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XRP7604 Datasheet(PDF) 8 Page - Exar Corporation |
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XRP7604 Datasheet(HTML) 8 Page - Exar Corporation |
8 / 13 page X XR RP P7 76 60 04 4 1 1A A 2 29 9V V N No on n--S Sy yn nc c.. B Bu uc ck k H Hiig gh h P Po ow we err L LE ED D D Drriiv ve err © 2009 Exar Corporation 8/13 Rev. 1.0.0 THEORY OF OPERATION The XRP7604 is a fixed frequency, Voltage- mode, non-synchronous buck PWM regulator optimized for driving LEDs. Constant LED current is achieved using resistor RFB as shown in the page 1 schematic. A low 0.2V reference voltage minimizes power dissipation in RFB. A tight reference voltage tolerance of ±3%, over full operating conditions, helps accurately program the LED current. High switching frequency of 1.2MHz (nominal) reduces the size of passive components. Dimming and power sequencing is achieved using a logic-level PWM signal applied to FB pin via a diode. Overcurrent protection (OCP) is based on high-side MOSFET’s Rds(on) and is programmable via a resistor placed at LX node. PROGRAMMING THE LED CURRENT Use the following equation to program the LED current: Equ.1: LED I V RFB 2 . 0 = The output voltage will adjust as needed to ensure average ILED is supplied. For example if the output current has to be set at 0.35A then RFB=0.57 Ohm. If the output LED has a corresponding Vf of 3.5V then XRP7604 will step down the VIN to 3.5V. If two of these LEDs are placed in series then XRP7604 will step down the voltage to 7V. Superimposed on ILED there is a current ripple that is equal in magnitude to inductor current ripple. Current ripple will be nominally set to 10% of ILED by proper sizing of inductor. Note that throughout this datasheet ILED and IO will be used interchangeably. DIMMING SIGNAL A logic-level PWM signal applied through a small-signal diode to the feed-back (FB) pin can be used for dimming control of the LED. This external signal we call DIM turns the MOSFET gate drive on/off, thereby modulating the average current delivered to the LED. The DIM signal connects to the VFB pin through a 1N4148 diode and will shutdown the XRP7604 when DIM = H and turn-on the XRP7604 when DIM = L. The DIM signal needs to be greater than 600mV minimum to turn-off the XRP7604 and less than 200mV to fully turn-on the XRP7604. It is recommended to use a signal with DIM = 1V or more for OFF and 0V for ON. The user should note that the logic is reversed relative to many other PWM controlled LED drivers. In other words a logic level high at 20% duty cycle will result in approximately an 80% duty cycle for the LED. Recommended modulation frequencies are from 100Hz to 200Hz with 10 – 90% duty cycle, 500Hz with 10 – 80% duty cycle, and 1000Hz with 10 - 70% duty cycle. Figures 15 & 16 show the output response at the maximum PWM DIM signal of 1000Hz. See figure 17 for 100Hz to 1000Hz duty cycle response for two Luxeon K2 LEDs in parallel at 0.75A total current. Fig. 14: 1.1KHz, 10% Duty Cycle Dimming Signal Dimming Signal is ~70% LED Duty Cycle Fig. 15: 1KHz, 70% Duty Cycle Dimming Signal is ~10% LED Duty Cycle Ch1: DIM Signal Ch2: VFB – 0.75A IOUT/div Ch1: DIM Signal Ch2: VFB – 0.75A IOUT/div |
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