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LM2593HVT-ADJ Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2593HVT-ADJ Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 21 page  Block Diagram 10133330 PIN FUNCTIONS +V IN (Pin 1) — This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator. Output (Pin 2) — Internal switch. The voltage at this pin switches between approximately (+V IN −VSAT) and approxi- mately −0.5V, with a duty cycle of V OUT/VIN. Error Flag (Pin 3) — Open collector output that goes active low ( ≤ 1.0V) when the output of the switching regulator is out of regulation (less than 95% of its nominal value). In this state it can sink maximum 3mA. When not low, it can be pulled high to signal that the output of the regulator is in regulation (power good). During power-up, it can be pro- grammed to go high after a certain delay as set by the Delay pin (Pin 5). The maximum rating of this pin should not be exceeded, so if the rail to which it will be pulled-up to is higher than 45V, a resistive divider must be used instead of a single pull-up resistor, as indicated in Figure 1. Ground (Pin 4) — Circuit ground. Delay (Pin 5) — This sets a programmable power-up delay from the moment that the output reaches regulation, to the high signal output (power good) on Pin 3. A capacitor on this pin starts charging up by means on an internal () 3 µA) current source when the regulated output rises to within 5% of its nominal value. Pin 3 goes high (with an external pull-up) when the voltage on the capacitor on Pin 5 exceeds 1.3V. The voltage on this pin is clamped internally to about 1.7V. If the regulated output drops out of regulation (less than 95% of its nominal value), the capacitor on Pin 5 is rapidly discharged internally and Pin 3 will be forced low in about 1/1000 th of the set power-up delay time. Feedback (Pin 6) — Senses the regulated output voltage to complete the feedback loop. This pin is directly connected to the Output for the fixed voltage versions, but is set to 1.23V by means of a resistive divider from the output for the Adjustable version. If a feedforward capacitor is used (Ad- justable version), then a negative voltage spike is generated on this pin whenever the output is shorted. This happens because the feedforward capacitor cannot discharge fast enough, and since one end of it is dragged to Ground, the other end goes momentarily negative. To prevent the energy rating of this pin from being exceeded, a small-signal Schot- tky diode to Ground is recommended for DC input voltages above 40V whenever a feedforward capacitor is present (See Figure 1). Feedforward capacitor values larger than 0.1 µF are not recommended for the same reason, whatever be the DC input voltage. Shutdown /Soft-start (Pin 7) — The regulator is in shut- down mode, drawing about 90 µA, when this pin is driven to a low level ( ≤ 0.6V), and is in normal operation when this Pin is left floating (internal pull-up) or driven to a high level ( ≥ 2.0V). The typical value of the threshold is 1.3V and the pin is internally clamped to a maximum of about 7V. If it is driven higher than the clamp voltage, it must be ensured by means of an external resistor that the current into the pin does not exceed 1mA. The duty cycle is minimum (0%) if this Pin is below 1.8V, and increases as the voltage on the pin is increased. The maximum duty cycle (100%) occurs when this pin is at 2.8V or higher. So adding a capacitor to this pin produces a softstart feature. An internal current source will charge the capacitor from zero to its internally clamped value. The charging current is about 5 µA when the pin is below 1.3V but is reduced to only 1.6 µA above 1.3V, so as to allow the use of smaller softstart capacitors. www.national.com 9 |
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