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MAX16821B Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX16821B Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 25 page High-Power Synchronous HBLED Drivers with Rapid Current Pulsing 10 ______________________________________________________________________________________ Synchronization The MAX16821A/MAX16821B/MAX16821C synchronize to an external clock connected to RT/SYNC. The appli- cation of an external clock at RT/SYNC disables the internal oscillator. Once the MAX16821A/MAX16821B/ MAX16821C are synchronized to an external clock, the external clock cannot be removed if reliable operation is to be maintained. Control Loop The MAX16821A/MAX16821B/MAX16821C use an average current-mode control scheme to regulate the output current (Figure 2). The main control loop con- sists of an inner current regulation loop for controlling the inductor current and an outer current regulation loop for regulating the LED current. The inner current regulation loop absorbs the double pole of the inductor and output capacitor combination reducing the order of the outer current regulation loop to that of a single-pole system. The inner current regulation loop consists of a current-sense resistor (RS), a current-sense amplifier (CSA), a current-error amplifier (CEA), an oscillator pro- viding the carrier ramp, and a PWM comparator (CPWM) (Figure 2). The MAX16821A/MAX16821B/ MAX16821C outer LED-current control loop consists of a differential amplifier (DIFF), a reference voltage, and a voltage-error amplifier (VEA). Inductor Current-Sense Amplifier The differential current-sense amplifier (CSA) provides a 34.5V/V DC gain. The typical input offset voltage of the current-sense amplifier is 0.1mV with a 0 to 5.5V common- mode voltage range (VIN = 7V to 28V). The current-sense amplifier senses the voltage across RS. The maximum common-mode voltage is 3.2V when VIN = 5V. Inductor Peak-Current Comparator The peak-current comparator provides a path for fast cycle-by-cycle current limit during extreme fault condi- tions, such as an inductor malfunction (Figure 3). Note the average current-limit threshold of 27.5mV still limits the output current during short-circuit conditions. To prevent inductor saturation, select an inductor with a saturation current specification greater than the aver- age current limit. The 60mV threshold for triggering the peak-current limit is twice the full-scale average cur- rent-limit voltage threshold. The peak-current compara- tor has only a 260ns delay. CF L RIN DIFF SENSE+ SENSE- EAN EAOUT CSN CSP CLP RF CCP CCZ RCF RS RLS LED STRING COUT DIFF VREF VEA CEA CPWM CA DRIVER MODE = SGND VIN Figure 2. MAX16821A/MAX16821B/MAX16821C Control Loop |
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