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ISL80101IR08Z-T Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL80101IR08Z-T Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 15 page 7 FN6931.0 December 21, 2009 Functional Description Enable Operation The Enable turn-on threshold is typically 0.8V with a hysteresis of 80mV. The Enable pin doesn't have an internal pull-up or pull-down resistor. As a result, this pin must not be left floating. This pin must be tied to VIN if it is not used. A pull-up resistor (typically 1kΩ to 10kΩ) will be required for applications that use open collector or open drain outputs to control the Enable pin. The Enable pin may be connected directly to VIN for applications that are always on. Soft-Start Operation The soft-start circuit controls the rate at which the output voltage comes up to regulation at power-up or coming out of a chip disable. A constant current charges an external soft-start capacitor. The external capacitor always gets discharged to 0V at start-up of after coming out of a chip disable. The discharge rate is the RC time constant of an internal resistance and CSS. The soft-start function effectively limits the amount of in-rush current below the programmed current limit during start-up or an enable sequence to avoid an overcurrent fault condition. This can be an issue for applications that require large, external bulk capacitances on VOUT where high levels of charging current can be seen for a significant period of time. High in-rush currents can cause VIN to drop below minimum which could cause VOUT to shutdown. Equation 3 can be used to calculate CSS for a desired in-rush current. Where VOUT is the output voltage, COUT is the total capacitance on the output and IINRUSH is the desired in-rush current. The following scope in Figure 3 captures the response for the soft-start function.The output voltage is set to 1.8V. Also The rise time of the regulator output voltage for a given CSS value can be calculated using Equation 2. Power-Good Operation The PGOOD circuit monitors VOUT and signals a fault condition when VOUT is below 85% of the nominal output voltage. The PGOOD flag is an open-drain NMOS that can sink 10mA during a fault condition. The PGOOD pin requires an external pull up resistor which is typically connected to the VOUT pin. The PGOOD pin should not be pulled up to a voltage source greater than VIN. During a fault condition, the PGOOD output is pulled low. The PGOOD fault can be caused by the current limit fault or low input voltage. The PGOOD does not function during thermal shutdown and when the part is disabled. C SS V OUT xCOUTx2μA ) () I INRUSH x0.5 V ----------------------------------------------------------- = (EQ. 1) FIGURE 3. IN-RUSH CURRENT WITH NO CSS, COUT = 1000µF, IN-RUSH CURRENT = 1.8A FIGURE 4. IN-RUSH CURRENT WITH CSS = 15nF, COUT = 1000µF, IN-RUSH CURRENT = 0.5A FIGURE 5. IN-RUSH CURRENT WITH CSS = 33nF, COUT = 1000µF, IN-RUSH CURRENT = 0.2A t RAMP C SSx0.5V 2 μA ---------------------------- = (EQ. 2) ISL80101 |
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