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NCP45521 Datasheet(PDF) 5 Page - ON Semiconductor |
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NCP45521 Datasheet(HTML) 5 Page - ON Semiconductor |
5 / 10 page NCP45520, NCP45521 http://onsemi.com 5 APPLICATIONS INFORMATION Enable Control Both the NCP45520 and the NCP45521 have two part numbers, NCP4552x-H and NCP4552x-L, that only differ in the polarity of the enable control. The NCP4552x-H devices allow for enabling the MOSFET in an active-high configuration. When the VCC supply pin has an adequate voltage applied and the EN pin is at a logic high level, the MOSFET will be enabled. Similarly, when the EN pin is at a logic low level, the MOSFET will be disabled. An internal pull down resistor to ground on the EN pin ensures that the MOSFET will be disabled when not being driven. The NCP4552x-L devices allow for enabling the MOSFET in an active-low configuration. When the VCC supply pin has an adequate voltage applied and the EN pin is at a logic low level, the MOSFET will be enabled. Similarly, when the EN pin is at a logic high level, the MOSFET will be disabled. An internal pull up resistor to VCC on the EN pin ensures that the MOSFET will be disabled when not being driven. Power Sequencing The NCP4552x devices will function with any power sequence, but the output turn−on delay performance may vary from what is specified. To achieve the specified performance, there are two recommended power sequences: 1) VCC → VIN → VEN 2) VIN → VCC → VEN Load Bleed (Quick Discharge) The NCP4552x devices have an internal bleed resistor, RBLEED, which is used to bleed the charge off of the load to ground after the MOSFET has been disabled. In series with the bleed resistor is a bleed switch that is enabled whenever the MOSFET is disabled. The MOSFET and the bleed switch are never concurrently active. It is required that the BLEED pin be connected to VOUT either directly (as shown in Figures 4 and 7) or through an external resistor, REXT (as shown in Figures 3 and 6). REXT should not exceed 1 k W and can be used to increase the total bleed resistance. Care must be taken to ensure that the power dissipated across RBLEED is kept at a safe level. REXT can be used to decrease the amount of power dissipated across RBLEED. Power Good The NCP45520 devices have a power good output (PG) that can be used to indicate when the gate of the MOSFET is fully charged. The PG pin is an active-high, open-drain output that requires an external pull up resistor, RPG, greater than or equal to 1 k W to an external voltage source, VTERM, that is compatible with input levels of all devices connected to this pin (as shown in Figures 3 and 4). The power good output can be used as the enable signal for other active−high devices in the system (as shown in Figure 5). This allows for guaranteed by design power sequencing and reduces the number of enable signals needed from the system controller. If the power good feature is not used in the application, the PG pin should be tied to GND. Slew Rate Control The NCP4552x devices are equipped with controlled output slew rate which provides soft start functionality. This limits the inrush current caused by capacitor charging and enables these devices to be used in hot swap applications. The slew rate of the NCP45521 can be decreased with an external capacitor added between the SR pin and ground (as shown in Figures 6 and 7). With an external capacitor present, the slew rate can be determined by the following equation: Slew Rate + KSR CSR [V s] (eq. 1) where KSR is the specified slew rate control constant, found in Table 4, and CSR is the slew rate control capacitor added between the SR pin and ground. The slew rate of the device will always be the lower of the default slew rate and the adjusted slew rate. Therefore, if the CSR is not large enough to decrease the slew rate more than the specified default value, the slew rate of the device will be the default value. The SR pin can be left floating if the slew rate does not need to be decreased. Short−Circuit Protection The NCP4552x devices are equipped with short−circuit protection that is used to help protect the part and the system from a sudden high−current event, such as the output, VOUT, being shorted to ground. This circuitry is only active when the gate of the MOSFET is fully charged. Once active, the circuitry monitors the difference in the voltage on the VIN pin and the voltage on the BLEED pin. In order for the VOUT voltage to be monitored through the BLEED pin, it is required that the BLEED pin be connected to VOUT either directly (as shown in Figures 4 and 7) or through a resistor, REXT (as shown in Figures 3 and 6), which should not exceed 1 k W. With the BLEED pin connected to VOUT, the short−circuit protection is able to monitor the voltage drop across the MOSFET. If the voltage drop across the MOSFET is greater than or equal to the short−circuit protection threshold voltage, the MOSFET is immediately turned off and the load bleed is activated. The part remains latched in this off state until EN is toggled or VCC supply voltage is cycled, at which point the MOSFET will be turned on in a controlled fashion with the normal output turn−on delay and slew rate. The current through the MOSFET that will cause a short−circuit event can be calculated by dividing the short−circuit protection threshold by the expected on−resistance of the MOSFET. Thermal Shutdown The thermal shutdown of the NCP4552x devices protects the part from internally or externally generated excessive |
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