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EN5330DC Datasheet(PDF) 8 Page - Enpirion, Inc. |
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EN5330DC Datasheet(HTML) 8 Page - Enpirion, Inc. |
8 / 11 page Rev 1.0 – July 2005 www.enpirion.com EN5330 8 Capacitor Selection The EN5330 needs about 20-40uF of input capacitance. Low-cost, low-ESR ceramic capacitors can be used as input capacitors for this converter and it is strongly recommended that they be rated X5R or X7R. In some applications, lower value capacitors are needed in parallel with the larger, lossy capacitors in order to provide high frequency decoupling. The EN5330 has been optimized for use with about 50µF of ceramic output capacitance. It is strongly recommended that these be low-cost, low-ESR, ceramic capacitors rated X5R or X7R. (See the Enpirion application note on ripple comparison for optimum selection of number and value of these capacitors based on ripple requirements.) In order to eliminate high-frequency switching spikes on the output ripple, usually a low-value, low-ESR ceramic capacitor is used in parallel with the larger capacitors right at the load. Enable Operation The ENABLE pin provides a means to shut down the power FET switching or enable normal operation. A logic low will disable the converter and cause it to shut down. A logic high will enable the converter into normal operation. Soft-Start Operation The SS pin in conjunction with a small capacitor between this pin and AGND provides the soft start function to limit the in-rush current during start-up. During start-up of the converter the reference voltage to the error amplifier is gradually increased to its final level by an internal current source of typically 10uA. The whole soft-start procedure is designed to take 1ms - 3ms with a 15-30nF soft start capacitor, but can be programmed by capacitor selection using the following equation: Rise Time: TR = Css* 80k During the soft-start cycle, when the soft-start capacitor reaches 0.8V, the output has reached its programmed regulation range. Note that the soft-start current source will continue to operate and during normal operation, the soft-start capacitor will charge up to a final value of 2.5V. Power Up Sequencing The sequencing of AVIN, PVIN and ENABLE should meet the following requirements: 1. ENABLE should not be asserted before PVIN. 2. PVIN should not be applied before AVIN. Note that tying AVIN, PVIN and ENABLE together and brought up at the same time does meet these requirements. POK Operation The POK signal is an open drain signal from the converter indicating the output voltage is within the specified range. The POK signal will be a logic high when the output voltage is within 90% - 120% of the programmed output voltage. If the output voltage goes outside of this range, the POK signal will be a logic low until the output voltage has returned to within this range. In the event of an over-voltage condition the POK signal will go low and will remain in this condition until the output voltage has dropped to 95% of the programmed output voltage before returning to the high state (see also Over Voltage Protection) Over-Current Protection The cycle-by-cycle current limit function is achieved by sensing the current flowing through the sense P- MOSFET and a signal generated by a differential amplifier with a preset over-current threshold. During a particular cycle, if the over-current threshold is exceeded, the power P-MOSFET is turned off and the power N-MOSFET is turned on to protect the P- MOSFET. If the over-current condition is removed, the over-current protection circuit will enable the PWM operation. If the over-current condition persists, the converter will eventually go through a full soft- start cycle. This circuit is designed to provide high noise immunity. It is possible to raise the over-current set-point by ~50% by connecting a 4.99k Ω resistor between ROCP and GND. Over-Voltage Protection When the output voltage exceeds 120% of the programmed output voltage, the PWM operation stops, the lower N-MOSFET is turned on and the POK signal goes low. When the output voltage drops below 95% of the programmed output voltage, normal |
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