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RC5054A Datasheet(PDF) 5 Page - Fairchild Semiconductor |
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RC5054A Datasheet(HTML) 5 Page - Fairchild Semiconductor |
5 / 13 page PRODUCT SPECIFICATION RC5054A 5 Functional Description Initialization The RC5054A automatically initializes upon receipt of power. Special sequencing of the input supplies is not neces- sary. The Power-On Reset (POR) function continually moni- tors the input supply voltages. The POR monitors the bias volt-age at the VCC pin and the input voltage (VIN) on the OCSET pin. The level on OCSET is equal to VIN less a fixed voltage drop (see over-current protection). The POR function initiates soft start operation after both input supply voltages exceed their POR thresholds. For operation with a single +12V power source, VIN and VCC are equivalent and the +12V power source must exceed the rising VCC threshold before POR initiates operation. Soft Start The POR function initiates the soft start sequence. An inter- nal 10 µA current source charges an external capacitor (CSS) on the SS pin to 4V. Soft start clamps the error amplifier out- put (COMP pin) and reference input (+ terminal of error amp) to the SS pin voltage. Figure 1 shows the soft start interval with CSS = 0.1µF. Initially the clamp on the error amplifier (COMP pin) controls the converter’s output volt- age. At t1 in Figure 1, the SS voltage reaches the valley of the oscillator’s triangle wave. The oscillator’s triangular wave- form is compared to the ramping error amplifier voltage. This generates PHASE pulses of increasing width that charge the output capacitor(s). This interval of increasing pulse width continues to t2. With sufficient output voltage, the clamp on the reference input controls the output voltage. This is the interval between t2 and t3 in Figure 1. At t3 the SS voltage exceeds the DACOUT voltage and the output voltage is in regulation. This method provides a rapid and controlled output voltage rise. The PGOOD signal toggles ‘high’ when the output voltage (VSEN pin) is within ±5% of DACOUT. The 2% hysteresis built into the power good comparators prevents PGOOD oscillation due to nominal output voltage ripple. Figure 1. Soft Start Interval 0V 0V 0V Time (5ms/DIV) SOFT-START (1V/DIV) OUTPUT (1V/DIV) VOLTAGE t2 t3 PGOOD (2V/DIV) t1 Over-Current Protection The over-current function protects the converter from a shorted output by using the upper MOSFET’s on-resistance, RDS(ON) to monitor the current. This method enhances the converter’s efficiency and reduces cost by eliminating a current sensing resistor. The over-current function cycles the soft- start function in a hiccup mode to provide fault protection. A resistor (ROCSET) programs the over-current trip level. An internal 200 µA current sink develops a voltage across ROCSET that is referenced to VIN. When the voltage across the upper MOSFET (also referenced to VIN) exceeds the voltage across ROCSET , the over-current function initiates a soft-start sequence. The soft-start function discharges CSS with a 10µA current sink and inhibits PWM operation. The soft-start func- tion recharges CSS, and PWM operation resumes with the error amplifier clamped to the SS voltage. Should an over- load occur while recharging CSS, the soft start function inhibits PWM operation while fully charging CSS to 4V to complete its cycle. Figure 2 shows this operation with an overload condition. Note that the inductor current increases to over 15A during the CSS charging interval and causes an over-current trip. The converter dissipates very little power with this method. The measured input power for the condi- tions of Figure 2 is 2.5W. The over-current function will trip at a peak inductor current (IPEAK) determined by: Figure 2. Over-Current Operation where IOCSET is the internal OCSET current source (200µA typical). The OC trip point varies mainly due to the MOSFET’s RDS(ON) variations. To avoid over-current tripping in the normal operating load range, find the ROCSET resistor from the equation above with: • The maximum RDS(ON) at the highest junction temperature. • The minimum IOCSET from the specification table. • Determine IPEAK for IPEAK > IOUT(MAX) + (∆I)/2, where ∆I is the output inductor ripple current. I PEAK I OCSET R OCSET • R DS ON () -------------------------------------------- = 0A 0V Time (20ms/DIV) 5A 10A 15A 2V 4V |
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