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TPS40052PWP Datasheet(PDF) 11 Page - Texas Instruments |
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TPS40052PWP Datasheet(HTML) 11 Page - Texas Instruments |
11 / 30 page TPS40052 SLUS563B − AUGUST 2003 − REVISED APRIL 2004 11 www.ti.com APPLICATION INFORMATION PROGRAMMING SOFT START TPS40052 uses a closed-loop approach to ensure a controlled ramp on the output during start-up. Soft-start is programmed by charging an external capacitor (CSS) via an internally generated current source. The voltage on CSS is fed into a separate non-inverting input to the error amplifier (in addition to FB and EA_REF). The loop is closed on the lower of the CSS voltage or the the external reference voltage EA_REF. Once the CSS voltage rises above the external reference voltage, regulation is based on the external reference. To ensure a controlled ramp-up of the output voltage the soft-start time should be greater than the L-CO time constant as described in equation (9). t START w 2p L C O (seconds) There is a direct correlation between tSTART and the input current required during start-up. The faster tSTART, the higher the input current required during start-up. This relationship is describe in more detail in the section titled, Programming the Current Limit which follows. The soft-start capacitance, CSS, is described in equation (10). For applications in which the VIN supply ramps up slowly, (typically between 50 ms and 100 ms) it may be necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO tripping. The soft-start time should be longer than the time that the VIN supply transitions between 8 V and 9 V. C SS + 2.3 mA 0.7 V t START (Farads) PROGRAMMING CURRENT LIMIT The TPS40052 uses a two-tier approach for overcurrent protection. The first tier is a pulse-by-pulse protection scheme. Current limit is implemented on the high-side MOSFET by sensing the voltage drop across the MOSFET when the gate is driven high. The MOSFET voltage is compared to the voltage dropped across a resistor connected from VIN pin to the ILIM pin when driven by a constant current sink. If the voltage drop across the MOSFET exceeds the voltage drop across the ILIM resistor, the switching pulse is immediately terminated. The MOSFET remains off until the next switching cycle is initiated. The second tier consists of a fault counter. The fault counter is incremented on an overcurrent pulse and decremented on a clock cycle without an overcurrent pulse. When the counter reaches seven (7) a restart is issued and seven soft-start cycles are initiated. Both the upper and lower MOSFETs are turned off during this period. The counter is decremented on each soft-start cycle. When the counter is decremented to zero, the PWM is re-enabled. If the fault has been removed the output starts up normally. If the output is still present the counter counts seven overcurrent pulses and re-enters the second-tier fault mode. See Figure 3 for typical overcurrent protection waveforms. The minimum current limit setpoint (ILIM) depends on tSTART, CO, VO, and the load current at turn-on (IL). I LIM + C O V O t START ) I L (Amperes) (9) (10) (11) |
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