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LM25010 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM25010 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 17 page Current Limit (Continued) mode. The load current (I OCL) is equal to the current limit threshold plus half the ripple current ( ∆I/2). The ripple ampli- tude ( ∆I) is calculated from: (9) The current limit threshold can be increased by connecting an external resistor (R CL) between SGND and ISEN. RCL typically is less than 1 Ω, and the calculation of its value is explained in the Applications Information section. If the cur- rent limit threshold is increased by adding R CL, the maximum continuous load current should not exceed 1.5A, and the peak current out of the SW pin should not exceed 2A. N - Channel Buck Switch and Driver The LM25010 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak cur- rent through the buck switch should not exceed 2A, and the load current should not exceed 1.5A. The gate driver circuit is powered by the external bootstrap capacitor between BST and SW (C4), which is recharged each off-time from V CC through the internal high voltage diode. The minimum off- time, nominally 260 ns, ensures sufficient time during each cycle to recharge the bootstrap capacitor. A 0.022 µF ce- ramic capacitor is recommended for C4. Soft-Start The soft-start feature allows the regulator to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. At turn-on, while V CC is below the under-voltage threshold (t1 in Figure 1), the SS pin is internally grounded, and V OUT is held at 0V. When VCC exceeds the under-voltage threshold (UVLO) an internal 11.5 µA current source charges the external capacitor (C6) at the SS pin to 2.5V (t2 in Figure 1). The increasing SS voltage at the non-inverting input of the regulation compara- tor gradually increases the output voltage from zero to the desired value. The soft-start feature keeps the load inductor current from reaching the current limit threshold during start-up, thereby reducing inrush currents. An internal switch grounds the SS pin if V CC is below the under-voltage lock-out threshold, if a thermal shutdown oc- curs, or if the circuit is shutdown using the RON/SD pin. Thermal Shutdown The LM25010 should be operated below the Maximum Op- erating Junction Temperature rating. If the junction tempera- ture increases during a fault or abnormal operating condi- tion, the internal Thermal Shutdown circuit activates typically at 175˚C. The Thermal Shutdown circuit reduces power dissipation by disabling the buck switch and the on-timer, and grounding the SS pin. This feature helps prevent cata- strophic failures from accidental device overheating. When the junction temperature reduces below approximately 155˚C (20˚C typical hysteresis), the SS pin is released and normal operation resumes. Applications Information EXTERNAL COMPONENTS The procedure for calculating the external components is illustrated with a design example. Referring to the Block Diagram, the circuit is to be configured for the following specifications: • V OUT =5V • V IN =6Vto40V • F S = 175 kHz • Minimum load current = 200 mA • Maximum load current = 1.0A • Softstart time=5ms. R1 and R2: These resistors set the output voltage, and their ratio is calculated from: R1/R2 = (V OUT/2.5V) - 1 (10) R1/R2 calculates to 1.0. The resistors should be chosen from standard value resistors in the range of 1.0 k Ω -10kΩ. A value of 1.0 k Ω will be used for R1 and for R2. R ON,FS: RON can be chosen using Equation 7 to set the nominal frequency, or from Equation 6 if the on-time at a particular V IN is important. A higher frequency generally means a smaller inductor and capacitors (value, size and cost), but higher switching losses. A lower frequency means 20172720 FIGURE 4. Inductor Current - Current Limit Operation www.national.com 11 |
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