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XRP7604 Datasheet(PDF) 9 Page - Exar Corporation |
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XRP7604 Datasheet(HTML) 9 Page - Exar Corporation |
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9 / 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 9/13 Rev. 1.0.0 LED current versus PWM Dimming Duty 0 10 20 30 40 50 60 70 80 90 100 0 10 20 3040 506070 8090 100 (1-D) PWM Dimming Duty (%) 100Hz 200Hz 500Hz 1kHz Fig. 16: Linearity, LED current vs (1-D) PWM Dimming Duty Cycle, Vin=12V, Io=0.75A, 2 LED in series MODULATOR OPERATION AND POWER SEQUENCING The XRP7604 has a unique modulator design which improves the device’s ability to operate at very high duty cycle. While seamless in operation as the duty cycle is increasing (input voltage falling), when the duty cycle is decreased (input voltage rising), the user will observe the switching frequency increasing in distinct fractions of the switching frequency. If the device is operating at 100% duty cycle, a unique advantage of using a p-channel pass device, and then the input voltage is increased, the frequency will start at 300kHz, then 600kHz, and then finally 1.2MHz. The frequency will tend to increase to the next higher fraction once the duty cycle reaches 75 to 65 percent. This is the normal operation of the device and should be expected. There is no impact on the LED current accuracy. If PWM dimming is being used as the input voltage is increased, one will see the frequency increasing when the duty cycle is < 90%. When power is initially applied the device will begin operating as if the input voltage is increasing and may start operation at one of the fractional operating frequencies. Many users will prefer to have the device start operating at the nominal operating frequency, thus it is recommended that Vin be applied after FB is set at the high state (>1.2V). The regulator is now in standby and once Vin has reached steady-state then FB is transitioned from a high to a low state. The regulator then starts operating at nominal frequency. Another benefit of using power sequencing for power up is that it ensures all internal circuitry is alive and fully operational before the device is required to regulate the current through the LEDs. Since the regulator was “Off” before power was applied, it is unlikely the LED is under any type of thermal stress. EXAR does not recommend using the XRP7604 in applications where dimming of the LED is achieved by PWM’ing the actual input power as is common in automotive dimming applications. BUCK OPERATION WITHOUT OUTPUT CAPACITOR In order to be able to apply the aforementioned dimming signal to the LED, the output filter capacitor that is normally used with a buck converter has to be removed from the circuit. Thus the LED current ripple equals the inductor current ripple. As a rule of thumb current ripple should be limited to 10% of ILED. Allowing for a higher current ripple, up to 30%, while staying within LED manufacturer ripple guidelines, will reduce inductance and possibly inductor size. OVERCURRENT PROGRAMMING Resistor Rs can be used to program Overcurrent Protection (OCP). Use the following equation for calculating the Rs value. Equ.2: A on Rds I V Rs OCP μ 33 )) ( 15 . 1 5 . 1 ( 35 . 0 × × × − = Where Iocp is the programmed overcurrent and is generally set 50% above nominal output current, and Rds(on) = 135mohms. Maximum value of Rs that can be used for programming OCP is 4k. INDUCTOR SELECTION Select the inductor L1 for inductance, Irms and Isat. Calculate inductance from |
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