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NCP1589D Datasheet(PDF) 6 Page - ON Semiconductor |
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NCP1589D Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 8 page NCP1589D http://onsemi.com 6 APPLICATIONS INFORMATION Overcurrent Protection (OCP) The NCP1589D monitors the voltage across the low side MOSFET and used this information to determine if there is excessive output current. The voltage across the low side MOSFET is measured from the LX pin when it is conducted, and is referenced to ground. The overcurrent measurement is timed to occur at the end of the low side MOSFET conduction period. If the voltage drop across the bottom MOSFET exceeds the overcurrent protection threshold, then an internal counter is triggered and incremented. If the voltage drop does not exceed the threshold for the next cycle, the internal counter will be reset. The NCP1589D will latch the over current protection fault condition after 4 consecutive cycles of overcurrent events. When the NCP1589D latches an overcurrent protection fault, both the high side and low side MOSFETs are turned off. To reset the overcurrent protection fault, the power to the VCC pin must be cycled. The overcurrent threshold can be set externally, by varying the ROCSET resistor shunted from low side gate pin to ground. During power on reset, after the VCC and BOOT pins both pass the undervoltage lockout threshold, the NCP1589D will source a 10 mA current from LG pin through the ROCSET resistor and produce a voltage. This voltage will be sampled and locked by the device as the overcurrent protection threshold. For example, if ROCSET is set to 10 kW, the 10 mA of current will yield a 100 mV threshold, and if the voltage across the low side MOSFET exceeds 100 mV at the end of the its conduction period, an overcurrent event will be detected. The OCP threshold is only associated with power on reset, and won’t be wiped out by pulling COMP pin down (disabling the part). If the ROCSET resistor is not present, the overcurrent protection threshold will max out at 640 mV. The recommended range for ROCSET is 5 kW to 60 kW which yields a threshold voltage range of 50 mV to 600 mV. Internal Soft−Start To prevent excess inrush current during startup, the NCP1589D uses a calibrated current source with an internal soft−start capacitor to ramp the reference voltage from 0 V to 800 mV over a period of around 4 ms. The soft−start ramp generator will reset if the input power supply voltages reach the undervoltage lockout threshold, or if the NCP1589D is disabled by having the COMP pin pulled low. Startup into a Precharged Load During a startup, the NCP1589D will detect the residual charge on the output capacitors. Instead of fully discharging the capacitors, the soft−start will begin from the precharged output voltage level. For example, if the NCP1589D is configured to provide a regulated output voltage of 2.5 V, the normal soft−start sequence will ramp the output voltage from 0 to 2.5 V in 4.2 ms; however if the output capacitors already has 1.2 V voltage, the NCP1589D will not discharge the capacitors, instead the soft−start sequence will begin at 1.2 V and then ramp the output up to 2.5 V. Power Good The PGOOD pin is an open drain connection, with an active high output to signal the condition of the converter. PGOOD is pulled low during soft−start cycle, and if there is overvoltage or undervoltage fault. If the voltage on the FB pin is within ±10% of Vref (800 mV) then the PGOOD pin will not be pulled low. The PGOOD pin does not have an internal pull-up resistor. Overvoltage Protection (OVP) If the voltage on the FB pin exceeds the overvoltage threshold (1000 mV, 125% of Vref), the NCP1589D will latch an overvoltage fault. During an overvoltage fault event the UG pin will be pulled low, and the LG pin will stay high until the voltage on the FB pin goes below Vref/2 (400 mV). If the overvoltage fault condition stays, the NCP1589D will continue drive the LG pin, LG will go high if FB exceeds 1000 mV, then go low when FB is below 400 mV. The power of the NCP1589D needs to be cycled up to clear the overvoltage fault. Undervoltage Protection (UVP) If the voltage on the FB pin falls below the undervoltage threshold after the soft−start cycle completes, then the NCP1589D will latch an undervoltage fault. During an undervoltage fault, both the UG and LG pins will be pulled low. Toggling power or COMP pin will reset the undervoltage protection unit. VORPM (RPM threshold) The NCP1589D runs in RPM mode, its switching frequency is controlled by COMP ripple voltage and RPM threshold. The VORPM pin is connected to the output voltage through an external divider. This voltage value is proportional to the output voltage and sets the RPM threshold voltage internally with input voltage information obtained through the switch node. The internal RPM threshold voltage (DTH) is a function of both Vout and Vin. DTH + Vout R9 R10)R9 Vin Vramp ) Voffset (eq. 1) Where R9/R10 (Figure 1) is the input voltage divider of VORPM pin Vramp is the internal ramp amplitude, Voffset is the offset voltage of the threshold. Each time when COMP voltage exceeds RPM threshold voltage, an internal ramp signal is started and UG is driven high. When the internal ramp intercepts with COMP voltage, the UG pin is reset low. The NCP1589D system operates at pseudo-fixed frequency in continuous current |
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