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SP7601_EB Datasheet(PDF) 4 Page - Exar Corporation |
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SP7601_EB Datasheet(HTML) 4 Page - Exar Corporation |
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4 / 9 page  Sept25-07 Rev D SP7601 29V High Voltage LED Driver © 2007 Exar Corporation Page 4 GENERAL OVERVIEW The SP7601 is a fixed frequency, Voltage- mode, non-synchronous buck PWM controller optimized for driving LEDs. Constant LED current is achieved using resistor RSET as shown in the page 1 schematic. A low 0.2V reference voltage minimizes power dissipation in RSET. A tight reference voltage tolerance of +/-7%, 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 the FB pin via a diode. Overcurrent protection (OCP) is based on the high-side MOSFET’s RDS(ON) and is programmable via a resistor placed at the LX node. Programming the LED Current Use the following equation to program the LED current: LED I V Rset 2 . 0 = …………………………… (1) Where 0.2 is SP7601 reference voltage The output voltage will adjust as needed to ensure average ILED is supplied. For example, if the output current is to be set at 0.35A, then RSET = 0.57Ω. If the output LED has a corresponding VF of 3.5V, then the SP7601 will step down the VIN to 3.5V. If two of these LEDs are placed in series, then SP7601 will step down the voltage to 7V. Superimposed on ILED current is a current ripple that is equal in magnitude to the inductor current ripple. Current ripple will be nominally set to 10% of ILED by proper sizing of the inductor. Note that throughout this datasheet ILED and IOUT 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 to control dimming of the LED. This signal turns the MOSFET gate drive on/off, thereby modulating the average current delivered to the LED. For turning the MOSFET off, the minimum signal amplitude at the FB node must be 1.2V; and to turn it back on it must fall below 0.7V. The frequency of the PWM signal can be up to 1kHz. Figure1- 1kHz, 10% duty cycle dimming signal applied to SP7601, ch1:LX, ch2: dimming signal, ch4: ILED Figure2- 1kHz, 80% duty cycle dimming signal applied to SP7601, ch1:LX, ch2: dimming signal, ch4: ILED |
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