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NCP1410DMR2G Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1410DMR2G Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 14 page NCP1410 http://onsemi.com 10 Low−Battery Detection A comparator with 30 mV hysteresis is applied to perform the low−battery detection function. When pin 2 (LBI) is at a voltage, which can be defined by a resistor divider from the battery voltage, lower than the internal reference voltage, 1.190 V, the comparator output will cause a 50 Ohm low side switch to be turned ON. It will pull down the voltage at pin 3 (LBO) which has a hundreds kilo−Ohm of pull−high resistance. If the pin 2 voltage is higher than 1.190 V + 30 mV, the comparator output will cause the 50 Ohm low side switch to be turned OFF, pin 3 will become high impedance, and its voltage will be pulled high by the external resistor. APPLICATIONS INFORMATION Output Voltage Setting The output voltage of the converter is determined by the external feedback network comprised of RFB1 and RFB2 and the relationship is given by: VOUT + 1.190 V 1 ) RFB1 RFB2 where RF2 and RF1 are the upper and lower feedback resistors respectively. Low Battery Detect Level Setting The Low Battery Detect Voltage of the converter is determined by the external divider network comprised of RLB1 and RLB2 and the relationship is given by: VLB + 1.190 V 1 ) RLB1 RLB2 where RLB1 and RLB2 are the upper and lower divider resistors respectively. Inductor Selection The NCP1410 is tested to produce optimum performance with a 22 mH inductor at VIN = 3 V, VOUT = 3.3 V supplying output current up to 250 mA. For other input/output requirements, inductance in the range 10 mH to 47 mH can be used according to end application specifications. Selecting an inductor is a compromise between output current capability and tolerable output voltage ripple. Of course, the first thing we need to obey is to keep the peak inductor current below its saturation limit at maximum current and the ILIM of the device. In NCP1410, ILIM is set at 1 A. As a rule of thumb, low inductance values supply higher output current, but also increase the ripple at output and reducing efficiency, on the other hand, high inductance values can improve output ripple and efficiency, however it also limit the output current capability at the same time. One other parameter of the inductor is its DC resistance, this resistance can introduce unwanted power loss and hence reduce overall efficiency, the basic rule is selecting an inductor with lowest DC resistance within the board space limitation of the end application. Capacitors Selection In all switching mode boost converter applications, both the input and output terminals sees pulsating voltage/current waveforms. The currents flowing into and out of the capacitors multiplying with the Equivalent Series Resistance (ESR) of the capacitor producing ripple voltage at the terminals. During the syn−rect switch off cycle, the charges stored in the output capacitor is used to sustain the output load current. Load current at this period and the ESR combined and reflected as ripple at the output terminals. For all cases, the lower the capacitor ESR, the lower the ripple voltage at output. As a general guide line, low ESR capacitors should be used. Ceramic capacitors have the lowest ESR, but low ESR tantalum capacitors can also be used as a cost effective substitute. Optional Startup Schottky Diode for Low Battery Voltage In general operation, no external Schottky diode is required, however, in case you are intended to operate the device close to 1 V level, a Schottky diode connected between the LX and OUT pins as shown in Figure 27 can help during startup of the converter. The effect of the additional Schottky was shown in Figure 8. Figure 27. Schottky Device Between LX and OUT Pins NCP1410 OUT LX COUT VOUT MBR0502 L PCB Layout Recommendations Good PCB layout plays an important role in switching mode power conversion. Careful PCB layout can help to minimize ground bounce, EMI noise and unwanted feedback that can affect the performance of the converter. Hints in the following paragraphs, can be used as guidelines in most situations. Grounding Star−ground connection should be used to connect the output power return ground, the input power return ground and the device power ground together at one point. All high current running paths must be thick enough for current flowing through and producing insignificant voltage drop along the path. Feedback signal path must be separated with the main current path and sensing directly at the anode of the output capacitor. |
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