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LM2695MHX 데이터시트(PDF) 11 Page - National Semiconductor (TI) |
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LM2695MHX 데이터시트(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page 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 12.3 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 under-voltage lockout threshold, if a thermal shutdown oc- curs, or if the RON/SD pin is grounded. Thermal Shutdown The LM2695 should be operated so the junction temperature does not exceed 125˚C. If the junction temperature in- creases, an internal Thermal Shutdown circuit, which acti- vates (typically) at 175˚C, takes the controller to a low power reset state by disabling the buck switch and the on-timer, and grounding the Softstart pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below 155˚C (typical hysteresis = 20˚C), the Softstart pin is released and normal operation resumes. Applications Information EXTERNAL COMPONENTS The following guidelines can be used to select the external components. R1 and R2: The ratio of these resistors is calculated from: R1/R2 = (V OUT/2.5V) - 1 R1 and R2 should be chosen from standard value resistors in the range of 1.0 k Ω -10kΩ which satisfy the above ratio. R ON: The minimum value for RON is calculated from: Equation 1 can be used to select R ON if a specific frequency is desired as long as the above limitation is met. L1: The main parameter affected by the inductor is the output current ripple amplitude (I OR). The limits for IOR must be determined at both the minimum and maximum nominal load currents. a) If the maximum load current is less than the current limit threshold (1.25A), the minimum load current is used to de- termine the maximum allowable ripple. To maintain continu- ous conduction mode the lower peak should not reach 0 mA. For this case, the maximum ripple current is: I OR(MAX1) =2xIO(min) (7) The ripple calculated in Equation 7 is then used in the following equation: (8) where V IN is the maximum input voltage and Fs is deter- mined from equation 1. This provides a minimum value for L1. The next larger standard value should be used, and L1 should be rated for the I PK current level. b) If the maximum load current is greater than the current limit threshold (1.25A), the LM2695 ensures the lower peak reaches 1.25A each cycle, requiring that I OR be at least twice the difference. The upper peak, however, must not exceed 2A. For this case, the ripple limits are: I OR(MAX2) =2x(2A -IO(max)) (9) and I OR(MIN1) =2x(IO(max) - 1.25A) (10) The lesser of Equations 9 and 10 is then used in Equation 8. If I OR(MAX2) is used, the maximum VIN is used in Equation 8. The next larger value should then be used for L1. If I OR(MIN1) is used, the minimum V IN is used in Equation 8. The next smaller value should then be used for L1. L1 must be rated for the peak value of the current waveform (I PK in Figure 7). C3: The capacitor at the V CC output provides not only noise filtering and stability, but also prevents false triggering of the V CC UVLO at the buck switch on/off transitions. For this reason, C3 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic capacitor. C2, and R3: Since the LM2695 requires a minimum of 25 mV p-p of ripple at the FB pin for proper operation, the re- quired ripple at V OUT1 is increased by R1 and R2. This necessary ripple is created by the inductor ripple current acting on C2’s ESR + R3. The minimum ripple current is calculated using equation 8, rearranged to solve for I OR at minimum V IN. The minimum ESR for C2 is then equal to: (11) If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Figure 3. Generally R3 is less than 1 Ω. C2 should generally be no smaller than 3.3 µF, although that is dependent on the frequency and the allow- able ripple amplitude at V OUT1. Experimentation is usually necessary to determine the minimum value for C2, as the nature of the load may require a larger value. A load which creates significant transients requires a larger value for C2 than a non-varying load. D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage drop is significant in the event the output is short-circuited as it is mainly this diode’s voltage (plus the voltage across the current limit sense resistor) which forces the inductor current to decrease during the off-time. For this reason, a higher voltage is better, although that affects efficiency. A reverse recovery time of )30 ns, and a forward voltage drop of )0.75V are preferred. The reverse leakage specification is important as that can significantly affect efficiency. D1’s reverse voltage rating must be at least as great as the maximum V IN, and its current rating must equal or exceed I PK Figure 7. C1 and C5: C1’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero. If the source’s dy- namic impedance is high (effectively a current source), it supplies the average input current, but not the ripple current. www.national.com 11 |
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