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LM5008SD データシート(PDF) 10 Page - National Semiconductor (TI) |
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LM5008SD データシート(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Current Limit The LM5008 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.5A the present cycle is immediately terminated, and a non-resetable OFF timer is initiated. The length of off-time is controlled by an external resistor (R CL) and the FB voltage (see Figure 5). When FB = 0V, a maximum off-time is required, and the time is preset to 35µs. This condition occurs when the output is shorted, and during the initial part of start-up. This amount of time ensures safe short circuit operation up to the maximum input voltage of 95V. In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time will be less than 35µs. Reducing the off-time during less severe overloads reduces the amount of fold- back, recovery time, and the start-up time. The off-time is calculated from the following equation: T OFF =10 -5 / (0.285 + (V FB /6.35x10 -6 xR CL)) (3) The current limit sensing circuit is blanked for the first 50- 70ns of each on-time so it is not falsely tripped by the current surge which occurs at turn-on. The current surge is required by the re-circulating diode (D1) for its turn-off recovery. N - Channel Buck Switch and Driver The LM5008 integrates an N-Channel Buck switch and as- sociated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap ca- pacitor and an internal high voltage diode. A 0.01µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from Vcc through the inter- nal diode. The minimum OFF timer, set to 300ns, ensures a minimum time each cycle to recharge the bootstrap capaci- tor. An external re-circulating diode (D1) carries the inductor current after the internal Buck switch turns off. This diode must be of the Ultra-fast or Schottky type to minimize turn-on losses and current over-shoot. Thermal Protection The LM5008 should be operated so the junction temperature does not exceed 125˚C during normal operation. An internal Thermal Shutdown circuit is provided to protect the LM5008 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, disabling the buck switch and the V CC regulator. This feature prevents catastrophic failures from accidental device overheating. When the junction tem- perature reduces below 140˚C (typical hysteresis = 25˚C), the Vcc regulator is enabled, and normal operation is re- sumed. Applications Information SELECTION OF EXTERNAL COMPONENTS A guide for determining the component values will be illus- trated with a design example. Refer to Figure 1. The follow- ing steps will configure the LM5008 for: • Input voltage range (Vin): 12V to 95V • Output voltage (V OUT1): 10V • Load current (for continuous conduction mode): 100 mA to 300 mA • Maximum ripple at V OUT2: 100 mVp-p at maximum input voltage R1 and R2: From Figure 1,V OUT1 =VFB x (R1 + R2) / R2, and since V FB = 2.5V, the ratio of R1 to R2 calculates as 3:1. Standard values of 3.01 k Ω (R1) and 1.00 kΩ (R2) are chosen. Other values could be used as long as the 3:1 ratio is maintained. The selected values, however, provide a small amount of output loading (2.5 mA) in the event the main load is disconnected. This allows the circuit to maintain regulation until the main load is reconnected. F s and RON: The recommended operating frequency range for the LM5008 is 50kHz to 600 kHz. 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 fre- quency, based on a minimum on-time of 400 ns, is calcu- lated from: F MAX =VOUT /VINMAX x 400ns For this exercise, Fmax = 263kHz. From equation 1, R ON calculates to 304 k Ω. A standard value 357 kΩ resistor will be used to allow for tolerances in equation 1, resulting in a frequency of 224kHz. 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 conduc- tion at minimum Io (100 mA), the ripple amplitude (I OR) 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: At Vin = 95V, L1(min) calculates to 200 µH. The next larger standard value (220 µH) is chosen and with this value I OR calculates to 181 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 LM5008’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 calcula- tion. 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 rat- ing, it also must be capable of carrying the maximum guar- anteed value of the LM5008’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 V CC output provides not only noise filtering and stability, but its primary purpose is to prevent www.national.com 10 |
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