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LM34919TLX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM34919TLX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 18 page  30004414 FIGURE 4. Inductor Current - Current Limit Operation N - Channel Buck Switch and Driver The LM34919 integrates an N-Channel buck switch and as- sociated floating high voltage gate driver. The peak current allowed through the buck switch is 1.5A, and the maximum allowed average current is 1A. The gate driver circuit works in conjunction with an external bootstrap capacitor and an in- ternal high voltage diode. A 0.022 µF capacitor (C4) connect- ed between BST and SW provides the voltage to the driver during the on-time. During each off-time, the SW pin is at ap- proximately -1V, and C4 charges from V CC through the inter- nal diode. The minimum off-time forced by the LM34919 ensures a minimum time each cycle to recharge the bootstrap capacitor. Softstart The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, after V CC reaches the under-voltage threshold, an internal 10.5 µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner. An internal switch grounds the SS pin if V CC is below the un- der-voltage lockout threshold, or if the RON/SD pin is ground- ed. Thermal Shutdown The LM34919 should be operated so the junction temperature does not exceed 125°C. If the junction temperature increases, an internal Thermal Shutdown circuit, which activates (typi- cally) at 175°C, takes the controller to a low power reset state by disabling the buck switch. This feature helps prevent catas- trophic failures from accidental device overheating. When the junction temperature reduces below 155°C (typical hysteresis = 20°C) normal operation resumes. Applications Information EXTERNAL COMPONENTS The procedure for calculating the external components is il- lustrated with the following design example. Referring to the Block Diagram, the circuit is to be configured for the following specifications: - VOUT = 5V - VIN = 8V to 40V - Minimum load current = 200 mA - Maximum load current = 600 mA - Switching Frequency = 800 kHz - Soft-start time = 5 ms R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from: R1/R2 = (V OUT/2.5V) - 1 For this example, R1/R2 = 1. R1 and R2 should be chosen from standard value resistors in the range of 1.0 k Ω - 10 kΩ which satisfy the above ratio. For this example, 2.49k Ω is chosen for R1 and R2. R ON: This resistor sets the on-time, and (by default) the switching frequency. The switching frequncy must be less than 1.6 MHz to ensure the minimum forced off-time does not interfere with the circuit's proper operation. The R ON resistor is calculated from the following equation, using the minimum input voltage. Check that this value resistor does not set an on-time less than 120 ns at maximum V IN. A standard value 43.2 k Ω resistor is used, resulting in a nom- inal frequency of 806 kHz. The minimum on-time is ≊231 ns at Vin = 40V, and the maximum on-time is ≊875 ns at Vin = 8V. Alternately, R ON can be determined using Equation 4 if a specific on-time is required. 11 www.national.com |
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