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LM3406 Datasheet(PDF) 11 Page - Texas Instruments |
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LM3406 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 38 page LM3406, LM3406HV, LM3406HV-Q1 www.ti.com SNVS512E – SEPTEMBER 2008 – REVISED MARCH 2014 8.7 MINIMUM ON-TIME The minimum on-time for the LM3406 family is 280 ns (typical). One practical example of reaching the minimum on-time is when dimming the LED light output with a power FET placed in parallel to the LEDs. When the FET is on, the output voltage drops to 200 mV. This results in a small duty cycle and in most circuits requires an on-time that would be less than 280 ns. In such a case the LM3406 family keeps the on-time at 280 ns and increases the off-time as much as needed, which effectively reduces the switching frequency. 8.8 HIGH VOLTAGE BIAS REGULATOR (VCC) The LM3406 family 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. The LM3406 family comes out of UVLO and begins operating when VCC crosses 5.3V. This is shown graphically in the Typical Performance curves. Connecting an external supply to VCC to power the gate drivers is not recommended. However, it may be done if certain precautions are taken. Be sure that the external supply will not violate any absolute maximum conditions and will at no point exceed the voltage applied to the VIN pins. Under certain conditions, some ringing may be present on the SW and BOOT pins when VCC is driven with an external supply. It is important to ensure that the absolute maximum ratings of these pins are not violated during the ringing or else damage to the device may occur. 8.9 INTERNAL MOSFET AND DRIVER The LM3406 family 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 9 nC. The high-side rail for the driver circuitry uses a bootstrap circuit consisting of an internal high-voltage diode and an external 22 nF capacitor, CB. 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 VCC voltage minus the diode drop to drive the MOSFET when its source voltage is equal to VIN. 8.10 FAST LOGIC PIN FOR PWM DIMMING The DIM pin is a TTL compatible input for 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 LM3406 family is on when DIM pin is open circuited, eliminating the need for a pull-up resistor. Dimming frequency, fDIM, 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, fDIM should be at least one order of magnitude lower than the steady state switching frequency in order to prevent aliasing. 8.11 INPUT VOLTAGE COMPARATOR FOR PWM DIMMING Adding an external input diode and using the internal VINS comparator allows the LM3406 family to sense and respond to dimming that is done by PWM of the input voltage. This method is also referred to as "Two-Wire Dimming", and a typical application circuit is shown in . If the VINS pin voltage falls 70% below the VIN pin voltage, the LM3406 family disables the internal power FET and shuts off the current to the LED array. The support circuitry (driver, bandgap, VCC) remains active in order to minimize the time needed to the turn the LED back on when the VINS pin voltage rises and exceeds 70% of VIN. This minimizes the response time needed to turn the LED array back on. Copyright © 2008–2014, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3406 LM3406HV LM3406HV-Q1 |
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