or greater. For buck mode LED drivers, the inductor value should be chosen to give a ripple current “ ΔI” of ~30% to 40% of the LED current. In the buck mode, the inductor value can be estimated using the formula: LµH Dt µs V V I D V V BUCK SW IN LED BUCK LED () = () = •( ) • – Δ IIN (6) VLED is the voltage across the LED string, VIN is the input voltage to the converter, and tSW is the switching period. In the boost configuration, the inductor can be estimated using the formula: LµH D t µs V I D VV V BOOST SW IN BOOST LED IN L () = = •( ) • – Δ E ED (7) |
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