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NCP3125ADR2G Datasheet(PDF) 8 Page - ON Semiconductor |
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NCP3125ADR2G Datasheet(HTML) 8 Page - ON Semiconductor |
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8 / 22 page  NCP3125 http://onsemi.com 8 COMP 0.9 V BG TG Figure 17. Enable/Disable Driver State Diagram External Soft−Start The NCP3125 features an external soft−start function, which reduces inrush current and overshoot of the output voltage. Soft−start is achieved by using the internal current source of 10.5 mA (typ), which charges the external integrator capacitor of the OTA. Figure 18 is a typical soft−start sequence. The sequence begins once VIN and VBST surpass their UVLO thresholds and OCP programming is complete. The current sourced out of the COMP pin continually increases the voltage until regulation is reached. Once the voltage reaches 400 mV logic is enabled. When the voltage exceeds 900 mV, switching begins. Current is sourced out of the COMP pin, placing the regulator into open loop operation until 800 mV is sensed at the FB pin. Once 800 mV is sensed at the FB pin, open loop operation ends and closed loop operation begins. In closed loop operation, the OTA is capable of sourcing and sinking 120 mA. Figure 18. Soft−Start Sequence VCC COMP VFB BG TG BG Comparator BG Comparator Output Vout UVLO POR Delay Current COMP Delay Soft−Start Normal Operation UVLO 0.9 V 3.85 V 4.2 V DAC Voltage 500 mV 50 mV Trip Set Overcurrent Threshold Setting NCP3125 overcurrent threshold can be set from 50 mV to 550 mV, by adding a resistor (RSET) between ISET and GND. During a short period of time following VIN rising over UVLO threshold, an internal 10 mA current (IOCSET) is sourced from the ISET pin, creating a voltage drop across RSET. The voltage drop is compared against a stepped internal voltage ramp. Once the internal stepped voltage reaches the RSET voltage, the value is stored internally until power is cycled. The overall time length for the OC setting procedure is approximately 9 ms. Connecting an RSET resistor between ISET and GND, the programmed threshold will be: IOCth + IOCSET *RSET RDS(on) ³ 4.2 A + 10 mA*21 kW 50 mW (eq. 1) IOCSET = Sourced current IOCth = Current trip threshold RDS(on) = On resistance of the low side MOSFET RSET = Current set resistor The RSET values range from 5 kW to 55 kW. If RSET is not connected, the device switches the OCP threshold to a fixed 375 mV value. An internal safety clamp on ISET is triggered as soon as ISET voltage reaches 700 mV, enabling the 375 mV fixed threshold and ending the OCP setting period. The current trip threshold tolerance is ±25 mV. The accuracy is best at the highest set point (550 mV). The accuracy will decrease as the set point decreases. MOSFET tolerances with temperature and input voltage will vary the over current set threshold operating point. A graph of the typical current limit set thresholds at 4.5 V and 12 V is shown in Figure . 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 5 6 7 8 9 1011 1213 1415 161718 1920 21 RSET (kW) Figure 19. RSET Value for Output Current 5 V 12 V |
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