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MAX8595X-MAX8596X Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX8595X-MAX8596X Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 8 page Design Procedure Adjusting LED Current Adjusting the output current of the MAX8595X/ MAX8596X changes the brightness of the LEDs. An analog input (CTRL) and the sense-resistor value set the output current. Output current is given by: ILED = VCTRL / (5 x RSENSE) The VCTRL voltage range for adjusting output current is 0.24V to 1.65V (or 1.72V for the MAX8596X). To set the maximum current, calculate RSENSE when VCTRL is at its maximum as follows: For the MAX8595X, RSENSE = 1.65 / (5 x ILED(MAX)) For the MAX8596X, RSENSE = 1.72 / (5 x ILED(MAX)) Power dissipation in RSENSE is typically less than 10mW, allowing the use of a small surface-mount resistor. 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 the need for any additional RC filters; see the Typical Operating Characteristics. Capacitor Selection Ceramic capacitors with X5R, X7R, or better dielectric are recommended for stable operation over the entire operating temperature range. 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. Higher 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 0.1µF capacitor from COMP to GND. For stable operation, COUT must not exceed 10 times CCOMP. Inductor Selection Inductor values range from 10µH to 47µH. A 22µH inductor optimizes the efficiency for most applications while maintaining low 12mVP-P input ripple. With input voltages near 5V, a larger value of inductance can 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 high switching frequency of the MAX8595X/ MAX8596X demands a high-speed rectification diode (D1) for optimum efficiency. A Schottky diode is recom- mended due to its fast recovery time and low forward- voltage drop. Ensure that the diode’s average and peak current rating exceed the average output current and peak inductor current. In addition, the diode’s reverse breakdown voltage must exceed VOUT. The RMS diode current can be approximated 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 (MAX8596XEVKIT) is available to speed design. When laying out a board, minimize trace lengths between the IC and RSENSE, the inductor, the diode, the input capacitor, and the output capacitor. Keep traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from CS. The IN bypass capacitor (CIN) should be placed as close to the IC as possible. PGND and GND should be connected direct- ly to the exposed paddle underneath the IC. The ground connections of CIN and COUT should be as close together as possible. The traces from IN to the inductor and from the Schottky diode to the LEDs can be longer. Chip Information TRANSISTOR COUNT: 2143 PROCESS: BiCMOS II I DIODE RMS OUT PEAK () =× I VI V Vs L PEAK OUT MAX LED MAX IN MIN IN MIN = × × + × × () () () () . . 09 09 2 µ High-Efficiency, 36V Step-Up Converters with TA Derating Option for 2 to 9 White LEDs _______________________________________________________________________________________ 7 |
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