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LTC1530IS8-2.8 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1530IS8-2.8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  9 LTC1530 Typically, thermal shutdown is activated if the LTC1530’s junction temperature exceeds 150 °C. G1 and G2 resume switching when the junction temperature drops below 100 °C. Soft-Start and Current Limit Unlike other PWM parts, the LTC1530 includes an on-chip soft-start capacitor that is used during start-up and cur- rent limit operation. On power-up, an internal 4 µA pull-up at COMP brings the LTC1530 out of shutdown mode. An internal current source then charges the internal CSS capacitor. The COMP pin is clamped to one VGS above the voltage on CSS during start-up. This prevents the error amplifier from forcing the loop to maximum duty cycle. The LTC1530 operates at low duty cycle as the COMP pin voltage increases above about 2.4V. The slew rate of the soft-start capacitor is typically 0.4V/ms. As the voltage on CSS continues to increase, MSS eventually turns off and the error amplifier regulates the output. The MIN comparator is disabled if soft-start is active to prevent an override of the soft-start function. The LTC1530 includes another feedback loop to control operation in current limit. Before each falling edge of G1, the current comparator, CC, samples and holds the volt- age drop across external MOSFET Q1 with the LTC1530’s IFB pin. CC compares the voltage at IFB to the voltage at the IMAX pin. As peak current rises, the voltage across the RDS(ON) of Q1 increases. If the voltage at IFB drops below IMAX, indicating that Q1’s drain current has exceeded the maximum desired level, CC pulls current out of CSS. Duty cycle decreases and the output current is controlled. The CC comparator pulls current out of CSS in proportion to the voltage difference between IFB and IMAX. Under minor overload conditions, the voltage at CSS falls gradually, creating a time delay before current limit activates. Very short, mild overloads may not affect the output voltage at all. Significant overload conditions allow the voltage on CSS to reach a steady state and the output remains at a reduced voltage until the overload is removed. Serious overloads generate a large overdrive and allow CC to pull the CSS voltage down quickly, thus preventing damage to the external components. By using the RDS(ON) of Q1 to measure output current, the current limit circuit eliminates the sense resistor that would otherwise be required. This minimizes the number of components in the high current power path. The current limit circuitry is not designed to be highly accurate. It is primarily meant to prevent damage to the power supply circuitry during fault conditions. The exact current level where current limiting takes effect will vary from unit to unit as the RDS(ON) of Q1 varies. Figure 5a illustrates the basic connections for the current limit circuitry. For a given current limit level, the external resistor from IMAX to VIN is determined by: LTC1530 + + CIN COUT VOUT 1530 F05 VIN LO 20 Ω IFB G1 Q1 Q2 G2 IMAX RIMAX 200 µA + – CC Maximum load current I Inductor ripple current = V f oscillator frequency = 300kHz L value R n-r tance of Q1 at I 200 A sink current RIPPLE IN OSC O DS(ON)Q1 LMAX R IR I where II I I VV LV f LTC Inductor O esis I IMAX LMAX DS ON Q IMAX LMAX LOAD RIPPLE LOAD OUT OUT OIN OSC IMAX = () =+ = = − ()( ) ()( )( ) = = = =µ () , 1 2 1530 Figure 5a. Current Limit Setting (Use Kelvin-Sense Connections Directly at the Drain and Source of Q1) APPLICATIO S I FOR ATIO |
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