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NCP1589AMNTWG Datasheet(PDF) 6 Page - ON Semiconductor |
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NCP1589AMNTWG Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 10 page NCP1589A, NCP1589B www.onsemi.com 6 APPLICATIONS INFORMATION Over Current Protection (OCP) The NCP1589A/B monitors the voltage drop across the low side mosfet and uses this information to determine if there is excessive output current. The voltage across the low side mosfet is measured from the LX pin, and is referenced to ground. The over current measurement is timed to occur at the end of the low side mosfet conduction period, just before the bottom mosfet is turned off. If the voltage drop across the bottom mosfet exceeds the over current protection threshold, then an internal counter is incremented. If the voltage drop does not exceed the over current protection threshold, then the internal counter is reset. The NCP1589A/B will latch the over current protection fault condition only if the over current protection threshold is exceeded for four consecutive cycles. When the NCP1589A/B latches an over current protection fault, both the high side and low side mosfets are turned off. To reset the over current protection fault, the power to the VCC pin must be cycled. The over current threshold voltage can be externally, by varying the value of the ROCSET resistor. The ROCSET resistor is a resistor connected between the LG pin (low side mosfet gate) and ground. During startup, after the VCC and BOOT pins reach the under voltage lock out threshold, the NCP1589A/B will source 10 mA of current out of the LG pin. This current will flow through the ROCSET resistor and produce a voltage that is sampled and then used as the over current protection threshold voltage. For example, if ROCSET is set to 10 k W, the 10 mA of current will yield a 100 mV threshold, and if the voltage drop across the low side mosfet exceeds 100 mV at the end of its conduction period, then an over current event will be detected. If the ROCSET resistor is not present, then the over current protection threshold will max out at 640 mV. The valid range for ROCSET is 5 k W to 55 kW which yields a threshold voltage range of 50 mV to 550 mV. Internal Soft-Start To prevent excess inrush current during startup, the NCP1589A/B uses a calibrated current source with an internal soft start capacitor to ramp the reference voltage from 0 to 800 mV over a period of 4 ms. The softstart ramp generator will reset if the input power supply voltages reach the under voltage lockout threshold, or if the NCP1589A/B is disabled by having the COMP pin pulled low. Startup into a Precharged Load During a startup and soft start sequence the NCP1589A will detect a residual charge on the output capacitors and not forcefully discharge the capacitors before beginning the softstart sequence, instead, the softstart ramping of the output will begin at the voltage level of the residual charge. For example, if the NCP1589A/B is configured to provide a regulated output voltage of 2.5 V, the normal softstart sequence will ramp the output voltage from 0 to 2.5 V in 4.2 ms; however if the output capcitors already have a 1.2 V charge on them, the NCP1589A/B will not discharge the capacitors, instead the softstart sequence will begin at 1.2 V and then ramp the output to 2.5 V. Power Good The PGOOD pin is an open drain active high output pin that signals the condition of the VOS (Voltage Output Sense) pin. PGOOD is pulled low during soft start cycle, and if there is a latched over current, over voltage, or under voltage fault. If the voltage on the VOS 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 If the voltage on the VOS pin exceeds the over voltage threshold the NCP1589A/B will latch an over voltage fault. During an over voltage fault the UG pin will be pulled low, and the LG pin will be high while the until the voltage on the VOS pin goes below Vref/2 (400 mV). The NCP1589A will continue drive the LG pin, LG will go high if VOS exceeds 1 V and then go low when VOS goes below 400 mV. The power to the NCP1589 must be cycled to reset the over voltage protection fault. Under Voltage Protection If the voltage on the VOS pin falls below the under voltage threshold after the soft start cycle completes, then the NCP1589A/B will latch an under voltage fault. During an under voltage fault, both the UG and LG pins will be pulled low. The power to the NCP1589 must be cycled to reset the under voltage protection fault. |
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Similar Description - NCP1589AMNTWG |
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