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LM3402HV Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM3402HV Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 22 page Application Information (Continued) MINIMUM OUTPUT VOLTAGE The minimum recommended on-time for the LM3402/02HV is 300 ns. This lower limit for t ON determines the minimum duty cycle and output voltage that can be regulated based on input voltage and switching frequency. The relationship is determined by the following equation: HIGH VOLTAGE BIAS REGULATOR The LM3402/02HV contains an internal linear regulator with a 7V output, connected between the VIN and the VCC pins. The VCC pin should be bypassed to the GND pin with a 0.1 µF ceramic capacitor connected as close as possible to the pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical) and then regulates at 7V as VIN increases. Opera- tion begins when VCC crosses 5.25V. INTERNAL MOSFET AND DRIVER The LM3402/02HV features an internal power MOSFET as well as a floating driver connected from the SW pin to the BOOT pin. Both rise time and fall time are 20 ns each (typical) and the approximate gate charge is 3 nC. The high-side rail for the driver circuitry uses a bootstrap circuit consisting of an internal high-voltage diode and an external 10 nF capacitor, C B.VCC charges CB through the internal diode while the power MOSFET is off. When the MOSFET turns on, the internal diode reverse biases. This creates a floating supply equal to the V CC voltage minus the diode drop to drive the MOSFET when its source voltage is equal to V IN. FAST SHUTDOWN FOR PWM DIMMING The DIM pin of the LM3402/02HV is a TTL logic compatible input for low frequency PWM dimming of the LED. A logic low (below 0.8V) at DIM will disable the internal MOSFET and shut off the current flow to the LED array. While the DIM pin is in a logic low state the support circuitry (driver, bandgap, VCC) remains active in order to minimize the time needed to turn the LED array back on when the DIM pin sees a logic high (above 2.2V). A 75 µA (typical) pull-up current ensures that the LM3402/02HV is on when DIM pin is open circuited, eliminating the need for a pull-up resistor. Dimming fre- quency, f DIM, and duty cycle, DDIM, are limited by the LED current rise time and fall time and the delay from activation of the DIM pin to the response of the internal power MOSFET. In general, f DIM should be at least one order of magnitude lower than the steady state switching frequency in order to prevent aliasing. PEAK CURRENT LIMIT The current limit comparator of the LM3402/02HV will en- gage whenever the power MOSFET current (equal to the inductor current while the MOSFET is on) exceeds 735 mA (typical). The power MOSFET is disabled for a cool-down time that is 10x the steady-state on-time. At the conclusion of this cool-down time the system re-starts. If the current limit condition persists the cycle of cool-down time and restarting will continue, creating a low-power hiccup mode, minimizing thermal stress on the LM3402/02HV and the external circuit components. OVER-VOLTAGE/OVER-CURRENT COMPARATOR The CS pin includes an output over-voltage/over-current comparator that will disable the power MOSFET whenever V SNS exceeds 300 mV. This threshold provides a hard limit for the output current. Output current overshoot is limited to 300 mV / R SNS by this comparator during transients. The OVP/OCP comparator can also be used to prevent the output voltage from rising to V O(MAX) in the event of an output open-circuit. This is the most common failure mode for LEDs, due to breaking of the bond wires. In a current regulator an output open circuit causes V SNS to fall to zero, commanding maximum duty cycle. Figure 2 shows a method using a zener diode, Z1, and zener limiting resistor, R Z,to limit output voltage to the reverse breakdown voltage of Z1 plus 200 mV. The zener diode reverse breakdown voltage, V Z, must be greater than the maximum combined VF of all LEDs in the array. The maximum recommended value for R Z is1k Ω. As discussed in the Maximum Output Voltage section, there is a limit to how high V O can rise during an output open- circuit that is always less than V IN. If no output capacitor is used, the output stage of the LM3402/02HV is capable of withstanding V O(MAX) indefinitely, however the voltage at the output end of the inductor will oscillate and can go above V IN or below 0V. A small (typically 10 nF) capacitor across the LED array dampens this oscillation. For circuits that use an output capacitor, the system can still withstand V O(MAX) in- definitely as long as C O is rated to handle VIN. The high current paths are blocked in output open-circuit and the risk of thermal stress is minimal, hence the user may opt to allow the output voltage to rise in the case of an open-circuit LED failure. www.national.com 11 |
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