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LM5008A Datasheet(PDF) 11 Page - Texas Instruments |
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LM5008A Datasheet(HTML) 11 Page - Texas Instruments |
11 / 22 page L1 = VOUT x (VIN - VOUT) IOR x Fs x VIN LM5008A www.ti.com SNVS583E – MARCH 2009 – REVISED APRIL 2011 Thermal Protection The LM5008A should be operated so the junction temperature does not exceed 125°C during normal operation. An internal Thermal Shutdown circuit is provided to shutdown the LM5008A in the event of a higher than normal junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state by disabling the buck switch. This feature prevents catastrophic failures from accidental device overheating. When the junction temperature reduces below 140°C (typical hysteresis = 25°C) normal operation is resumed. Applications Information SELECTION OF EXTERNAL COMPONENTS A guide for determining the component values will be illustrated with a design example. Refer to the Block Diagram. The following steps will configure the LM5008A for: • Input voltage range (Vin): 12V to 95V • Output voltage (VOUT1): 10V • Load current (for continuous conduction mode): 100 mA to 300 mA RFB1, RFB2: VOUT = VFB x (RFB1 + RFB2) / RFB1, and since VFB = 2.5V, the ratio of RFB2 to RFB1 calculates as 3:1. Standard values of 3.01 k Ω and 1.00 kΩ are chosen. Other values could be used as long as the 3:1 ratio is maintained. Fs and RT: The recommended operating frequency range for the LM5008A is 50 kHz to 1.1 MHz. Unless the application requires a specific frequency, the choice of frequency is generally a compromise since it affects the size of L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time of 400 ns, is calculated from: FMAX = VOUT / (VINMAX x 400 ns) (6) For this exercise, Fmax = 263 kHz. From equation 1, RT calculates to 274 kΩ. A standard value 324 kΩ resistor will be used to allow for tolerances in equation 1, resulting in a frequency of 223 kHz. L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum ripple current occurs at maximum Vin. a) Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude (IOR) must be less than 200 mA p-p so the lower peak of the waveform does not reach zero. L1 is calculated using the following equation: (7) At Vin = 95V, L1(min) calculates to 200 µH. The next larger standard value (220 µH) is chosen and with this value IOR calculates to 182 mA p-p at Vin = 95V, and 34 mA p-p at Vin = 12V. b) Maximum load current: At a load current of 300 mA, the peak of the ripple waveform must not reach the minimum guaranteed value of the LM5008A’s current limit threshold (410 mA). Therefore the ripple amplitude must be less than 220 mA p-p, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum Vin and Io, the peak of the ripple will be 391 mA. While L1 must carry this peak current without saturating or exceeding its temperature rating, it also must be capable of carrying the maximum guaranteed value of the LM5008A’s current limit threshold (610 mA) without saturating, since the current limit is reached during startup. The DC resistance of the inductor should be as low as possible. For example, if the inductor’s DCR is one ohm, the power dissipated at maximum load current is 0.09W. While small, it is not insignificant compared to the load power of 3W. C3: The capacitor on the VCC output provides not only noise filtering and stability, but its primary purpose is to prevent false triggering of the VCC UVLO at the buck switch on/off transitions. C3 should be no smaller than 0.47 µF. C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its ESR, ripple voltage due to its capacitance, and the nature of the load. Copyright © 2009–2011, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM5008A |
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