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NCP1508 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1508 Datasheet(HTML) 10 Page - ON Semiconductor |
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10 / 15 page  NCP1508 http://onsemi.com 10 DETAILED OPERATING DESCRIPTION Overview The NCP1508 is a monolithic micro−power high frequency PWM step−down DC−DC converter specifically optimized for applications requiring high efficiency and a small PCB footprint such as portable battery powered products. It integrates synchronous rectification to improve efficiency as well as eliminate the external Schottky diode. High switching frequency allows for a low profile inductor and capacitors to be used. Four digital selectable output voltages (1.0, 1.3, 1.5 and 1.89 V) can be generated from the input supply that can range from 2.7−5.2 V. All loop compensation is integrated as well further reducing the external component count as well. The DC−DC converter has two operating modes (normal PWM, pulsed switching), which are intended to allow for optimum efficiency under either light (up to 30 mA) or heavy loads. The user determines the operating mode by controlling the SYNC input. In addition the SYNC input can be used to synchronize the PWM to an external system clock signal in the range of 500−1000 kHz. PWM Operating Mode The NCP1508 can be set to current mode PWM operation by connecting SYNC pin to VCC. In this mode, the output voltage is regulated by modulating on−time pulse width of the main switch Q1 at a fixed frequency of 1 MHz. The switching of the PMOS Q1 is controlled by a flip−flop driven by the internal oscillator and a comparator that compares the error signal from an error amplifier with the sum of the sensed current signal and compensation ramp. At the beginning of each cycle, the main switch Q1 is turned ON by the rising edge of the internal oscillation clock. The inductor current ramps up until the sum of the current sense signal and compensation ramp becomes higher than the error voltage amplifier. Once this has occurred, the PWM comparator resets the flip−flop, Q1 is turned OFF and the synchronous switch Q2 is turned ON. Q2 replaces the external Schottky diode to reduce the conduction loss and improve the efficiency. To avoid overall power loss, a certain amount of dead time is introduced to ensure Q1 is completely turned OFF before Q2 is being turned ON. In continuous conduction mode (CCM), Q1 is turned ON after Q2 is completely turned OFF to start a new clock cycle. In discontinuous conduction mode (DCM), the zero crossing comparator (ZLC) will turn off Q2 when the inductor current drops to zero. Overvoltage Protection The overvoltage protection circuit is present in PWM mode to prevent the output voltage from going too high under light load or fast load transient conditions. The output overvoltage threshold is 5% above nominal set value. If the output voltage rises above 5% of the nominal value, the OVP comparator is activated and switch Q1 is turned OFF. Switching will continue when the output voltage falls below the threshold of OVP comparator. Pulsed Mode (PM) Under light load conditions (< 30 mA), NCP1508 can be configured to enter a low current pulsed mode operation to reduce power consumption. This is accomplished by SYNC pin held LOW. The output regulation is implemented by pulse frequency modulation. If the output voltage drops below the threshold of PM comparator (typically Vnom−2%), a new cycle will be initiated by the PM comparator to turn on the switch Q1. Q1 remains ON until 200 mA inductor peak current is reached. Then ILIM comparator goes high to switch off Q1. After a short dead time delay, switch rectifier Q2 is turn ON. The zero crossing comparator will detect when the inductor current drops to zero and send the signal to turn off Q2. The output voltage continues to decrease through discharging the output capacitor. When the output voltage falls below the threshold of PM comparator again, a new cycle starts immediately. Cycle−by−Cycle Current Limit From the block diagram (Figure 3), an ILIM comparator is used to realize cycle−by−cycle current limit protection. The comparator compares the LX pin voltages with the reference voltage from the SENFET, which is biased by constant current. If the inductor current reaches the limit, ILIM comparator detects the LX voltage falling below the reference voltage from SENFET and releases the signal to turn off the switch Q1. The cycle−by−cycle current limit is set at 800 mA in PWM and 200 mA in PM. Frequency Synchronization and Operating Mode Selection The SYNC pin can also be used for frequency synchronization by connecting it with an external clock signal. It operates in PWM mode when synchronized to an external clock. The switching cycle initiates by the rising edge of the clock. The synchronization clock signals between 0.4 V and 1.2 V from 500 kHz to 1000 kHz. Gating on and off the clock, the SYNC pin can also be used to select between PM and PWM modes. It allows efficient dynamical power management by adjusting the converter operation to the specific system requirement. Set SYNC pin low to select PM mode at light load conditions (up to 30 mA) and set SYNC pin high or connect with external clock to select PWM mode at heavy load condition to achieve optimum efficiency. Table 1 shows the mode selection with three different SYNC pin states. |
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