Datasheet NCP1421 (ON Semiconductor) - 10

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NCP1421
reference voltage of the controller is disabled and the (LBI/EN) is at a voltage (defined by a resistor divider from controller typically consumes only 50 nA of current. If the the battery voltage) lower than the internal reference pin 2 voltage is raised to higher than 0.5 V (for example, by voltage of 1.20 V, the comparator output turns on a 50 a resistor connected to VIN), the IC is enabled again, and the low side switch. It pulls down the voltage at pin 3 (LBO) internal circuit typically consumes 8.5 A of current from which requires a hundred to a thousand k of external the OUT pin during normal operation. pull−high resistance. If the pin 2 voltage is higher than 1.20 V + 30 mV, the comparator output turns off the 50
Low−Battery Detection
low side switch. When this occurs, pin 3 becomes high A comparator with 30 mV hysteresis is applied to impedance and its voltage is pulled high again. perform the low−battery detection function. When pin 2
APPLICATIONS INFORMATION Output Voltage Setting
voltage/current waveforms. The currents flowing into and A typical application circuit is shown in Figure 26. The out of the capacitors multiply with the Equivalent Series output voltage of the converter is determined by the Resistance (ESR) of the capacitor to produce ripple voltage external feedback network comprised of R1 and R2. The at the terminals. During the Syn−Rect switch−off cycle, the relationship is given by: charges stored in the output capacitor are used to sustain the output load current. Load current at this period and the ESR VOUT + 1.20 V ǒ1 ) R1Ǔ R2 combine and reflect as ripple at the output terminals. For where R1 all cases, the lower the capacitor ESR, the lower the ripple and R2 are the upper and lower feedback resistors, respectively. voltage at output. As a general guideline, low ESR capacitors should be used. Ceramic capacitors have the
Low Battery Detect Level Setting
lowest ESR, but low ESR tantalum capacitors can also be The Low Battery Detect Voltage of the converter is used as an alternative. determined by the external divider network that is comprised of R3 and R4. The relationship is given by:
PCB Layout Recommendations
Good PCB layout plays an important role in switching VLB + 1.20 V ǒ1 ) R3Ǔ mode power conversion. Careful PCB layout can help to R4 minimize ground bounce, EMI noise, and unwanted where R3 and R4 are the upper and lower divider resistors feedback that can affect the performance of the converter. respectively. Hints suggested below can be used as a guideline in most situations.
Inductor Selection
The NCP1421 is tested to produce optimum performance
Grounding
with a 5.6 H inductor at VIN = 2.5 V and VOUT = 3.3 V, A star−ground connection should be used to connect the supplying an output current up to 600 mA. For other output power return ground, the input power return ground, input/output requirements, inductance in the range 3 H to and the device power ground together at one point. All 10 H can be used according to end application high−current paths must be as short as possible and thick specifications. Selecting an inductor is a compromise enough to allow current to flow through and produce between output current capability, inductor saturation insignificant voltage drop along the path. The feedback limit, and tolerable output voltage ripple. Low inductance signal path must be separated from the main current path values can supply higher output current but also increase and sense directly at the anode of the output capacitor. the ripple at output and reduce efficiency. On the other hand, high inductance values can improve output ripple
Components Placement
and efficiency; however, it is also limited to the output Power components (i.e., input capacitor, inductor and current capability at the same time. output capacitor) must be placed as close together as Another parameter of the inductor is its DC resistance. possible. All connecting traces must be short, direct, and This resistance can introduce unwanted power loss and thick. High current flowing and switching paths must be reduce overall efficiency. The basic rule is to select an kept away from the feedback (FB, pin 1) terminal to avoid inductor with the lowest DC resistance within the board unwanted injection of noise into the feedback path. space limitation of the end application. In order to help with
Feedback Network
the inductor selection, reference charts are shown in Feedback of the output voltage must be a separate trace Figure 27 and 28. detached from the power path. The external feedback network must be placed very close to the feedback (FB,
Capacitors Selection
In all switching mode boost converter applications, both pin 1) pin and sense the output voltage directly at the anode the input and output terminals see impulsive of the output capacitor.
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