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LT3956EUHE-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3956EUHE-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3956 3956f applicaTions inForMaTion 20mV should not cause misoperation, but may lead to noticeable offset between the average value and the user- programmed value. Output Current Capability An important consideration when using a switch with a fixed current limit is whether the regulator will be able to supply the load at the extremes of input and output voltage range. Several equations are provided to help determine this capability. Some margin to data sheet limits is included. For boost converters: I A V V OUT MAX IN MIN OUT MAX ( ) ( ) ( ) . ≤ 2 5 For buck mode converters: IOUT(MAX) ≤ 2.5A For SEPIC and buck-boost mode converters: I A V V V OUT MAX IN MIN OUT MAX IN MIN ( ) ( ) ( ) ( ) . ( ) ≤ + 2 5 These equations assume the inductor value and switch- ing frequency have been selected so that inductor ripple current is ~600mA. Ripple current higher than this value will reduce available output current. Be aware that current limited operation at high duty cycle can greatly increase inductor ripple current, so additional margin may be required at high duty cycle. If some level of analog dimming is acceptable at minimum supplylevels,thentheCTRLpincanbeusedwitharesistor divider to VIN (as shown on page 1) to provide a higher output current at nominal VIN levels. FB LT3956 VOUT R4 3956 F02 R3 Figure 2. Feedback Resistor Connection for Boost or SEPIC LED Drivers FB LT3956 100k VOUT COUT R4 3956 F03 R3 LED ARRAY RLED + – Figure 3. Feedback Resistor Connection for Buck Mode or Buck-Boost Mode LED Driver Programming Output Voltage (Constant-Voltage Regulation) or Open LED/Overvoltage Threshold For a boost or SEPIC application, the output voltage can be set by selecting the values of R3 and R4 (see Figure 2) according to the following equation: V R R R OUT = + 1 25 3 4 4 . • For a boost type LED driver, set the resistor from the output to the FB pin such that the expected voltage level during normal operation will not exceed 1.1V. For an LED driver of buck mode or a buck-boost mode configuration, the output voltage is typically level-shifted to a signal with respect to GND as illustrated in Figure 3. The output can be expressed as: V V R R OUT BE = + 1 25 3 4 . • ISP/ISN Short-Circuit Protection Feature for SEPIC The ISP and ISN pins have a protection feature indepen- dent of the LED current sense feature that operates at ISN below 3V. The purpose of this feature is to provide continuous current sensing when ISN is below the LED current sense common mode range (during start-up or an output short-circuit fault) to prevent the development of excessive switching currents that could damage the power components in a SEPIC converter. The action threshold (335mV, typ) is above the default LED current sense threshold, so that no interference will occur over the ISN voltage range where these two functions overlap. This feature acts in the same manner as switch-current limit — it prevents switch turn-on until the ISP/ISN differ- ence falls below the threshold. |
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