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NCP1450ASN27T1 Datasheet(PDF) 17 Page - ON Semiconductor |
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NCP1450ASN27T1 Datasheet(HTML) 17 Page - ON Semiconductor |
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17 / 22 page  NCP1450A http://onsemi.com 17 DETAILED OPERATING DESCRIPTION Operation The NCP1450A series are monolithic power switching controllers optimized for battery powered portable products where large output current is required. The NCP1450A series are low noise fixed frequency voltage−mode PWM DC−DC controllers, and consist of startup circuit, feedback resistor divider, reference voltage, oscillator, loop compensation network, PWM control circuit, and low ON resistance driver. Due to the on−chip feedback resistor and loop compensation network, the system designer can get the regulated output voltage from 1.8 V to 5.0 V with 0.1 V stepwise with a small number of external components. The quiescent current is typically 93 mA (VOUT = 2.7 V, fOSC = 180 kHz), and can be further reduced to about 1.5 mA when the chip is disabled (VCE t 0.3 V). The NCP1450A operation can be best understood by referring to the block diagram in Figure 2. The error amplifier monitors the output voltage via the feedback resistor divider by comparing the feedback voltage with the reference voltage. When the feedback voltage is lower than the reference voltage, the error amplifier output will decrease. The error amplifier output is then compared with the oscillator ramp voltage at the PWM controller. When the ramp voltage is higher than the error amplifier output, the high−side driver is turned on and the low−side driver is turned off which will then switch on the external transistor; and vice versa. As the error amplifier output decreases, the high−side driver turn−on time increases and duty cycle increases. When the feedback voltage is higher than the reference voltage, the error amplifier output increases and the duty cycle decreases. When the external power switch is on, the current ramps up in the inductor, storing energy in the magnetic field. When the external power switch is off, the energy stored in the magnetic field is transferred to the output filter capacitor and the load. The output filter capacitor stores the charge while the inductor current is higher than the output current, then sustains the output voltage until the next switching cycle. As the load current is decreased, the switch transistor turns on for a shorter duty cycle. Under the light load condition, the controller will skip switching cycles to reduce power consumption, so that high efficiency is maintained at light loads. Soft Start There is a soft start circuit in NCP1450A. When power is applied to the device, the soft start circuit first pumps up the output voltage to approximately 1.5 V at a fixed duty cycle. This is the voltage level at which the controller can operate normally. In addition to that, the startup capability with heavy loads is also improved. Oscillator The oscillator frequency is internally set to 180 kHz at an accuracy of "20% and with low temperature coefficient of 0.11%/ °C. Regulated Converter Voltage (VOUT) The VOUT is set by an integrated feedback resistor network. This is trimmed to a selected voltage from 1.8 V to 5.0 V range in 100 mV steps with an accuracy of "2.5%. Compensation The device is designed to operate in continuous conduction mode. An internal compensation circuit was designed to guarantee stability over the full input/output voltage and full output load range. Enable/Disable Operation The NCP1450A series offer IC shutdown mode by chip enable pin (CE pin) to reduce current consumption. When voltage at pin CE is equal or greater than 0.9 V, the chip will be enabled, which means the controller is in normal operation. When voltage at pin CE is less than 0.3 V, the chip is disabled, which means IC is shutdown. Important: DO NOT apply a voltage between 0.3 V to 0.9 V to pin CE as this is the CE pin’s hysteresis voltage range. Clearly defined output states can only be obtained by applying voltage out of this range. |
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