Datasheet LT3517 (Analog Devices) - 9

LT3517
applications inForMation
the PWM high state. When the CTRL pin voltage is less For a buck or a buck-boost configuration, the output than 1V, the LED current is: voltage is typically level-shifted to a signal with respect to GND, as illustrated in the Figure 4. The output can be I VCTRL LED = (2) expressed as: 10 •R SENSE V R1 •1.01V+V When V OUT = BE(Q1) (5) CTRL is higher than 1V, the LED current is clamped R2 to be: + I 100mV R1 RSENSE LED = (3) VOUT RSENSE LED – ARRAY The LED current programming feature possibly increases LT3517 Q2 total dimming range by a factor of ten. FB R2 The CTRL pin should not be left open (tie to VREF if not 3517 F04 used). The CTRL pin can also be used in conjunction with a PTC thermistor to provide overtemperature protection
Figure 4
for the LED load.
Inductor Selection
2V VREF The inductor used with the LT3517 should have a satura- 45.3k 49.9k CTRL tion current rating of 2A or greater. For buck mode LED drivers, the inductor value should be chosen to give a 5k ripple current “ΔI” of ~30% to 40% of the LED current. PTC In the buck mode, the inductor value can be estimated 3517 F02 using the formula:
Figure 2
D L µ ( H) ( ) = BUCK • tSW(µs) • VIN – VLED (6) ΔI
Setting Output Voltage
VLED For a boost application, the output voltage can be set by DBUCK = V selecting the values of R1 and R2 (see Figure 3) according IN to the following equation: VLED is the voltage across the LED string, VIN is the input voltage to the converter, and tSW is the switching period. V ⎛ R1 ⎞ OUT = ⎜ +1⎟ • 1.01V (4) In the boost configuration, the inductor can be estimated ⎝R2 ⎠ using the formula: D V BOOST • tSW (µs) • VIN OUT L µ ( H) = (7) ΔI LT3517 R1 FB D VLED – VIN BOOST = R2 VLED 3517 F03
Figure 3
3517fh For more information www.linear.com/LT3517 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts