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MAX158 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX158 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 10 page between the input and the LEDs through the inductor and Schottky diode. To ensure the LEDs remain off when the MAX1582 is in shutdown, the minimum input forward voltage of the LED array must exceed the maxi- mum input voltage. Typically, the leakage current through the LEDs in shutdown is less than 1µA. Soft-Start The MAX1582 attains soft-start by charging CCOMP gradually with a current source. When VCOMP rises above 1.25V, the internal MOSFET begins switching, but at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle is at its maximum. Overvoltage Protection OVLO occurs when VOUT1 rises above 27V. The protec- tion circuitry stops the internal MOSFET from switching and causes VCOMP to decay to GND. The device comes out of OVLO and into soft-start when VOUT1 falls below 25V. Design Procedure Adjusting LED Current Adjusting the MAX1582’s output current changes the brightness of the LEDs. The LED current is set by the voltage at CTRL (VCTRL) and the sense-resistor value (RSENSE): The VCTRL voltage range for adjusting output current is 0 to +1.62V. To set the maximum current, calculate RSENSE when VCTRL is at its maximum as follows: Power dissipation in RSENSE is typically less than 5mW; therefore, a standard chip resistor is sufficient. PWM Dimming Control CTRL is also used as a digital input, allowing LED brightness control with a logic-level PWM signal applied directly to CTRL. The frequency range is from 200Hz to 200kHz, while 0% duty cycle corresponds to zero current and 100% duty cycle corresponds to full current. The error amplifier and compensation capaci- tor form a lowpass filter, so PWM dimming results in DC current to the LEDs without any additional RC filters required. Capacitor Selection The exact values of input and output capacitors are not critical. The typical value for the input capacitor is 2.2µF, and the typical value for the output capacitor is 0.1µF. Larger value capacitors can be used to reduce input and output ripple, but at the expense of size and higher cost. CCOMP stabilizes the converter and controls soft-start. Connect a 0.022µF capacitor from COMP to GND. The minimum value for CCOMP is COUT / 10. The soft-start time is found from: Inductor Selection Recommended inductor values range from 10µH to 47µH. A 22µH inductor optimizes the efficiency for most applications, while maintaining a low 15mVP-P input rip- ple. With input voltages near 5V, a larger value of induc- tance may be more efficient. To prevent core saturation, ensure that the inductor saturation current rating exceeds the peak inductor current for the application. Calculate the peak inductor current with the following formula: Schottky Diode Selection The MAX1582’s high switching frequency demands a high-speed rectification diode (D1) for optimum efficiency. A Schottky diode is recommended due to its fast recovery time and low forward-voltage drop. Ensure that the diode’s average and peak current rating exceeds the average output current and peak inductor current. In addition, the diode’s reverse break- down voltage must exceed VOUT1. The RMS diode current can be calculated from: Applications Information PC Board Layout Due to fast-switching waveforms and high-current paths, careful PC board layout is required. An evalua- tion kit (MAX1582EVKIT) is available to speed design. II I DIODE RMS OUT PEAK () ≅× 1 I VI V Vs L PEAK OUT MAX LED MAX IN MIN IN MIN ≅ × × + ×µ × . . () () () () 1 08 08 2 tC V A SS COMP . =× µ 125 5 R I SENSE LED MAX . () = × 162 10 I V R LED CTRL SENSE = × 10 High-Efficiency, 26V Step-Up Converter for White LED Main and Subdisplay Backlighting _______________________________________________________________________________________ 7 |
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